Potential decoupling of CO_2 and Hg uptake process by global vegetation in the 21st century

Domain: earth_science
Published: 2024-05-27
Editorial outcome: major_revision

Abstract
Mercury (Hg), a potent neurotoxin posing risks to human health, is cycled through vegetation uptake, which is susceptible to climate change impacts. However, the extent and pattern of these impacts are largely unknown, obstructing predictions of Hg’s fate in terrestrial ecosystems. Here, we evaluate the effects of climate change on vegetation elemental Hg [Hg(0)] uptake using a state-of-the-art global terrestrial Hg model (CLM5-Hg) that incorporates plant physiology. In a business-as-usual scenario, the terrestrial Hg(0) sink is predicted to decrease by 1870 Mg yr−1 in 2100, that is ~60% lower than the present-day condition. We find a potential decoupling between the trends of CO2 assimilation and Hg(0) uptake process by vegetation in the 21st century, caused by the decreased stomatal conductance with increasing CO2. This implies a substantial influx of Hg into aquatic ecosystems, posing an elevated threat that warrants consideration during the evaluation of the effectiveness of the Minamata Convention.

Introduction
Mercury (Hg) is a pervasive toxic pollutant causing adverse effects on human health at a global scale1. Additionally, it endangers ecosystems by bioaccumulating in food chains, affecting biodiversity and disrupting ecological balances2. Anthropogenic activities, such as fossil fuel combustion and metal mining, have significantly increased Hg emissions and caused widespread environmental Hg contamination since the industrial era3,4. Vegetation within terrestrial ecosystems can absorb large amounts of atmospheric gaseous elemental Hg [Hg(0)] (2200–3600 Mg year−1), acting as a major sink for the atmosphere in the present-day Hg cycles5,6. Long-living vegetation not only stores present-day Hg but also Hg emitted into the atmosphere decades ago7.

Climate-related factors, such as rising temperatures and elevated carbon dioxide (eCO2), are profoundly affecting the growth and physiological processes of vegetation8,9. However, their cascading effects on the terrestrial Hg cycle, especially the uptake of Hg(0) by vegetation, remain unclear. Here, we evaluate this effect by using a coupled climate–land–mercury model running for the twenty-first century.

Previous research has revealed complex impact pathways of climate change on vegetation Hg(0) uptake. The higher temperature was found to boost vegetation’s ability to absorb atmospheric Hg(0) in glacier retreat areas10. Yet, the rising temperatures often lead to localized droughts, which are likely to weaken the Hg sink in terrestrial ecosystems11. Furthermore, the alteration in global precipitation patterns can affect the Hg sink of forests12. The future increases in vegetation density driven by CO2 fertilization are expected to enhance the Hg(0) dry deposition velocity13,14. Stomatal uptake of Hg(0) by foliage was found to be inhibited under high vapor pressure deficit (VPD) conditions and proved to be sensitive to extreme climate events15–17. However, previous studies have not systematically considered the interaction of climate change, vegetation dynamics, and Hg processes, and some have primarily focused only on specific regions14,18. Therefore, our research aims to exam how vegetation-regulated atmospheric Hg(0) deposition will change under the impact of future climate change. We hypothesize that the climate will influence global vegetation Hg(0) uptake by altering the plant physiology such as the stomatal activities, with CO2 and other meteorological factors as important driver factors. This study uses the Community Land Model–Hg (CLM5-Hg) within the Community Earth System Model (CESM), which incorporates a dynamic plant growth framework that includes the stomatal uptake process of Hg(0) and various plant functional types (PFTs)19. This model also encompasses the comprehensive biogeochemical cycling of Hg in terrestrial ecosystems. It is forced by different future climate scenarios throughout the twenty-first century: (i) the Shared Socioeconomic Pathway (SSP) 1-2.6, a.k.a. the “2°C scenario” representing a sustainability framework; (ii) the SSP3-7.0, representing a medium-high reference within the socio-economic context of “regional rivalry”; (iii) SSP5-8.5, a.k.a. the “business-as-usual”, considered as the worst-case scenario within a high fossil fuel-intensive world20. These future scenarios are compared against the present-day simulation (baseline case). We also include a pre-industrial scenario (ca. 1850) for comparison. Furthermore, we design sensitivity experiments by alternatively changing specific climate-related factors for the SSP5-8.5 scenario while maintaining others consistent with the baseline scenario (Supplementary Table 1). These experiments can diagnose and compare the influence of individual factors on the uptake of Hg(0) by vegetation in terrestrial ecosystems. We consider factors including atmospheric CO2 concentration (with a focus on biogeochemical effects only), precipitation, temperature, humidity, pressure, radiation, and wind. We keep the anthropogenic Hg emissions and the atmospheric Hg concentrations constant for all scenarios to highlight the impact of climate factors, and remove the effects of land use and land cover change (LULCC) and aerosols (see “Methods”).

Results
Our results indicated that, under future climate change scenarios, the biogeochemical effects of elevated CO2 emerge as the dominant factor influencing vegetation Hg(0) uptake. This uptake represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage. In the SSP5-8.5 (business-as-usual) scenario where only CO2 concentration is altered and other climatological factors kept as present day, the global Hg(0) uptake decreases by 1870 Mg year−1 or 59.6% in 2100, in comparison to the present-day condition of 3138 Mg year−1 (Fig. 1). The most significant changes were simulated in East Asian and the Amazon forests, attributed to their high Hg(0) assimilation compared to other regions19. The global vegetation Hg(0) uptake was predicted to further decrease by only 88 Mg year−1 while accounting for the changes of all factors in the SSP5-8.5 scenarios (Supplementary Fig. 2). Other climate change factors, such as changing temperature, precipitation, radiation, pressure, and humidity account for a much smaller effect than the biogeochemical effects of eCO2 alone (Supplementary Figs. 3 and 4b). We also found no significant interaction between the eCO2 effect and other factors (Supplementary Fig. 5).

We predicted a higher global vegetation Hg(0) uptake for the other two future scenarios with lower CO2 levels: 2685 Mg year−1 for the SSP1-2.6 (2°C) scenario (CO2 = 445.6 p.p.m.) and 1570 Mg year−1 for SSP3-7.0 (regional rivalry) scenario (CO2 = 867.2 p.p.m.) vs. 1268 Mg year−1 for SSP5-8.5 (business-as-usual) scenario (CO2 = 1135.2 p.p.m.) (Fig. 2). A slight increase in Hg(0) uptake was simulated during the pre-industrial era: 3324 Mg year−1 when the global average CO2 is lower at 288 p.p.m. Unlike the future scenarios, we noted the largest impact is contributed by the lower atmospheric humidity in the pre-industrial era (Supplementary Figs. 3d and 4a). Changes in precipitation and temperature, as well as its interaction with the biogeochemical effects of eCO2, significantly affect the uptake of Hg(0) by global vegetation (see Supplementary Figs. 4a and 5a). This suggested that the biogeochemical impact of eCO2 on Hg(0) uptake has not yet become dominant when compared with other climate change factors in the pre-industrial era. When all scenarios were considered together, we observe a continuous decrease in the potential of vegetation to uptake Hg(0) in the future as CO2 levels increase (Supplementary Fig. 12).

Direct field evidence that integrated the various processes of terrestrial Hg cycling, and was responsive to multiple climate change factors across different spatiotemporal scales, remained scarce. Previous manipulative experiments focusing on CO2 enrichment have primarily concentrated on the biogeochemical impact of eCO2 on vegetation Hg concentrations (Fig. 3a). The CO2 concentrations were usually enhanced to 360–610 p.p.m. in these experiments, similar to our future scenarios. Data on Hgveg (Hg flux or Hg concentration of plant) from six eCO2 experimental studies were integrated into four eCO2 conditions based on the levels of increased CO2 concentration (Supplementary Fig. 10). Under experimental conditions of a 150 and 200 p.p.m. increase in CO2 concentration, Hgveg showed a significant decreasing trend (P < 0.05, Supplementary Fig. 10a). Despite the lack of significant differences at 253 and 360 p.p.m., even showing an opposite trend at 253 p.p.m. (Supplementary Fig. 10b), the overall effect of our meta-analysis (P < 0.01) suggested that eCO2 had a suppressive effect on vegetation Hg levels (Fig. 3b). The meta-analysis revealed a significant decrease in vegetation Hg levels as a result of eCO2, showing an average decrease in foliage Hg levels or Hg uptake of 5.87% per 100 p.p.m. increase (95% CI, −6.5% to −5.3%). If translated to the increased CO2 concentration level under our model’s SSP5-8.5 scenario, this change rate reached nearly 50%. The change in terrestrial Hg(0) sink (~60%) simulated by our model was fairly close to the experimentally observed values. Overall, our findings are consistent with existing evidences synthesized from experimental data worldwide, as atmospheric uptake is the major source of Hg in foliage6,21. Indeed, a multitude of studies, including those employing isotopic techniques, have demonstrated that a predominant portion of atmospheric Hg(0) is assimilated into terrestrial ecosystems via vegetation, primarily through stomatal uptake, accounting for over 80% of the total uptake22,23.

We found that this effect varies across different PFTs. The most substantial suppression by eCO2 was observed in crops (−12.2 ± 1.1% per 100 p.p.m. CO2), while the suppression in grasses (−5.8 ± 1.1% per 100 p.p.m. CO2) and trees (−5.2 ± 0.3% per 100 p.p.m. CO2) was relatively similar (Fig. 3b). In our model, similar pattern was simulated. We also extracted the Plant Functional Types (PFTs) corresponding to the species used in the meta-analysis experimental data. We found that the modeled global Hg(0) uptake of trees, crops, and grasses in 2100 all showed a substantial decrease under the SSP5-8.5 scenario (Fig. 3c). Although trees still dominated the reduction of total Hg(0) uptake due to their largest global coverage (Supplementary Fig. 11), crops were the most affected by eCO2, showing a decline of nearly 60%. Indeed, massive grass species belong to C4 photosynthetic processes of carbon fixation in plants (C4 plants), while trees and crops are predominantly C3 photosynthetic processes of carbon fixation in plants (C3 plants)24. The photosynthesis of the former is less limited by ambient atmospheric CO2 concentrations and subsequently responds less to eCO2 than the latter25. As C3 plants dominate global vegetation and account for most of the Earth’s current plant life26, an overall significant weakening of global vegetation Hg(0) uptake by the biogeochemical effects of eCO2 was found (Supplementary Fig. 12).

We found the reduced Hg uptake predicted by the model for the future was caused by a decrease in stomatal conductance due to eCO2. Vegetation uptake Hg(0) via diffusion through stomatal pores, which is subsequently fixed by foliage19,23,27. Stomatal conductance depends on its aperture and is associated with plant physiological activities28–30 (see Eq. 1 in “Methods”). The Medlyn model in our CLM5-Hg model can effectively simulate the response of stomatal conductance to eCO2 (see Supplementary Figs. 7 and 8, details in the model validation section of the Supplementary Information). A 42.6% decrease in the stomatal conductance of global vegetation was projected for the year 2100 with eCO2 under the SSP5-8.5 (business-as-usual) scenario, compared to present-day levels (Fig. 4). Our analysis identified that the sunlit stomatal conductance emerges as the key driving factor influencing the observed reduction in Hg(0) uptake. Specifically, the changes of conductance on the sunlit side of leaves were more consistent with the distribution of vegetation Hg(0) uptake than the shaded sides, indicating the changes in the sunlit side as a primary factor contributing to the diminished global vegetation Hg(0) uptake (Fig. 1). In general, the sunlit side of leaves receives more direct sunlight and heat, prone to stomatal closure31. Conversely, the shaded side has a higher stomatal conductance density and is reserved for gas exchange with relatively stable aperture32.

Our study illustrated a complex and extensive feedback mechanism between the terrestrial Hg, water and carbon cycles. The enhancement of photosynthesis is caused by the biogeochemical effect of eCO2 is accompanied by the loss of plant water content33. During this process, plants adjust the stomatal aperture to reduce water transpiration and maximize water use efficiency34,35. The Medlyn model in our CLM5-Hg model is consistent with this optimal stomatal theory. With eCO2, the CO2 partial pressure at the leaf surface (Cs) also increased accordingly, leading to enhanced leaf photosynthesis (An) (Eq. (1)). However, An is constrained by water potential while Cs continues to increase in CLM529. This resulted in reduced stomatal conductance (gs) with eCO2 (Supplementary Fig. 13), leading to decreased evapotranspiration (Fig. 4d). This process induced increased soil water storage, enhancing water use efficiency (Supplementary Fig. 14). This adaptive mechanism ultimately led to a nonlinear relationship between atmospheric CO2 concentration and stomatal conductance. Stomatal conductance is directly related to vegetation uptake of Hg(0) in our model (Eqs. (1)–(4), for details refer to “Methods”). This relationship explained the gradual reduction in vegetation uptake of Hg(0). We observed this reduction from the pre-industrial and present-day periods to the SSP1-2.6 scenario (Supplementary Fig. 12). Yet, there was a significant decline in the Hg(0) uptake by vegetation from SSP1-2.6 to SSP3-7.0 due to the dramatic increase in atmospheric CO2 concentration (Fig. 2). Intriguingly, the sensitivity of stomata to eCO2 diminished gradually under the influence of long-term eCO2 conditions. This occurred because guard cells and mesophyll tissues, which mediate stomatal movements, lead to decreases in stomatal aperture and size, culminating in physiological adaptation to higher concentrations36,37. Consequently, this resulted in a less pronounced decline from the SSP3-7.0 to SSP5-8.5 scenarios (Fig. 2).

A tight coupling between carbon and Hg in terrestrial ecosystems has been observed as a paradigm over the past two decades10,38. Conventionally, it has been postulated that rising atmospheric CO2 levels would increase vegetation’s photosynthesis rate, leading to the beneficial impact on plant growth, known as the CO2 fertilization effect11. This effect is believed to enhance the concurrent absorption of both CO2 and Hg(0), as suggested by Jiskra et al.39, Obrist40, and Schaefer et al.41. For example, atmospheric Hg and CO2 have similar seasonal fluctuation patterns in both hemispheres, regulated by vegetation photosynthetic activity. An increase in terrestrial net primary production has been also speculated to contribute to a diminishing trend in atmospheric Hg(0) levels in the Northern Hemisphere over the past two decades39,42,43. Contrarily, we predicted a potential decoupling between the trends of CO2 assimilation and Hg(0) uptake process by vegetation in the twenty-first century, when considering the dynamic response of vegetation physiological activities to climate change. The CLM5 model projected an increased greening of vegetation in many regions in the twenty-first century resulting from eCO2 (a.k.a. fertilization effect), evidenced by the increased leaf area index (LAI) in the northern mid-to-low latitudes and certain regions of the Southern Hemisphere (Fig. 4c). The increase in photosynthesis can simultaneously induce a state of water deficit and nutrient saturation within the plant’s internal environment44. Therefore, under climate change, the increase in vegetation LAI may only represent an increase in leaf density or even stomatal numbers, but stomatal conductance may not necessarily increase accordingly. However, our model suggested a discernible decrease in the flux of Hg(0) uptake by vegetation in these areas (Fig. 1), reflecting the differences in CO2 and Hg element during plant physiological processes especially those related to water dynamics in terrestrial ecosystems45.

Methods
CLM5-Hg
We applied a state-of-the-art global terrestrial Hg model to explore the impact of climate change on global vegetation Hg(0) uptake. We used the CLM5-Hg model, which comprehensively contains the biogeophysical and biogeochemical processes that control terrestrial Hg cycling (Supplementary Fig. 1). CLM5-Hg was tested against observational field data by Yuan et al.19. The model simulates the migration, transformation, accumulation, and emission processes of Hg in terrestrial ecosystems. This includes processes such as the stomatal and non-stomatal uptake of Hg(0) in leaves, the throughfall of divalent Hg [Hg(II)], and the formation of litter and soil Hg. It also includes processes such as the leaching of soil Hg, photo-reduction, as well as microbial decomposition, thermal evaporation, and emissions from wildfires.

The CLM5 model represents its surface heterogeneity with multi-layer nested grid cells. The first layer of the sub-grid consists of land units, including five types: vegetation, lakes, cities, glaciers, and crops. The second layer of the sub-grid represents soil columns, indicating the state changes of the soil within the same land unit. The third layer of the sub-grid is plant functional types (PFTs) with different biogeochemical processes. Sub-grids within the same model grid use the same atmospheric forcing dataset, but the diagnostic variables for each sub-grid are simulated independently. The vegetation covering the land surface is composed of 16 different PFTs (temperate-needleleaf evergreen tree, boreal-needleleaf evergreen tree, boreal-needleleaf deciduous tree, tropical-broadleaf evergreen tree, temperate-broadleaf evergreen tree, tropical-broadleaf deciduous tree, temperate-broadleaf deciduous tree, boreal-broadleaf deciduous tree, temperate-broadleaf evergreen shrub, temperate-Broadleaf deciduous shrub, boreal-broadleaf deciduous shrub, C3 arctic grass, C3 grass, C4 grass, and crops)19. Their differences in leaves and stems determine the uniqueness of different vegetation in reflection, transmittance, and solar radiation absorption. Root distribution parameters control soil moisture absorption, aerodynamic parameters determine thermal resistance, moisture, and momentum transfer, and photosynthetic parameters determine stomatal resistance, photosynthesis, and evapotranspiration processes. All PFTs are divided into three different types of phenology: perennial evergreen types, seasonal deciduous types determined by temperature and daylight length, and multi-seasonal stress deciduous types determined by temperature and soil moisture29.

We used the offline version of CLM5 (0.90° latitude × 1.25° longitude) with coupled biogeochemical cycles (BGC), which is forced by the dataset of Global Soil Wetness Project (GSWPS), a 3-hourly 0.5° global forcing product based on 20th Century Reanalysis version. The biogeochemical model was run for 200 years, and the CLM5-Hg was run for 10 years in each scenario simulation (1841–1850 for the pre-industrial era, 1991–2000 for the present day, and 2091-2100 for the future). The results for the last year of each simulation were used for data analysis, as this time point reached a steady state, as indicated by two representative variables (see Supplementary Fig. 17). We used the simulated atmospheric Hg(0) concentrations and the dry and wet deposition fluxes of Hg(II) from the CAM6-Chem model as its upper boundary conditions75. To isolate the impact of climate change, anthropogenic Hg emissions were maintained at the present-day level76.

Stomatal model
In CLM5-Hg, maximum stomatal conductance was obtained from the Medlyn “empirical‐optimal” conductance model30. This stomatal model calculates stomatal conductance (gs) based on net leaf photosynthesis, the CO2 concentration at the leaf surface, and the VPD. The stomatal resistance of the leaf is:

1 = gs = gs0 + 1:6 (1 + g1 / \sqrt{D}) An / Cs / Patm (1)

where rs is the stomatal resistance, gs0 is the minimum stomatal conductance, An is leaf net photosynthesis, Cs is the CO2 partial pressure at the leaf surface, Patm is the atmospheric pressure, and D is the VPD at the leaf surface. The value of g1 depends on the PFTs following the CABLE model49. The model further have corrected rs by partitioning to sunlit and shaded side leaf stomatal resistance and the condition of snow cover29:

Rs = (f_sun * elai) / rsun_s + ((1 - f_sun) * elai) / rsha_s (2)

where Rs represents the adjusted stomatal resistance, f_sun is the sunlit fraction of canopy, elai represents one-sided LAI buried by snow, rsun_s is the sunlit leaf stomatal resistance, and rsha_s is the shaded leaf stomatal resistance. We used the dry deposition flux is used to calculate the absorption of atmospheric Hg(0) by global vegetation77:

Fd(z) = Vd C(z) (3)

where Fd(Z) represents the Hg(0) dry deposition flux at height z, C(z) is the atmospheric Hg(0) concentration, and vd is the dry deposition velocity calculated following the Wesely scheme78:

Vd = 1 / (Ra + Rb + Rc) (4)

where Ra is the aerodynamic resistance between a specific height and the surface, Rb is the quasi-laminar sublayer resistance, and Rc is the bulk surface resistance. Rc is mainly determined by the adjusted stomatal resistance (Rs).

Experiment design
We simulated the terrestrial Hg cycling that is representative of the present-day conditions to serve as the baseline, and 1850 as the pre-industrial climate condition. We selected three different SSPs to represent three different future CO2 emission scenarios in 2100: SSP1-2.6 represents the lowest, SSP5-8.5 represents the highest, and SSP3-7.0 represents a moderate scenario. To better unravel the effects of individual factors on global vegetation uptake of Hg(0), a suite of simulations was performed in the pre-industrial era and the hypothesized twenty-first century under the SSP5-8.5 scenario. To mitigate high computational costs, we adopted an alternative approach known as “anomaly forcing” for land-only simulations in CLM5 in line with future climate projections, enabling the generation of climate data to drive CLM579. This method used data from a fully coupled simulation to produce monthly changes in near-surface atmospheric states and fluxes, relative to current conditions79. We isolated the effects of individual factors by sequentially altering only one variable to reflect future states while maintaining other variables in line with present-day conditions. To eliminate the impact of land use and land cover changes, as well as aerosols, we preprocessed the vegetation patterns and aerosol deposition in all simulation scenarios using the specified file provided by CLM5. These climate change factors specifically include atmospheric CO2 concentration (limited to its biogeochemical effects and its climate effects are reflected in other factors), precipitation, radiation, meteorological factors such as temperature, humidity, pressure, and wind. In the “all” scenario, all these factors were modified simultaneously to simulate the overall effect. Additionally, we examined the interactions between the biogeochemical effect of CO2 and temperature and precipitation. We also have intentionally maintained constant levels of anthropogenic emissions and atmospheric Hg concentrations across all scenarios. This methodological choice was made to isolate and underscore the influence of climatic variables on Hg dynamics. By controlling for anthropogenic inputs, we aim to provide a clear assessment of how climate change alone can affect the biogeochemical cycling of Hg.

Data and meta-analysis
We utilized a global vegetation Hg(0) flux dataset to constrain and validate our simulated global vegetation assimilation of Hg(0) at present day. This database contained 79 publications with measurements ranging from 1987 to 202018. These data were from 37 different measurement sites across the world covering the East Asia, Western Europe, and North America regions. The modeled Hg(0) vegetation uptake flux was constrained by the 60 individual data points and agrees well with the observations (r2 = 0.38)19. The datasets of global stomatal conductance were used to validate the accuracy of stomatal uptake process by CLM5-Hg37.

We used meta-analysis to assess the biogeochemical effects of experimentally elevated CO2 (eCO2) on foliar Hg concentration. We searched for journal articles using the ISI Web of Science with the following keyword combinations: (elevated CO2 concentration OR CO2 enrichment OR increasing CO2 concentration) AND (mercury OR mercury concentration OR mercury uptake) AND (tree OR grass OR plants OR vegetation OR leaf OR leaves) from 1990 to 2023. Papers have to meet the following criteria to be included in our dataset: (i) eCO2 experiments were conducted in terrestrial ecosystems; (ii) initial environmental factors in control plots were the same as those in eCO2 plots; (iii) at least two CO2 concentration regimes were compared. Finally, our dataset included 368 observations from six studies80–85. To make sure we include all important studies, we did another search using Google Scholar and sort the studies based on their relevance.

The weighted mean response ratio (lnRR) is employed to analyze the treatment effect on vegetation mercury (Hg)37,86, the effect size is estimated as:

lnRR = ln (xeCO2 / xaCO2) (5)

where xeCO2 and xaCO2 represent the mean of the elevated CO2 level and ambient CO2 level, respectively.

The individual observations are assigned weights calculated from the experimental replications:

weight = (neCO2 * naCO2) / (neCO2 + naCO2) (6)

where neCO2 and naCO2 indicate the numbers of replications at elevated CO2 level and ambient CO2 level, respectively.

Finally, lnRR is transformed to percentage change (%) as:

RR% = (exp(lnRR) − 1) * 100 (7)

The natural log-transformed foliar Hg concentration or flux (Hgveg) sensitivity (lnSens) is calculated as:

lnSens = lnRR / Δ (8)

where lnRR represents the natural log-transformed response ration, and the Δ is the magnitude of eCO2 (per 100 p.p.m. increase). The weighted means of lnSens and percentage sensitivity are calculated using equations similar to those presented in Eqs. (6) and (7) above.

A negative effect size indicate a decline in the Hg (response variable) for the treatment plots compared to the control plots. A variable is considered significantly different between the treatment and control plots (P < 0.05) if the 95% confidence intervals (CI) of the effect size for that variable does not overlap with zero. Differences between subgroups are deemed significant if their CIs do not overlap. Furthermore, due to the limitations of the data, it is not feasible to specifically subdivide PFTs for a direct one-to-one comparison between the model output and the observational data. Therefore, we have roughly categorized these studies into three subgroups: tree, grass, and crop (Supplementary Table 2). This categorization aims to observe the impact of eCO2 on plants from different PFTs, enabling comparison with our model results.

Uncertainty analysis
We conducted an uncertainty analysis on the Hg(0) uptake and gs using a perturbation experiment. This was based on the process of stomatal uptake, which is a crucial part in the assimilation of atmospheric Hg(0) by global vegetation55. Our focus was on a set of five plant physiology-related parameters within the stomatal conductance model (Medlyn model in CLM5), selected due to their mechanical impacts on responses to elevated CO2 levels and their significance in representing important ecological processes in vegetation47 (Supplementary Table 3). The Medlyn_slope is the slope of the Medlyn model (Eq. 1), where the slope parameter dictates the extent of stomatal opening based on a given mix of assimilation capacity, CO2 concentration, and VPD. Slatop measures the leaf area per gram of leaf biomass. Higher SLA values, indicating thinner and more efficient leaves, lead to a greater LAI from the same biomass. Leaf_cn denotes the ideal leaf carbon to nitrogen (C:N) ratio, with higher leaf nitrogen enhancing photosynthesis but increasing respiration costs. Psi50 is the water potential at which there is a 50% loss of conductivity, a hydraulic trait of plant that effectively reflects the vegetation’s state in response to water-deficient conditions. Stem_leaf determines the biomass distribution between stem and leaves. As this ratio increases, it diminishes the achievable LAI per unit of carbon and nitrogen dedicated to growth, while concurrently increasing the equilibrium woody biomass. These parameters were selected from the broader set of CLM5 parameters through a method primarily guided by the model’s structure and thorough iterative testing during CLM5’s development47. To simplify understanding and pinpointing model behavior changes due to parameter variations, we employed one-at-a-time (OAAT) perturbations from the default settings. This approach, unlike a comprehensive global parameter sensitivity analysis, enables easier visualization and interpretation of results. We assessed the sensitivity of simulations in the CLM5-Hg model under a standard scenario, testing four levels for each parameter as outlined in Supplementary Table 3, resulting in a total of 20 distinct physical perturbation ensembles. In addition, we also considered a condition in which all parameters are altered simultaneously at each level. We estimated the relative uncertainty in Hg(0) uptake and stomatal conductance using the CVs, defined as the relative degree of change in the model output compared to the proportion of parameter changes48.

Model validation
We curated a dataset within the CO2 concentration ranges projected by our model under three SSP scenarios, which included 462 paired observations of treated versus control groups for various species across different vegetation biomes (Supplementary Figs. 6 and 7). First, the comparison results under the current scenario indicated that the simulation can captures the global pattern of stomatal conductance (r > 0.5, Supplementary Fig. 8). Second, we performed point-by-point validations against matched observational data for projected eCO2 concentrations in each scenario, achieving an r value of 0.50 (Supplementary Fig. 9).

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review for “Decoupling of CO2 and Hg uptake by global vegetafion in the 21st century” by Yuan et. al. (NCOMMS-24-07636-T).\nThe manuscript describes the modeling findings on the change of carbon dioxide (CO2) and gaseous elemental mercury (Hg0) uptake by plants under several climate scenarios using the NCAR Community Land Model 5 with Hg processors (CLM5-Hg). The primary conclusion from the model results is that Hg uptake by vegetafion is expected to decrease significantly through 2100 under a business-as-usual climate scenario because of decreasing stomatal conductance with increasing concentrafion of CO2; and that global Hg uptake by vegetafion, which has been thought to be closely associated with the global carbon cycle, will be gradually “decoupled” from CO2 assimilafion caused by vegetafion.\nThis is an interesfing study that aftempts to describe the long-term trend of an important sink term of atmospheric Hg(0) facilitated by foliar uptake in the terrestrial ecosystem. The work is relevant considering the concerted efforts devoted to source eliminafion of atmospheric Hg release regulated by the Minamata Convenfion of Mercury. However, it is not completely clear what scienfific processors are incorporated in the CLM5-Hg (not even in Yuan et. al., 2023, hftps://doi.org/10.1016/j.envint.2023.107904, where the model was described), making the results of such long-term simulafion doubfful and somewhat difficult to evaluate. Another technical concern is that the stated “Hg(0) uptake by vegetafion” is not clearly defined and the presented “preset-day uptake (3,138 Mg/yr)” appears to be much greater than what is currently understood. It is also unclear if the release and re-emission from the bare soil surface are included in the calculafion. Furthermore, since the biogeochemical cycle of Hg in vegetated ecosystems is sfill relafively poorly understood, biases in model results can propagate over a long simulafion period, which is the case in this work. These concerns, and the associated uncertainfies of the modeling framework, do not project confidence that the conclusion would be valid. Finally, there is a general lack of testable hypotheses in this work, and the study objecfives are not laid out specifically.\nOverall, I recommend a major revision of the manuscript before considerafion for publicafion. Specific comments are as follows:\n1. In the introducfion, provide the study hypotheses and the associated study objecfives to jusfify the need for this work.\n2. Clearly describe what processors are included in the CLM5-Hg model. What is equally important is what fluxes are being simulated. Is it only Hg(0) uptake by vegetafion, or the underlying soil release and re-emission are also included? What are the vegetafion types? What soil chemistry and surface redox chemistry are considered in the model? Are wet deposifion terms of Hg(II) included in the model? How are throughfall fluxes calculated? Is the immediate reflux of deposited Hg considered? Substanfial modeling details are missing in this work and Yuan et. al. (2023). Without understanding the parameterizafions, it is difficult to evaluate and verify the model results.\n3. In Figure 1a, what are the “uptake” terms included in the graph? It appears that there is substanfial “vegetafion uptake” in regions where there is no vegetafion. The reported value is much greater than what is currently known (e.g., about 1200 Mg/yr) based on lifterfall data. If throughfall is included, how is it esfimated given the general lack of through measurement for model verificafion?\n4. It is unclear how the elevated CO2 would lead to changes in the surface resistance terms (parficularly the stomatal resistance) and how it would lead to the decoupling claimed in the manuscript. A mechanisfic explanafion will benefit understanding of the underlying processes forced by the changed climate.\n5. The trend shown in Figure 2, in addifion to the uptake quanfity in quesfion, does not appear to be consistent with the current understanding of the future trend of vegetafion Hg(0) uptake under a warming climate. A warming climate will lead to melfing of surface ice and permafrost soil. The vegetafive succession after the melfing and the increased intensity of precipitafion will lead to a substanfial increase of vegetafive biomass that increases the Hg(0) uptake by vegetafion. The model results are counter-intuifive and there is liftle plausible explanafion provided in the manuscript.\n6. Similarly in Figure 3, it is possible the land-cover changes under the changed climate would lead to significantly different vegetafion types and distribufion. Is the effect of landuse changes considered in the modeling?\n7. In the implicafion secfion, it is not clear how the “complex and extensive feedback mechanisms between the terrestrial Hg, water and C cycles” are illustrated in this work. The discussion can benefit from a more specific discussion on what processes are responsible for the simulated changes of the model results, and why the simulated results and the temporal trends appear to be significantly different from the assessment in earlier studies.", "reviewer_2": "I liked that the authors of this modeling paper were very honest about the limitafions of their model calling this effort a diagnosfic exercise. They have provided a nice summary of how changes in CO2 concentrafions and climate change could impact Hg uptake. I feel they were very honest about the limitafions of this study.\nSome limitafions not addressed clearly include impacts of deforestafion that is only menfioned briefly in the implicafions secfion.\nThey note that plants physiologically could adapt to higher concentrafions. Is there any work to indicate how plant behavior could change?\nThey are clear that the experimental work was done with seedlings, and there will be differences associated with different plant species.\nLine 287 it has been demonstrated in mulfiple studies that Hg does not enter the plants via the roots and that roots act as a barrier accumulafing soil Hg\nLine 320 I would change this to read “bypass sequestrafion of Hgo by plants and deposifion of foliar Hg to the soil causing increasing concentrafions in the atmosphere, where it could be converted to HgII that is deposited to ecosystems, where it can subsequently be methylated.\nGaseous elemental Hg is the form taken up by foliage. HgII is not taken up see Arnold et al. (2018)\nSome general housekeeping\nLine 28 which should be that.\nLine 48 previous research has\nLine 84 and throughout the text what was done should be described in past tense. Here indicated should be used. Line 89 were instead of are.\nReferences for Supplemental Informafion are not in alphabefical order and\nArnold J, Gusfin MS, Weisberg PJ. Evidence for Nonstomatal Uptake of Hg by Aspen and Translocafion of Hg from Foliage to Tree Rings in Austrian Pine. Environmental Science & Technology 2018; 52: 1174-1182.", "reviewer_3": "Review of the manuscript NCOMMS-24-07636-T fitled \"Decoupling of CO2 and Hg Uptake by Global Vegetafion in the 21st Century.\"\nThe manuscript addresses a crucial topic, given the pivotal role of plants in the global cycle of toxic mercury and the current and anficipated changes in CO2 emissions and budget. Its significance is heightened by the imperafive to effecfively implement the UN Minamata Convenfion on Mercury.\nDespite numerous studies on the intricate impact pathways of climate change on vegetafion Hg(0) uptake, the cascading effects of climate change-related factors on the terrestrial Hg cycle, parficularly the uptake of Hg(0) by vegetafion, remain unclear.\nThe study employs a compelling combinafion of the Community Land Model - Hg (CLM5-Hg) and the Community Earth System Model (CESM). Although the novel method has been previously applied by the authors (Yuan et al., 2023), the current manuscript ufilizes a disfinct approach, yielding enfirely new results. The presented findings are highly valuable and align with the global trend of research on pollutant cycling and its connecfions to climate change. The publicafion is meficulously and thoughffully prepared, and its content is certain to capture readers' interest.\nThe introducfion secfion is succinct yet effecfively arficulates the rafionale for the research topic. The results and discussion secfion is well-organized, comprising four subsecfions that sequenfially address changes in Hg uptake by vegetafion, the coupling of terrestrial Hg and CO2 uptake, model uncertainfies, and potenfial implicafions. In this secfion, my comments primarily relate to the part associated with model uncertainty. However, they do not quesfion the substanfive value of the paper. My comments are driven more by scienfific curiosity in this context. A separate secfion consfitutes the Materials and Methods part, in which the authors provide a detailed descripfion of the CLM5-Hg model. This model simulates the migrafion, transformafion, accumulafion, and emission processes of Hg in terrestrial ecosystems. It encompasses both stomatal and non-stomatal uptake in leaves, throughfall, soil formafion and leaching, photo-reducfion, microbial decomposifion, thermal evaporafion, and emissions from wildfires. In my assessment, the model's components allow for a comprehensive tracking of the Hg cycle involving land plants. Parficularly interesfing is the applicafion of disfinguishing numerous plant funcfional types (PFTs) characterized by different leaf, stem, and root properfies. This approach has been successfully tested against observafional field data in a previous study by the authors (Yuan et al., 2023). The data used regarding simulated atmospheric Hg(0) concentrafions, as well as the dry and wet deposifion fluxes of Hg(II), do not raise concerns, according to Zhang & Zhang (2022). Similarly, the ufilizafion of the global vegetafion Hg(0) flux dataset (Feinberg et al., 2022; Yuan et al., 2023), as well as CO2 dataset (Millhollen et al., 2006; Sañudo-Wilhelmy et al., 2008; Stamenkovic and Gusfin, 2009; Duval et al., 2011; Demers et al., 2013; Tang et al., 2021), is adequate. The uncertainty analysis appears to have been conducted thoroughly and comprehensively in my opinion.\nIn conclusion, the manuscript is presented in a clear and generally grammafically correct manner.\nHowever, I advise authors to carefully check through the text to ensure consistency in verb tenses, especially when discussing model results or future research direcfions.\nBelow, you will find a list of my suggesfions.\nIntroducfion\nLines 35-36: Consider adding informafion on the adverse effects of Hg on the ecosystem\nLines 39-42: Consider adding informafion that long-living vegetafion can store not only present-day Hg but also Hg emifted into the atmosphere decades ago\nLine 42: Change \"warming temperatures\" to \"rising temperatures\". The same applies to line 51\nLine 45: Add a comma after \"vegetafion\"\nLines 51-52: The symbol \"Hg\" has already been explained at the beginning of the Introducfion\nLine 60: Correct the citafion for \"Zhang et al., 2016\"\nLine 74: What is the source of pre-industrial data? Also, correct \"c.a\" to \"ca.\"\nLine 82: Change to \"Materials and Methods\"\nResults and Discussion\nLine 86: Consider specifying what kind of scenarios you are referring to, in this case “business-as-usual”. The same applies to line 188\nLines 106-107: Consider adding “2°C scenario” and “regional rivalry”. The same applies to line 213\nLine 138: Provide a clear understanding of what the percentages represent, e.g., a change in Hg uptake or a decrease in foliage Hg levels\nLine 179: Change to \"Decoupled CO2 and Hg\" since you are referring to CO2 specifically rather than total carbon. The same applies to lines 232, 305, and 333\nLines 189-192: Consider providing a concise statement that summarizes the key finding before delving into the specific details. This can help guide the reader's understanding\nLines 204-207: Consider breaking down this sentence into shorter ones for befter clarity. The same applies to lines 208-212\nLines 217-220: Consider providing a brief transifion sentence. Also, please explain “CO2 ferfilizafion effect”\nLines 225-227: Please clarify whether you mean a decoupling in trends or a decoupling in the magnitude of the trends\nLines 247-249: Please rewrite this sentence for befter clarity, e.g., \"This occurs in part because the anomaly forcing method assumes that future changes (anomalies) can overlay present-day variability.\"\nLines 252-253: Consider providing a brief transifion sentence, e.g., \"Addifionally, it's important to note that some data sources in our meta-analysis originate from seedling experiments.\"\nLines 271-274: Please rewrite this sentence for befter clarity. Also, please be consistent in terminology used, either \"Medlyn\" or \"medlyn\"\nLine 283-285: The statement lacks specificity. Please provide more details on the specific uncertainfies in the model representafion of land-atmosphere Hg exchange and its implicafions for future predicfions\nLines 285-287: Please clarify the significance of these processes in the context of your study. How do they contribute to the uncertainfies menfioned earlier?\nLines 287-288: Please explain the rafionale behind excluding the absorpfion of Hg from underground root systems and root secrefions. This omission could have implicafions for the overall accuracy of your model\nLine 288-289: Consider elaborafing on why anthropogenic and legacy Hg emissions remain unchanged. Provide jusfificafion or discuss potenfial impacts on the study's outcomes.\nLines 317-319: Consider rephrasing the beginning for smoother flow, e.g., \"Our findings reveal a suppression of atmospheric Hg(0) uptake by plants across most regions in the 21st century due to reduced stomatal conductance in vegetafion caused by increased CO2.\"\nLines 321-325: Consider breaking down this sentence into shorter ones for befter clarity\nLines 330-335: Consider rephrasing to enhance clarity, e.g., \"The terrestrial ecosystem, recognized as a significant Hg sink, may face disrupfions under future climate change scenarios, parficularly with rising atmospheric CO2 concentrafions. It becomes crucial to comprehensively consider the fight coupling among Hg, CO2, and water cycles when assessing the effecfiveness of the Minamata Convenfion within the context of climate change.\"\nFigures\nFigure 1: Please add in the capfion the unit of fime (yr-1) for the global total values of Hg(0) uptake. In line 101 change to \"Materials and Methods\"\nFigure 2: Please add explanafions to the scenario numbers in the legend.\nFigure 3: What does the purple triangle on Figure 3a mean?\nFigure 4: It seems there is an error in the color scale legend of figures 4a and 4c. Now it is 0 - 3 - 2 - 1 instead of 0 - 1 - 2 - 3\nMaterials and Methods\nLines 613-619: From what does the division of the soil pool into specific layers in the model result? Has this approach been applied before, and if so, could you please provide references?\nLines 630-631: One of the model assumpfions is that anthropogenic mercury (Hg) will persist at the current level (Zhang et al., 2016). However, some esfimafions indicate that global anthropogenic emissions of Hg will increase in the forthcoming decades under the current legislative scenario, e.g. Rafaj et al. (2013, hftp://dx.doi.org/10.1016/j.atmosenv.2013.06.042), Pacyna et al. (2016, hftps://doi.org/10.5194/acp-16-12495-2016), Brocza et al. (2024, hftps://doi.org/10.5194/egusphere-2024-41). Even if this rise won’t substanfial compared to the present-day level, I believe it would be worthwhile to consider this aspect somewhere in the manuscript."}, "author_response": "Response: Thank you very much for your recognition and detailed and constructive feedback on our manuscript. We appreciate the time and effort you have devoted to reviewing our work. Your comments have provided us with valuable insights that will undoubtedly improve the quality and clarity of our research. We have carefully considered each point you raised and have made corresponding revisions to address these concerns in the revised manuscript. Additionally, we have included diagrams and equations in the supplementary materials, as detailed in our responses to your specific comments below:\nSpecific comments are as follows:\n1. In the introduction, provide the study hypotheses and the associated study objectives to justify the need for this work.\nResponse: We thank you for pointing out this issue. We have added the following sentences to the beginning of the last paragraph in the introduction.\nLines 63-66 “Therefore, our research aims to examine how vegetation-regulated atmospheric Hg(0) deposition will change under the impact of future climate change. We hypothesize that future climate change will increase the global atmospheric Hg(0) deposition, as suggested by previous studies (Wang 2020a; Wu et al. 2012).”\nAdditionally, we have added the following conclusive sentence at the beginning of the implications section.\nLines 357-359: “We found that, in the climate change scenario, the atmospheric Hg(0) uptake by terrestrial vegetation in 2100 will be likely to decrease by more than half compared to present-day conditions.”\n2. Clearly describe what processors are included in the CLM5-Hg model. What is equally important is what fluxes are being simulated. Is it only Hg(0) uptake by vegetation, or the underlying soil release and re-emission are also included? What are the vegetation types? What soil chemistry and surface redox chemistry are considered in the model? Are wet deposition terms of Hg(II) included in the model? How are throughfall fluxes calculated? Is the immediate reflux of deposited Hg considered? Substantial modeling details are missing in this work and Yuan et. al. (2023). Without understanding the parameterizations, it is difficult to evaluate and verify the model results.\nResponse: Thank you very much for this great suggestion, which we believe helps the readers to better understand our work. Our currently developed CLM5-Hg is a mechanistic model based on a relatively complete understanding of the Hg processes in terrestrial ecosystems, with the underlying soil release and re-emission already included (see Supplementary Fig.1, as added below). However, our current investigation is primarily concentrated on vegetative uptake of Hg(0), since vegetation within terrestrial ecosystems absorbs gaseous elementary Hg [Hg(0)], serving as the largest removal mechanism of atmospheric Hg and the pivotal process within the terrestrial Hg cycle, given that vegetation typically responds more sensitively to climate change than soil does. Regarding the description of the mechanistic processes of CLM5-Hg that you mentioned, I will provide a detailed explanation for each point as follows:\ni. Specifically, the CLM model includes simulations for more than 15 types of Plant Functional Types (PFTs). However, due to the limitations of observational data under our eCO control 2 experiment conditions, it is not feasible to subdivide PFTs for a direct one-to-one comparison. Therefore, in the validation process, we can only summarize the meta-sourced observational data according to their PFTs roughly and categorize them into three major groups: tree, grass, crop. Thus, referring to them as \"vegetation type\" was indeed inappropriate, and we have reverted to using \"PFTs\". The error has been corrected, and detailed descriptions have been added to the methods section to refine the corresponding part of the original manuscript:\nLines 537-542: “Furthermore, due to the limitations of the data, it is not feasible to specifically subdivide PFTs for a direct one-to-one comparison between the model output and the observational data. Therefore, we have roughly categorized these studies into three subgroups: tree, grass, and crop (Table S2). This categorization aims to observe the impact of eCO on plants from different 2 PFTs, enabling comparison with our model results.”\nLine 152: “vegetation type” has been revised to “PFTs”.\nii. We largely followed the Global Terrestrial Mercury Model (GTMM) by Smith-Downey et al. (2010) to simulate the soil chemistry and surface redox chemistry, with a few modifications. Compared with previous models, our model includes a vertically resolved soil biogeochemistry scheme. This scheme features base decomposition rates that vary with depth and are modified by soil temperature, water, and oxygen limitations. It also includes vertical mixing of soil carbon and nitrogen due to bioturbation, cryoturbation, and diffusion. The above supplementary content has been added to the revised Supplementary Information (SI) as follows:\nLines 48-75 (SI): “The decomposition process of soil Hg is tied to the soil carbon pool, assuming that the Hg in the soil binds with soil carbon pools of different ages (Smith-Downey et al., 2010). The transformation between different Hg pools and the microbial transformation rate are characterized using the conversion and respiration rates of the soil carbon pool in every layer (Lawrence et al., 2019). Upon each transfer of carbon and Hg between pools, a fraction is lost—carbon as CO respiration and Hg as Hg(0) evasion into the atmosphere. Following microbial decay, 2 we estimate that 16% of atmospheric Hg(0) evades, with the remainder being reincorporated into organic matter (Schaefer et al., 2020; Smith-Downey et al., 2010). In the framework of the single-level model structure, the foundational equation within decomposing Hg pools is as follows:\n(1)\nWhere C is the Hg content of pool i, R represent the Hg inputs from plant tissues directly to pool i i i (only non-zero for litter pools and CWD), k is the decay constant of carbon pool i; T indicates the i ji fraction of carbon directed from pool j to pool i, with a fraction rj being lost as a respiration flux along the way. f is the constant representing the evasion into the atmospehere as Hg(0).\nHg\nIncorporating the vertical dimension into the decomposition dynamics alters the balance equation as detailed below:\n(2)\nWhere Ci(z) is now defined at each model level in volumetric terms (gC m-3), along with Ri(z) and kj(z). Additionally, vertical transport is accounted for by the last two terms, representing diffusive and advective transport, respectively. In the base model, advective transport is set to zero, leaving only a diffusive flux, with diffusivity D(z) defined for all decomposing carbon and Hg pools.\nOur model includes a vertically resolved soil biogeochemistry scheme in CLM5 was introduced. This scheme features base decomposition rates that vary with depth and are modified by soil temperature, water, and oxygen limitations. It also includes vertical mixing of soil carbon and nitrogen due to bioturbation, cryoturbation, and diffusion (Lawrence et al., 2019).”\niii. Wet deposition of Hg(II) are also include in our model following the GTMM, and the monthly flux data are speciated from the CAM6-Chem-Hg (Zhang and Zhang, 2022). Additionally, we do not consider the immediate reflux of deposited mercury; instead, we focus on the gross uptake of Hg(0), although our model includes the re-emission process. The related supplementary content has been added to the revised SI and manuscript as follows:\nLines 36-40 (SI): “The wet deposition process of Hg(II) is simulated following the GTMM (Smith-Downey et al., 2010). The Hg(II) wet deposition fluxes are speciated from the CAM6-Chem-Hg (Zhang and Zhang, 2022). After being removed from the surfaces of leaves and soil, Hg(II) can penetrate the soil and has the ability to attach to reduced sulfur groups present in organic matter. Throughfall flux are calculated as the sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced (Smith-Downey et al., 2010).”\nLines 91-93: “This uptake represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage.”\nLines 108-110: “the value represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage.”\nLines 30-35 (SI): “The nonstomatal uptake pathway entails the direct absorption of atmospheric Hg vapor via the cuticles in the epidermis of the canopy's upper layers (Wesely, 1989). We assume a 10% re-emission of Hg from foliage due to the release and subsequent reduction of previously sequestered Hg(0) within leaf tissue, along with a 15% re-emission from Hg deposited on leaf surfaces, transformed to Hg(0) through photoreduction (Demers et al., 2013; Yu et al., 2016; Yuan et al., 2019).”\nLines 337-346: “In our CLM5-Hg model, throughfall primarily originates from the washing off of atmospheric divalent mercury (sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced ) (Paige Wright et al., 2016; Smith-Downey et al., 2010). Recent studies indicate that epiphytic vegetation on canopies absorbs atmospheric Hg(0) and decomposes into humus, adhering to tree trunks and canopies, where mercury is subsequently washed into throughfall by precipitation (Wang et al., 2020b). Additionally, research indicates that the temporal scale and frequency of sampling for throughfall mercury measurements can impact the accuracy of their estimates (Choi et al., 2008). Therefore, our model has limitations in this part, and more extensive experimental data covering broader spatiotemporal scales is needed to further constrain the model(e.g., flux measurements or isotope compositions).”\nSupplementary Fig.1 The terrestrial Hg cycle as simulated by CLM5-Hg, modified from Yuan et al. (2023).\n3. In Figure 1a, what are the “uptake” terms included in the graph? It appears that there is substantial “vegetation uptake” in regions where there is no vegetation. The reported value is much greater than what is currently known (e.g., about 1200 Mg/yr) based on litterfall data. If throughfall is included, how is it estimated given the general lack of through measurement for model verification?\nResponse: Thank you for pointing these out. \"uptake\" refers to the dry deposition of Hg(0) driven by global vegetation. It represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes, and does not account for immediate reflux of foliage. Our simulation results agree well with the global vegetation distribution as follows: areas represented with values in the figure indeed correspond to actual vegetation presence (Southworth et al., 2023).\nI wonder if you might be questioning the presence of vegetation in tundra regions? Previous studies have shown that vegetation in tundra areas, such as mosses, can absorb atmospheric Hg(0) (Obrist et al., 2017; Wang et al., 2020a). The 1200 Mg/yr value based on litterfall data is very likely underestimated since these data primarily represent foliar Hg over the period near the growing season and do not account for the amount from the tissues, mosses, and lichens (Feinberg et al., 2022). Therefore, it cannot accurately estimate reversely to the Hg(0) vegetation uptake. This point was mentioned in our previous study (Yuan et al., 2023).\nIn our CLM5-Hg model, throughfall flux are calculated as the sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced. It is partly refer to our response to previous comments.\nThe above-mentioned content has been added to the revised manuscript as follows:\nLines 91-93: “This uptake represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage.”\nLines 108-110: “a, Hg(0) vegetation uptake flux at present-day, the value represents the gross uptake of atmospheric Hg(0) through both stomatal and cuticular (non-stomatal) processes and does not include the immediate re-emission from foliage.”\nLines 337-346: “In our CLM5-Hg model, throughfall primarily originates from the washing off of atmospheric divalent mercury (sum of the Hg(II) dry deposition onto the canopy surface and the Hg(II) wet deposition that has not been reduced ) (Paige Wright et al., 2016; Smith-Downey et al., 2010). Recent studies indicate that epiphytic vegetation on canopies absorbs atmospheric Hg(0) and decomposes into humus, adhering to tree trunks and canopies, where mercury is subsequently washed into throughfall by precipitation (Wang et al., 2020b). Additionally, research indicates that the temporal scale and frequency of sampling for throughfall mercury measurements can impact the accuracy of their estimates (Choi et al., 2008). Therefore, our model has limitations in this part, and more extensive experimental data covering broader spatiotemporal scales is needed to further constrain the model(e.g., flux measurements or isotope compositions)”\n4. It is unclear how the elevated CO would lead to changes in the surface resistance terms 2 (particularly the stomatal resistance) and how it would lead to the decoupling claimed in the manuscript. A mechanistic explanation will benefit understanding of the underlying processes forced by the changed climate.\nResponse: Regarding the process by which elevated CO leads to changes in leaf stomatal 2 resistance (stomatal conductance), we elaborated in detail in the section 'Decoupled CO and Hg': 2\nLines 219-227: “The Medlyn model in our CLM5-Hg model is consistent with this optimal stomatal theory. With eCO , the CO partial pressure at the leaf surface (C ) also increases accordingly, 2 2 S leading to enhanced leaf photosynthesis (A ) (equation (1)). However, A is constrained by water n n potential while C continues to increase in CLM5 (Lawrence et al., 2019). This resulted in reduced s stomatal conductance (g) with eCO (Supplementary Fig. 13)…This adaptive mechanism s 2 ultimately leads to a nonlinear relationship between atmospheric CO concentration and stomatal 2 conductance.”\nAs for the decoupling claimed in our study, we elaborated it at the end of the section. We also added descriptions related to the trade-offs between the effects by increasing LAI and reducing g (stomatal s conductance) to the original text. The integrated explanation is as follows:\nLines 250-263: Contrarily, we predicted a decoupling between the trends of CO assimilation and 2 Hg(0) uptake by vegetation in the 21st century. The CLM5 model projects an increased greening of vegetation in many regions in the 21st century resulting from eCO2 (a.k.a. fertilization effect), evidenced by the increased leaf area index (LAI) in the northern mid-to-low latitudes and certain regions of the Southern Hemisphere (Fig. 4c). The increase in photosynthesis can simultaneously induce a state of water deficit and nutrient saturation within the plant's internal environment (Victoria et al., 2010). Therefore, under climate change, the increase in vegetation LAI may only represent an increase in leaf density or even stomatal numbers, but stomatal conductance may not necessarily increase accordingly. Our model suggests a discernible decrease in the flux of Hg(0) uptake by vegetation in these areas (Fig. 1), reflecting the differences in CO and Hg elements during 2 plant physiological processes especially those related to water dynamics in terrestrial ecosystems (Wang et al., 2021).\n5. The trend shown in Figure 2, in addition to the uptake quantity in question, does not appear to be consistent with the current understanding of the future trend of vegetation Hg(0) uptake under a warming climate. A warming climate will lead to melting of surface ice and permafrost soil. The vegetative succession after the melting and the increased intensity of precipitation will lead to a substantial increase of vegetative biomass that increases the Hg(0) uptake by vegetation. The model results are counter-intuitive and there is little plausible explanation provided in the manuscript.\nResponse: We appreciate the reviewer for pointing out the issues related to a warming climate and for providing many excellent and constructive suggestions. We completely agree that it is very likely that vegetative succession following melting and increased precipitation intensity will lead to an increase in vegetative biomass and, consequently, an increase in Hg(0) uptake by vegetation. Our study did not consider the impacts of Land Use and Land Cover Change (LULCC), because we focused more on the direct effects of climate change (e.g. eCO ) in this research. This point is very 2 insightful, and our next work is addressing this aspect. We clearly stated this point in the revised manuscript as follows:\nLines 86-87: “…and remove the effects of land use and land cover change (LULCC) and aerosols (see Materials and Methods).”\nLines 385-390: “We did not consider the impacts LULCC in this study. Under global warming, vegetative succession following melting and increased precipitation intensity is likely to lead to an increase in vegetative biomass and, consequently, an increase in Hg(0) uptake by vegetation (Wang et al., 2020a). Indeed, the interactive effects of climate change combined with changes in LULCC worth further examination.”\n6. Similarly in Figure 3, it is possible the land-cover changes under the changed climate would lead to significantly different vegetation types and distribution. Is the effect of landuse changes considered in the modeling?\nResponse: The results presented in Figure 3 are also based on OAAT analysis focusing solely on the influence of CO . In this study, we didn’t considered the effects of LULCC, please refer to our 2 response to last comment and\nLines 480-482: “To eliminate the impact of land use and land cover changes, as well as aerosols, we preprocessed the vegetation patterns and aerosol deposition in all simulation scenarios using the specified file provided by CLM5.” Your suggestion is very valuable, and in future research, we could explore the interactive effects of LULCC and climate change, which could be an intriguing study.”\n7. In the implication section, it is not clear how the “complex and extensive feedback mechanisms between the terrestrial Hg, water and C cycles” are illustrated in this work. The discussion can benefit from a more specific discussion on what processes are responsible for the simulated changes of the model results, and why the simulated results and the temporal trends appear to be significantly different from the assessment in earlier studies.\nResponse: Thank you for pointing this out. The sentence was originally intended to summarize the results and discussion sections from before, and placing it here was indeed somewhat inappropriate. Therefore, we have moved it to Line 215 of the \"Decoupled CO and Hg\" section in the revised 2 manuscript. In this section, we thoroughly discuss what processes are responsible for the simulated changes in the model results and why our findings differ significantly as follows:\nLines 215-263: “Our study illustrates a complex and extensive feedback mechanism between the terrestrial Hg, water and carbon cycles. The enhancement of photosynthesis caused by the biogeochemical effect of eCO is accompanied by the loss of plant water content (Katul et al., 2012). 2 During this process, plants adjust the stomatal aperture to reduce water transpiration and maximize water use efficiency (Gardner et al., 2023; Hsiao et al., 2019). The Medlyn model in our CLM5-Hg model is consistent with this optimal stomatal theory. With eCO , the CO partial pressure at the 2 2 leaf surface (C ) also increases accordingly, leading to enhanced leaf photosynthesis (A ) (equation S n (1)). However, A is constrained by water potential while C continues to increase in CLM5 n s (Lawrence et al., 2019). This results in reduced stomatal conductance (g) with eCO s 2 (Supplementary Fig. 13), leading to decreased transpiration (Fig. 4d). This process induces increased soil water storage, enhancing water use efficiency (Supplementary Fig. 14). …A tight coupling between carbon and Hg in terrestrial ecosystems has been observed as a paradigm over the past two decades (Smith-Downey et al., 2010; Wang et al., 2020a). Conventionally, it has been postulated that rising atmospheric CO2 levels would increase vegetation's photosynthesis rate, leading to the beneficial impact on plant growth, known as the CO fertilization effect (Liu et al., 2 2019). This effect is believed to enhance the concurrent absorption of both CO and Hg(0), as 2 suggested by Jiskra et al. (2018), Lawrence et al. (2019), Obrist (2007), and Schaefer et al. (2020). …Contrarily, we predict a decoupling between the trends of CO assimilation and Hg(0) 2 uptake by vegetation in the 21st century, when considering the dynamic response of vegetation physiological activities to climate change. …The increase in photosynthesis can simultaneously induce a state of water deficit and nutrient saturation within the plant's internal environment (Victoria et al., 2010). Therefore, under climate change, the increase in vegetation LAI may only represent an increase in leaf density or even stomatal numbers, but stomatal conductance may not necessarily increase accordingly. However, our model suggests a discernible decrease in the flux of Hg(0) uptake by vegetation in these areas (Fig. 1), reflecting the differences in CO and Hg element 2 during plant physiological processes especially those related to water dynamics in terrestrial ecosystems (Wang et al., 2021).”\nReviewer #2 (Remarks to the Author):\nI liked that the authors of this modeling paper were very honest about the limitations of their model calling this effort a diagnostic exercise. They have provided a nice summary of how changes in CO2 concentrations and climate change could impact Hg uptake. I feel they were very honest about the limitations of this study.\nResponse: Thank you very much for your positive and constructive feedback on our manuscript. We are grateful for your recognition of our research findings and our transparency of the model limitations. Please refer to our response to specific comments as follows:\n1. Some limitations not addressed clearly include impacts of deforestation that is only mentioned briefly in the implications section.\nResponse: Thank you for bringing this point up. In this study, our focus is on the effect of climate change on vegetation Hg(0) uptake. We haven't considered the effects of land use and land cover (LULCC), as described in the Methods section of the manuscript:\nLines 481-483: “To eliminate the impact of land use and land cover (LULCC) changes, as well as aerosols, we preprocessed the vegetation patterns and aerosol deposition in all simulation scenarios using the specified file provided by CLM5.”\nYour suggestion is very valuable, and in our future research, we plan to explore the interactive effects of LULCC and climate change, which could be an intriguing study. The added sentences in the revised manuscript are as follows:\nLines 386-391: “We did not consider the impacts LULCC in this study. Under global warming, vegetative succession following melting and increased precipitation intensity is likely to lead to an increase in vegetative biomass and, consequently, an increase in Hg(0) uptake by vegetation (Wang et al., 2020a). Indeed, the interactive effects of climate change combined with changes in LULCC worth further examination.”\nLines 86-87: “…and remove the effects of land use and land cover change (LULCC) and aerosols (see Materials and Methods).”\n2. They note that plants physiologically could adapt to higher concentrations. Is there any work to indicate how plant behavior could change?\nResponse: Thank you for bringing this point up. It is reported that under the influence of long-term eCO conditions, stomatal density and size will decrease, which are attributed to the guard 2 cells and mesophyll tissues that mediate stomatal movements. The improved sentences have been added to the revised manuscript as follows:\nLines 233-239 : “Intriguingly, the sensitivity of stomata to eCO diminishes gradually under the 2 influence of long-term eCO conditions. This occurs because guard cells and mesophyll tissues, 2 which mediate stomatal movements, lead to decreases in stomatal aperture and size, culminating in physiological adaptation to higher concentrations (Engineer et al., 2016; Liang et al., 2023). Consequently, this results in a less pronounced decline from the SSP3-7.0 to SSP5-8.5 scenarios (Fig. 2).”\n3. They are clear that the experimental work was done with seedlings, and there will be differences associated with different plant species.\nResponse: Thanks for your good suggestion. We have added and improved the following sentences as follows:\nLines 285- 288: “Additionally, there will be differences associated with different plant species. Thus, we suggest that future research should focus on this aspect, aiming to bridge the knowledge gap by including experiments across various growth stages and more species.”\n4. Line 287 it has been demonstrated in multiple studies that Hg does not enter the plants via the roots and that roots act as a barrier accumulating soil Hg\nResponse: Thank you for pointing this out. As you mentioned, Hg is hard to enters the plants via the root, as most previous studies have shown (Arnold et al., 2018; Chiarantini et al., 2016; Siwik et al., 2010). Regarding this, we have made the following supplementary explanation in the revised manuscript:\nLines 327-336: “Our model also did not consider the absorption of Hg from underground root systems and root secretions (Keuper et al., 2020). Indeed, Hg is hard to enter the plants via the root, as most previous studies have shown (Arnold et al., 2018; Chiarantini et al., 2016; Siwik et al., 2010). Meanwhile, our model does not account for the translocation of Hg among plant tissue organs. A recent study suggests that a significant proportion of Hg in roots may originate from absorption by leaves and subsequent translocation, with an estimation of up to 300 Mg yr-1 of atmospheric Hg⁰ stored in roots (Zhou et al., 2021), but the specific migration and distribution mechanisms are still unclear. These processes also have a potential influence on the amount of Hg(0) uptake by the vegetation, and could be incorporated in our model when more data is available.\n5. Line 320 I would change this to read “bypass sequestration of Hgo by plants and deposition of foliar Hg to the soil causing increasing concentrations in the atmosphere, where it could be converted to HgII that is deposited to ecosystems, where it can subsequently be methylated. Gaseous elemental Hg is the form taken up by foliage. HgII is not taken up see Arnold et al. (2018)\nResponse: Thank you very much for the valuable suggestion. We have rewritten the sentences as advised, as follows:\nLines 370-373: With climate change, the bypassing of atmospheric Hg(0) sequestration by plants and the deposition of foliar Hg to the soil lead to increasing concentrations in the atmosphere. This Hg can then be converted to HgII, which is deposited in aquatic ecosystems and can subsequently be methylated (Arnold et al., 2018; Feinberg et al., 2022; Zhou et al., 2021).\nSome general housekeeping\n6. Line 28 which should be that.\nResponse: Revised as suggested\n7. Line 48 previous research has\nResponse: Revised as suggested\n8. Line 84 and throughout the text what was done should be described in past tense. Here indicated should be used. Line 89 were instead of are.\nResponse: Thanks for your suggestions. We have thoroughly checked and corrected all the tenses that needed to be changed, and the changes have been highlighted in revised manuscript.\n9. References for Supplemental Information are not in alphabetical order and\nArnold J, Gustin MS, Weisberg PJ. Evidence for Nonstomatal Uptake of Hg by Aspen and Translocation of Hg from Foliage to Tree Rings in Austrian Pine. Environmental Science & Technology 2018; 52: 1174-1182.\nResponse: We have now revised all our references to conform to the format of Nature Communications.\nReviewer #3 (Remarks to the Author):\nThe manuscript addresses a crucial topic, given the pivotal role of plants in the global cycle of toxic mercury and the current and anticipated changes in CO2 emissions and budget. Its significance is heightened by the imperative to effectively implement the UN Minamata Convention on Mercury. Despite numerous studies on the intricate impact pathways of climate change on vegetation Hg(0) uptake, the cascading effects of climate change-related factors on the terrestrial Hg cycle, particularly the uptake of Hg(0) by vegetation, remain unclear.\nThe study employs a compelling combination of the Community Land Model - Hg (CLM5-Hg) and the Community Earth System Model (CESM). Although the novel method has been previously applied by the authors (Yuan et al., 2023), the current manuscript utilizes a distinct approach, yielding entirely new results. The presented findings are highly valuable and align with the global trend of research on pollutant cycling and its connections to climate change. The publication is meticulously and thoughtfully prepared, and its content is certain to capture readers' interest.\nThe introduction section is succinct yet effectively articulates the rationale for the research topic. The results and discussion section is well-organized, comprising four subsections that sequentially address changes in Hg uptake by vegetation, the coupling of terrestrial Hg and CO uptake, model 2 uncertainties, and potential implications. In this section, my comments primarily relate to the part associated with model uncertainty. However, they do not question the substantive value of the paper. My comments are driven more by scientific curiosity in this context. A separate section constitutes the Materials and Methods part, in which the authors provide a detailed description of the CLM5-Hg model. This model simulates the migration, transformation, accumulation, and emission processes of Hg in terrestrial ecosystems. It encompasses both stomatal and non-stomatal uptake in leaves, throughfall, soil formation and leaching, photo-reduction, microbial decomposition, thermal evaporation, and emissions from wildfires. In my assessment, the model's components allow for a comprehensive tracking of the Hg cycle involving land plants. Particularly interesting is the application of distinguishing numerous plant functional types (PFTs) characterized by different leaf, stem, and root properties. This approach has been successfully tested against observational field data in a previous study by the authors (Yuan et al., 2023). The data used regarding simulated atmospheric Hg(0) concentrations, as well as the dry and wet deposition fluxes of Hg(II), do not raise concerns, according to Zhang & Zhang (2022). Similarly, the utilization of the global vegetation Hg(0) flux dataset (Feinberg et al., 2022; Yuan et al., 2023), as well as CO dataset (Millhollen et al., 2006; Sañudo-Wilhelmy et al., 2008; Stamenkovic and 2 Gustin, 2009; Duval et al., 2011; Demers et al., 2013; Tang et al., 2021), is adequate. The uncertainty analysis appears to have been conducted thoroughly and comprehensively in my opinion.\nIn conclusion, the manuscript is presented in a clear and generally grammatically correct manner.\nHowever, I advise authors to carefully check through the text to ensure consistency in verb tenses, especially when discussing model results or future research directions.\nBelow, you will find a list of my suggestions.\nResponse: Thank you very much for your multifaceted recognition of our study, as well as for the detailed and thoughtful review. Your insightful comments and suggestions are invaluable to us. Please refer to our responses to the specific comments as follows:\nIntroduction\n1. Lines 36-37: Consider adding information on the adverse effects of Hg on the ecosystem.\nResponse: Thank you very much for the valuable suggestion. We have added the following sentence in revised manuscript.\nLines 38-39: “Additionally, it endangers ecosystems by bioaccumulating in food chains, affecting biodiversity and disrupting ecological balances (Eagles-Smith et al., 2016).”\n2. Lines 39-42: Consider adding information that long-living vegetation can store not only present-day Hg but also Hg emitted into the atmosphere decades ago\nResponse: Thank you very much for the valuable suggestion. We have added this sentence you advised in the revised manuscript as follows:\nLines 42-45: “Vegetation within terrestrial ecosystems can absorb large amounts of atmospheric gaseous elementary Hg [Hg(0)] (2200-3600 Mg yr-1), acting as a major sink for the atmosphere in the present-day Hg cycles (Obrist et al., 2021; Zhou et al., 2021). Long-living vegetation not only stores present-day Hg but also Hg emitted into the atmosphere decades ago (Bargagli, 2016).”\n3. Line 42: Change \"warming temperatures\" to \"rising temperatures\". The same applies to line 51\nResponse: Thanks for the suggestions, we fixed it in the revised manuscript.\n4. Line 45: Add a comma after \"vegetation\"\nResponse: Revised as suggested.\n5. Lines 51-52: The symbol \"Hg\" has already been explained at the beginning of the Introduction.\nResponse: Revised as suggested.\n6. Line 60: Correct the citation for \"Zhang et al., 2016\"\nResponse: Revised as suggested.\n7. Line 74: What is the source of pre-industrial data? Also, correct \"c.a\" to \"ca.\"\nResponse: Revised as suggested.\n8. Line 82: Change to \"Materials and Methods\"\nResponse: Revised as suggested.\nResults and Discussion\n9. Line 86: Consider specifying what kind of scenarios you are referring to, in this case “business-as-usual”. The same applies to line 188\nResponse: Thanks for the suggestions. “SSP5-8.5 scenario” has been revised to the “SSP5-8.5 (‘business-as-usual’) scenario” in the mentioned place.\n10. Lines 106-107: Consider adding “2°C scenario” and “regional rivalry”. The same applies to line 213\nResponse: Thanks for bringing it up. We have added them in the corresponding place.\n11. Line 138: Provide a clear understanding of what the percentages represent, e.g., a change in Hg uptake or a decrease in foliage Hg levels\nResponse: Thank you very much for the reviewer's suggestion. We have addressed it as follows::\nLines 155-158: “The meta-analysis revealed a significant decrease in vegetation Hg levels as a result of eCO2, showing an average decrease in foliage Hg levels or Hg uptake of 5.87% per 100 ppm increase (95%CI, -6.5% to -5.3%).”\n12. Line 179: Change to \"Decoupled CO2 and Hg\" since you are referring to CO2 specifically rather than total carbon. The same applies to lines 232, 305, and 333\nResponse: Thank you very much for the valuable suggestion. We fixed it in the revised manuscript.\n13. Lines 189-192: Consider providing a concise statement that summarizes the key finding before delving into the specific details. This can help guide the reader's understanding\nResponse: Thank you very much for the valuable suggestion. We have added the following sentence:\nLines 206-208: “Our analysis identified that the sunlit stomatal conductance emerges as the key driving factor influencing the observed reduction in Hg(0) uptake.”\n14. Lines 204-207: Consider breaking down this sentence into shorter ones for better clarity. The same applies to lines 208-212\nResponse: Thank you very much for the valuable suggestion. We have rewritten the following sentences:\nLines 223-226: “This results in reduced stomatal conductance (gs) with eCO (Supplementary Fig. 2 13), leading to decreased transpiration (Fig. 4d). This process induces increased soil water storage, enhancing water use efficiency (Supplementary Fig. 14).”\nLines 227-231: “Stomatal conductance is directly related to vegetation uptake of Hg(0) in our model (equations (1)-(4), for details refer to Methods). This relationship explains the gradual reduction in vegetation uptake of Hg(0). We observe this reduction from the pre-industrial and present-day periods to the SSP1-2.6 scenario (Supplementary Fig. 12).”\n15. Lines 217-220: Consider providing a brief transition sentence. Also, please explain “CO 2 fertilization effect”\nResponse: We appreciate the reviewer for pointing this point out. We added and improved the following sentences in the revised manuscript.\nLines 240-241: “A tight coupling between carbon and Hg in terrestrial ecosystems has been observed as a paradigm over the past two decades (Smith-Downey et al., 2010; Wang et al., 2020a).”\nLines 241-244: “Conventionally, it has been postulated that rising atmospheric CO levels would 2 increase vegetation's photosynthesis rate, leading to the beneficial impact on plant growth, known as the CO fertilization effect (Liu et al., 2019). This effect is believed to enhance the concurrent 2 absorption of both CO and Hg(0), as suggested by Jiskra et al. (2018), Lawrence et al. (2019), 2 Obrist (2007), and Schaefer et al. (2020).”\n16. Lines 225-227: Please clarify whether you mean a decoupling in trends or a decoupling in the magnitude of the trends\nResponse: Thank you for pointing this out. What we indeed meant is a decoupling in trends. We have clarified this in the last paragraph of the “Decoupled CO and Hg” section in the revised 2 manuscript:\nLines 252-263: “The CLM5 model projected an increased greening of vegetation in many regions in the 21st century resulting from eCO (a.k.a. fertilization effect), evidenced by the increased leaf 2 area index (LAI) in the northern mid-to-low latitudes and certain regions of the Southern Hemisphere (Fig. 4c). …Therefore, under climate change, the increase in vegetation LAI may only represent an increase in leaf density or even stomatal numbers, but stomatal conductance may not necessarily increase accordingly. However, our model suggests a discernible decrease in the flux of Hg(0) uptake by vegetation in these areas (Fig. 1), reflecting the differences in CO2 and Hg element during plant physiological processes especially those related to water dynamics in terrestrial ecosystems (Wang et al., 2021)”\n17. Lines 247-249: Please rewrite this sentence for better clarity, e.g., \"This occurs in part because the anomaly forcing method assumes that future changes (anomalies) can overlay present-day variability.\"\nResponse: Thank you very much for the valuable suggestion. We have rewritten this sentence in the lines 275-277 of the revised manuscript as you advised.\n18. Lines 252-253: Consider providing a brief transition sentence, e.g., \"Additionally, it's important to note that some data sources in our meta-analysis originate from seedling experiments.\"\nResponse: Thank you very much for the valuable suggestion. We refined this sentence as you advised.\nLines 279-280: “Additionally, it is important to note that some data sources in our meta-analysis originate from seedling experiments.”\n19. Lines 271-274: Please rewrite this sentence for better clarity. Also, please be consistent in terminology used, either \"Medlyn\" or \"medlyn\"\nResponse: Thank you for pointing this out. We have rewritten the sentence as follows and standardized the terminology to 'Medlyn'..\nLines 300-304: “In the Medlyn model within CLM5, the Medlyn slope, denoted as \"g1,\" plays a crucial role in controlling how stomata respond to CO levels. It does this by determining the extent 2 to which stomata open, based on the assimilation capacity, CO concentration, and vapor pressure 2 deficit (VPD).\n20. Line 283-285: The statement lacks specificity. Please provide more details on the specific uncertainties in the model representation of land-atmosphere Hg exchange and its implications for future predictions\nResponse: Thank you for pointing this out.We clarified this point as follows:\nLines 313-321: “There are still considerable uncertainties regarding the model representation of the land-atmosphere exchange of Hg at present-day, which serves as a baseline for our prediction for the future. The atmospheric Hg concentrations and deposition are specified as a boundary condition, not yet dynamically modeled in a two-way coupled fashion. The feedback between land Hg emissions and their atmospheric abundance and subsequent deposition onto the land are also not considered. Although our current framework can well diagnose the direct impact of changing climate on these exchange fluxes, an online land-atmosphere coupled model will be needed to reveal a more comprehensive and accurate changes in global Hg budget in future works.”\n21. Lines 285-287: Please clarify the significance of these processes in the context of your study. How do they contribute to the uncertainties mentioned earlier?\nResponse: Thank you for pointing this out. We have added the corresponding explanations to the revised manuscript as follows:\nLines 322-327: “Current isotopic evidence indicates that the photoreduction process is related to the re-emission of Hg(0) by vegetation leaves, with this re-emission ratio reaching nearly 30% in subtropical forest areas (Yuan et al., 2019). However, for the majority of other regions worldwide, we lack sufficient observational data to make estimates. In our model, we have only used median values as the reduction parameter. Therefore, in future research, we need to utilize more measured data to refine our parameterization scheme.”\n22. Lines 287-288: Please explain the rationale behind excluding the absorption of Hg from underground root systems and root secretions. This omission could have implications for the overall accuracy of your model\nResponse: Thank you for pointing this out. We have added the corresponding explanations to the revised manuscript as follows:\nLines 330-336: “Meanwhile, our model did not account for the translocation of Hg among plant tissue organs. A recent study suggests that a significant proportion of Hg in roots may originate from absorption by leaves and subsequent translocation, with an estimation of up to 300 Mg yr-1 of atmospheric Hg⁰ stored in roots (Zhou et al., 2021), but the specific migration and distribution mechanisms are still unclear. These processes also have a potential influence on the amount of Hg(0) uptake by the vegetation, and could be incorporated in our model when more data is available.”\n23. Line 288-289: Consider elaborating on why anthropogenic and legacy Hg emissions remain unchanged. Provide justification or discuss potential impacts on the study's outcomes.\nResponse: Thank you for your good suggestion. Because in our study, we focus primarily on the effect of climate change on the vegetation Hg(0) uptake, we have elaborated on this in the 'Experimental Design' section of our original manuscript.\nLines 682-685 of original manuscript: “This methodological choice was made to isolate and underscore the influence of climatic variables on Hg dynamics. By controlling for anthropogenic inputs, we aim to provide a clear assessment of how climate change alone can affect the biogeochemical cycling of Hg.”\nLines 315-321 of revised MS: “The atmospheric Hg concentrations and deposition are specified as a boundary condition, not yet dynamically modeled in a two-way coupled fashion. The feedback between land Hg emissions and their atmospheric abundance and subsequent deposition onto the land are also not considered. Although our current framework can well diagnose the direct impact of changing climate on these exchange fluxes, an online land-atmosphere coupled model will be needed to reveal a more comprehensive and accurate changes in global Hg budget in future works.”\nLines 352-353 of revised MS: “The model should be interpreted as a diagnostic tool designed to unveil the influence of individual factors.”\n24. Lines 317-319: Consider rephrasing the beginning for smoother flow, e.g., \"Our findings reveal a suppression of atmospheric Hg(0) uptake by plants across most regions in the 21st century due to reduced stomatal conductance in vegetation caused by increased CO .\"\n25. Lines 321-325: Consider breaking down this sentence into shorter ones for better clarity\nResponse: Thank you very much for the valuable suggestion. We have rewritten the following sentences:\nLines 373-377: “Furthermore, these inorganic Hg compounds are transformed into methylmercury by microbes. This process leads to the enrichment of methylmercury in riverine and marine food chains. As a result, a substantial threat to human health arises through the consumption of inland aquatic animals and seafood, including commercial fish.”\n26. Lines 330-335: Consider rephrasing to enhance clarity, e.g., \"The terrestrial ecosystem, recognized as a significant Hg sink, may face disruptions under future climate change scenarios, particularly with rising atmospheric CO concentrations. It becomes crucial to comprehensively 2 consider the tight coupling among Hg, CO , and water cycles when assessing the effectiveness of 2 the Minamata Convention within the context of climate change.\"\nResponse: Thank you very much for the valuable suggestion. We rephrased this paragraph as you advised in the revised manuscript.\nLines 391-395: “The terrestrial ecosystem, recognized as a significant Hg sink, may face disruptions under future climate change scenarios, particularly with rising atmospheric CO concentrations 2 (Daniel et al., 2021; Zhang et al., 2023). Therefore, it becomes crucial to comprehensively consider the tight coupling among Hg, CO , and water cycles when assessing the effectiveness of 2 the Minamata Convention within the context of climate change.”\nFigures\n27. Figure 1: Please add in the caption the unit of time (yr-1) for the global total values of Hg(0) uptake. In line 101 change to \"Materials and Methods\"\nResponse: Thanks for your suggestions. We fixed it in the revised manuscript.\n28. Figure 2: Please add explanations to the scenario numbers in the legend.\nResponse: Thank you for pointing this out. We have added the following sentences to the legend of Figure 2 in the revised manuscript\nLines 135-140: “SSP1-2.6 represents the lowest scenario, termed the “2°C scenario,” which aims for a sustainable future. SSP3-7.0 represents a moderate scenario, described as a medium-high reference scenario within the socio-economic context of “regional rivalry.” SSP5-8.5 represents the highest scenario, also known as ‘business-as-usual,’ considered the worst-case scenario in a high fossil fuel-intensive world.\n29. Figure 3: What does the purple triangle on Figure 3a mean?\nResponse: The purple triangle in Figure 3a represents the \"Δ\" in \"Δ CO levels\". Since it appears 2 for the first time, we have added \"Δ CO =\" in front of the numbers outside the parentheses to clarify 2 its meaning.\n30. Figure 4: It seems there is an error in the color scale legend of figures 4a and 4c. Now it is 0 - 3 - 2 - 1 instead of 0 - 1 - 2 - 3\nResponse: Thank you for pointing this out. We fixed it in the revised MS.\nMaterials and Methods\n31. Lines 613-619: From what does the division of the soil pool into specific layers in the model result? Has this approach been applied before, and if so, could you please provide references?\nResponse: Thank you for pointing this out. The rationale for layering is primarily based on the hydrologically and biogeochemically active differences at various soil depths (Lawrence et al., 2019). Additionally, the decomposition process of soil Hg is tied to the soil carbon pool, assuming that the Hg in the soil binds with soil carbon pools of different ages (Smith-Downey et al., 2010). The transformation between different Hg pools and the microbial transformation rate are characterized using the conversion and respiration rates of the soil carbon pool. Schaefer et al. (2020) have applied this method, although they did not consider as many soil layers due to the limited depth of soil simulated in their study. We have added this explanation in the corresponding place of the revised SI:\nLines 48-75 (SI): “The decomposition process of soil Hg is tied to the soil carbon pool, assuming that the Hg in the soil binds with soil carbon pools of different ages (Smith-Downey et al., 2010). The transformation between different Hg pools and the microbial transformation rate are characterized using the conversion and respiration rates of the soil carbon pool in every layer…This scheme features base decomposition rates that vary with depth and are modified by soil temperature, water, and oxygen limitations. It also includes vertical mixing of soil carbon and nitrogen due to bioturbation, cryoturbation, and diffusion (Lawrence et al., 2019).”\n32. Lines 630-631: One of the model assumptions is that anthropogenic mercury (Hg) will persist at the current level (Zhang et al., 2016). However, some estimations indicate that global anthropogenic emissions of Hg will increase in the forthcoming decades under the current legislative scenario, e.g. Rafaj et al. (2013,http://dx.doi.org/10.1016/j.atmosenv.2013.06.042), Pacyna et al. (2016,https://doi.org/10.5194/acp-16-12495-2016), Brocza et al. (2024,https://doi.org/10.5194/egusphere-2024-41). Even if this rise won’t substantial compared to the present-day level, I believe it would be worthwhile to consider this aspect somewhere in the manuscript.\nResponse: Thank you very much for your excellent suggestion. We have taken your advice into consideration and incorporated it into the ‘Implications’ section of the revised manuscript:\nLines 381-385: “Furthermore, although the impact of anthropogenic source emissions is not the focus of this study, some estimations indicate that global anthropogenic emissions of Hg will increase in the forthcoming decades under the current legislative scenario (Brocza et al., 2024; Pacyna et al., 2016; Rafaj et al., 2013). Therefore, under future climate change scenarios, it is possible that there will be a greater threat to human health.”"}, {"round": 2, "reviewer_comments": {"reviewer_1": "Review for “Decoupling of CO2 and Hg uptake by global vegetafion in the 21st century” by Yuan et. al. (NCOMMS-24-07636A).\nThe authors did an excellent job of addressing the technical concerns raised during the original review. The revised manuscript is significantly improved from the original manuscript. I recommend a few minor revisions before considerafion of publicafion:\n1. The hypothesis of this work “We hypothesize that future climate change will increase the global atmospheric Hg(0) deposifion” has been suggested by quite a few other studies in addifion to Wang 2020 and Wu et al. 2012, e.g., Wu et al. 2023 (DOI: 10.1021/acs.est.3c03107), Wang et al. 2022 (DOI: 10.1080/10643389.2021.1961505), and therefore does not reflect the innovafion of the work. It is recommended that the authors consider a more specific hypothesis that strengthens the novelty of the study.\n2. One of the primary conclusions of this work is the “decoupling between the trends of CO2 assimilafion and Hg(0) uptake by vegetafion.” This conclusion appears to be purely based on the increased stomatal resistance (or increased stomatal conductance) of foliage, proposed by earlier studies referenced in the manuscript. Although this proposed pathway is plausible, the detailed mechanism of vegetafive uptake and subsequent assimilafion in biomass has not been substanfiated with direct measurement or experimental data. A statement that the “decoupling” will occur would be an overstatement given the weight of available evidence. It is recommended that the tone of the “decoupling process” reflect the current understanding of the vegetafive uptake of Hg(0).\n3. The modeling processors implemented by Smith-Downey et al. (2010) in GTMM have been outdated. In parficular, the model did not consider the reducfion processes in soil proposed recently and the radiafion transfer in soil that contributes to the reemission. It is suspected that the net Hg sink calculated by the original CLM5-Hg model (Yuan et al. 2023, 2,576 Mg/yr) was a result of the weaker reducfion parameterized in the model. Addifional discussion in this regard is warranted.\n4. The CLM5-Hg model only considers the exchanges in terrestrial ecosystems. Based on Yuan et al. (2023), the geogenic source was not considered. Also, the contribufion of anthropogenic releases and the air-ocean exchange of Hg is not included. In a way, the air-land exchange alone does not close the global mass balance. How would the model results obtained from a constrained (i.e., only air-land exchange is considered) modeling system be related to the global Hg cycle should be discussed further.", "reviewer_2": "My comments have been addressed adequately.", "reviewer_3": "I wish to extend my grafitude for the thorough revisions made by the Authors to the manuscript fitled \"Decoupling of CO2 and Hg uptake by global vegetafion in the 21st century\" (NCOMMS-24-07636A).\nThe Authors have conscienfiously addressed all the remarks provided in my review. These efforts have resulted in significant improvements to the manuscript. Their aftenfion to detail and responsiveness have notably enhanced its quality, leaving no further suggesfions from my end.\nIn my opinion, the manuscript is now suitable for publicafion in its current form."}, "author_response": "Reviewer #1\nThe authors did an excellent job of addressing the technical concerns raised during the original review. The revised manuscript is significantly improved from the original manuscript. I recommend a few minor revisions before consideration of publication:\n1. The hypothesis of this work “We hypothesize that future climate change will increase the global atmospheric Hg(0) deposition” has been suggested by quite a few other studies in addition to Wang 2020 and Wu et al. 2012, e.g., Wu et al. 2023 (DOI: 10.1021/acs.est.3c03107), Wang et al. 2022 (DOI: 10.1080/10643389.2021.1961505), and therefore does not reflect the innovation of the work. It is recommended that the authors consider a more specific hypothesis that strengthens the novelty of the study.\nResponse: Thank you for pointing out this issue. We rewrote the sentence as follows:\nLines 63-66: “We hypothesize that the climate will influence global vegetation Hg(0) uptake by altering the plant physiology such as the stomatal activities, with CO and other meteorological 2 factors as important driver factors.”\n2. One of the primary conclusions of this work is the “decoupling between the trends of CO2 assimilation and Hg(0) uptake by vegetation.” This conclusion appears to be purely based on the increased stomatal resistance (or increased stomatal conductance) of foliage, proposed by earlier studies referenced in the manuscript. Although this proposed pathway is plausible, the detailed mechanism of vegetative uptake and subsequent assimilation in biomass has not been substantiated with direct measurement or experimental data. A statement that the “decoupling” will occur would be an overstatement given the weight of available evidence. It is recommended that the tone of the “decoupling process” reflect the current understanding of the vegetative uptake of Hg(0).\nResponse: Thank you for bringing this point up. We have revised the title and relevant sentences as follows:\nTitle: “Potential decoupling of CO and Hg uptake process by global vegetation in the 21st century”. 2\nThe word “Potential” indicates significant uncertainty of our results, while the word “process” limits the scope of our results: only a diagnosis of the uptake process, not a comprehensive prediction.\nLines 30-32: “We find a potential decoupling between the trends of CO assimilation and Hg(0) 2 uptake process by vegetation in the 21st century, caused by the decreased stomatal conductance with increasing CO .” 2\nLines 250-251: “Contrarily, we predicted a potential decoupling between the trends of CO 2 assimilation and Hg(0) uptake process by vegetation in the 21st century, when considering the dynamic response of vegetation physiological activities to climate change.”\n3. The modeling processors implemented by Smith-Downey et al. (2010) in GTMM have been outdated. In particular, the model did not consider the reduction processes in soil proposed recently and the radiation transfer in soil that contributes to the reemission. It is suspected that the net Hg sink calculated by the original CLM5-Hg model (Yuan et al. 2023, 2,576 Mg/yr) was a result of the weaker reduction parameterized in the model. Additional discussion in this regard is warranted.\nResponse: Thank you very much for the valuable suggestion. Indeed, the soil-related process of the GTMM has been outdated, which is also the focus of our future model developing. Therefore, in this study, we primary focus on the vegetation-related process. We included the following discussion in the revised manuscript:\nLines 335-338: “Furthermore, the model simplified the soil Hg processes, following GTMM (Global Terrestrial Mercury Model) (Smith-Downey et al., 2010), by focusing mainly on the microbial reduction process. It did not account for other processes like the radiative transfer in soil, photo-reduction, and other abiotic reduction processes (wang et al.,2016; Fritsche et al., 2008).”\n4. The CLM5-Hg model only considers the exchanges in terrestrial ecosystems. Based on Yuan et al. (2023), the geogenic source was not considered. Also, the contribution of anthropogenic releases and the air-ocean exchange of Hg is not included. In a way, the air-land exchange alone does not close the global mass balance. How would the model results obtained from a constrained (i.e., only air-land exchange is considered) modeling system be related to the global Hg cycle should be discussed further.\nResponse: Thanks for your valuable and constructive suggestions. In fact, the geogenic source was considered in CLM5-Hg, since the initial soil Hg concentration from Wang et al. (2019) included the fraction of geological sources. In addition, the atmospheric Hg concentration and deposition flux, which are the boundary condition for CLM5-Hg, also include the contribution of geogenic source. However, we completely agree with you that a further work combining air-land exchange, air-ocean exchange, and anthropogenic emissions is necessary to close the global mass balance. We included the following sentences in the revised manuscript:\nLines 399-406 in ‘implication section’: “From the perspective of the global Hg cycle, considering only the air-land exchange process is insufficient to achieve global mass balance. Both anthropogenic releases and the air-ocean exchange of Hg can potentially affect atmospheric Hg levels, thus influencing the air-land exchange process. Therefore, future research should aim to further incorporate time-varying anthropogenic emissions and develop a fully coupled land-atmosphere-ocean global Hg model within CESM2. This would enable a comprehensive understanding of the complete pathway of Hg from emission to deposition.”\nREVIEWERS' COMMENTS\nReviewer #1 (Remarks to the Author):\nThe authors have addressed the technical concerns and I have no further comment. I want to congratulate the authors on a work well done."}]

Decision letter