# Divergent responses of carbon fluxes to elevated temperature and precipitation: A meta-analysis in North Hemisphere grassland ecosystems

Weikang Zhang, Shan Cong, Zhaogang Liu, Ming Zhao, Zhaoxia Jiang, Congcong Zheng

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## To cite this version:

Weikang Zhang, Shan Cong, Zhaogang Liu, Ming Zhao, Zhaoxia Jiang, et al.. Divergent responses of carbon fluxes to elevated temperature and precipitation: A meta-analysis in North Hemisphere grassland ecosystems. Global Ecology and Conservation, 2024, 56, pp.e03289. 10.1016/j.gecco.2024.e03289. hal-04845952

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Original research article

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# Divergent responses of carbon fluxes to elevated temperature and precipitation: A meta-analysis in North Hemisphere grassland ecosystems

Weikang Zhang<sup>a</sup>, Shan Cong<sup>b</sup>, Zhaogang Liu<sup>c,d,e</sup>, Ming Zhao<sup>f</sup>, Zhaoxia Jiang<sup>g,h</sup>, Congcong Zheng<sup>i,*</sup>

## ARTICLE INFO

**Keywords:**

Carbon fluxes

Grassland

Elevated temperature

Increased precipitation

Meta-analysis

## ABSTRACT

The manipulation experiments involving increased precipitation and elevated temperature have elucidated a series of responses to the carbon fluxes (gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem productivity (NEP)) in grassland ecosystems. It is still unclear how the duration, magnitude or local climate conditions of these manipulation experiments can affect the response of GEP, ER, and NEP. Here, we conducted a meta-analysis using data from 50 published studies that manipulated precipitation or temperature, while measuring GEP, ER, and NEP in North Hemisphere grassland ecosystems. The results showed that increased precipitation significantly increased the GEP and ER of grassland ecosystems in the Northern Hemisphere, with an average increase of 9.8 % and 12.5 %, respectively. In contrast to increased precipitation, warming did not have a significant effect on GEP and ER. Meantime, there was no significant response of the NEP to both elevated temperature and increased precipitation. Furthermore, we found that the responses of carbon fluxes in North Hemisphere grassland ecosystems were associated with the magnitude and duration of increased precipitation, as well as to local temperature and precipitation changes induced by increased precipitation and elevated temperature. GEP was positively correlated with the local temperature under warming and increased precipitation. These findings emphasize the divergent response of carbon fluxes to climate factors change such as temperature and precipitation in North Hemisphere grassland ecosystems, which helps us better protect grasslands and predict ecosystem functions in the future.

<footnote><sup>a</sup> National Institute of Natural Hazards, Beijing 100085, China</footnote>

<footnote><sup>b</sup> College of Life Science, Tonghua Normal University, Tonghua 134002, China</footnote>

<footnote><sup>c</sup> Co-Innovation Center for Sustainable Forestry in Southern China, Laboratory of Biodiversity and Conservation, College of Ecology and Environment, Nanjing Forestry University, Nanjing 210037, China</footnote>

<footnote><sup>d</sup> Key Laboratory of Ecosystem Network Observation and Modeling, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China</footnote>

<footnote><sup>e</sup> College of Resources and Environment, University of Chinese Academy of Sciences, Beijing 100049, China</footnote>

<footnote><sup>f</sup> State Key Laboratory of Vegetation and Environmental Change, Institute of Botany, Chinese Academy of Sciences, Beijing 100093, China</footnote>

<footnote><sup>g</sup> School of Geographical Sciences, Northeast Normal University, Changchun 130024, China</footnote>

<footnote><sup>h</sup> Key Laboratory of Geographical Processes and Ecological Security of Changbai Mountains, Ministry of Education, Changchun 130024, China</footnote>

<footnote><sup>i</sup> INRAE, UMR ISPA, Villenave d'Ornon 33140, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: congcong.zheng@inrae.fr (C. Zheng).</footnote>
# 1. Introduction

The rise in atmospheric CO₂ concentrations, which cause global warming, will have a significant impact on global precipitation patterns (Borken et al., 2002; Knapp et al., 2015; Sun and Ao, 2013; Thomey et al., 2011; Yang et al., 2022). In turn, this will affect soil and plant processes (Pugnaire et al., 2019; Reichstein et al., 2013). The resulting cascade response could have a lasting effect on the terrestrial carbon cycle (Frank et al., 2015; Heimann and Reichstein, 2008; Wang et al., 2021; Wu et al., 2011; Xu et al., 2016). Due to the feedback relationship between climate change and carbon cycling, changes in carbon balance caused by climate change will in turn affect climate dynamics (Ciais et al., 2005). Given the complex effects of ongoing climate change on the terrestrial carbon balance, it is necessary to further understand how carbon fluxes respond to climate change in details (Cox et al., 2000).

Grasslands account for 40.5 % of global land area and store approximately 34 % of terrestrial carbon storage, which are important for carbon sequestration (Bai and Cotrufo, 2022; Booker et al., 2013). The interannual variations in global land carbon sinks are mainly caused by semi-arid grassland ecosystems, as they are very sensitive to climate change (Ahlstrom et al., 2015; Poulter et al., 2014). However, the response of carbon fluxes in grassland ecosystems to elevated temperature has yielded inconsistent results (Ganjurjav, 2015; Niu et al., 2008; Pendall et al.; Peng, 2014; Phillips et al., 2011; Wu et al., 2011). The net ecosystem productivity (NEP) of an ecosystem like forest and grassland can be defined as the difference between the gross ecosystem productivity (GEP) and ecosystem respiration (ER), which is the basis of the ecosystem carbon sinks and commonly used (Chen et al., 2021b; Grant et al., 2009). Accordingly, the universality and contingency of carbon fluxes in response to elevated temperature in grassland ecosystems at global level, remain to be investigated.

For many terrestrial ecosystems, water availability is one of the limiting factors for plant growth and productivity, and precipitation undoubtedly has a profound effect on the dynamics of grassland ecosystems. Taking precipitation events as an example, more frequent and intense precipitation change events are expected to occur globally, especially in arid and semi-arid regions (Spinoni et al., 2014; Touma et al., 2015). It is also estimated that regions characterised by high seasonal variability are likely to experience an even greater degree of variability and an increase in the frequency of extreme weather events (Konapala et al., 2020). Increased intra-factorial variation will undoubtedly reshape their interaction with other climate factors to carbon fluxes. Consequently, it is crucial to establish a causal relationship between increased or decreased precipitation patterns and corresponding ecosystem responses, while controlling other climate variables (Garbulsky et al., 2010). Recently, a large number of field experiments have shown that carbon fluxes of the grassland ecosystem can be very sensitive to the altered precipitation. A large-scale transect study in both temperate and alpine grasslands of China showed that precipitation consistently promotes their carbon fluxes (Liu et al., 2024), further study in the Eurasian grassland demonstrated that even small precipitation events can enhance its carbon sink through alter net ecosystem CO₂ exchange

<figure><img src="image_6.png" /><figcaption>**Fig. 1.** The geographical distribution of the study sites which is used in this study. Green circle represents warming studies while purple circle represents increased precipitation related studies.</figcaption></figure>
(Legesse et al., 2021). However, the grassland carbon fluxes response to water availability at large-scales can be different with the site experiment. Recently, a growing number of field studies have studied the effect of warming and increased precipitation on carbon fluxes, the interaction between local temperature and precipitation on carbon fluxes, as well as their magnitude and duration, considering increasing intensity and longer time-scales of altered precipitation (e.g., severe and prolonged drought) are frequent in vast areas of the world. Some recent studies showed that the combination or interaction of elevated temperature and increased precipitation may lead to significant carbon loss, while other study predicted the transition from terrestrial carbon sinks to carbon sources may accelerate in the coming decades due to temperature and precipitation changed (Taylor et al., 2017; Zhang et al., 2019). However, net carbon storage may also increase as soil respiration is suppressed by temperature and precipitation change (Saleska et al., 2003). No unified conclusion has been drawn so far about how the magnitude and duration of experimental treatments of elevated temperature and increased precipitation affect the response of carbon fluxes in grassland ecosystems globally.

The grassland ecosystem in the Northern Hemisphere is an important component of the global grassland ecosystem, with a rich variety of grassland types and the vast majority distributed in arid and semi-arid regions (Liu et al., 2023a, 2024). It is most sensitive to climate change and has also been a significant ecosystem that has dominated the significant interannual variations in global terrestrial carbon sinks over the past few decades (Ahlström et al., 2015). Considering the complex interplay of warming, increased precipitation and local climate, it is therefore necessary to examine the overall effects of increased temperature and precipitation on the carbon fluxes of grassland ecosystems. In this study, we used meta-analysis to integrate the response of carbon fluxes to elevated temperature and increased precipitation in grassland ecosystems in the Northern Hemisphere. The following questions will be addressed in this study: (1) What are the overall effects of warming and increased precipitation on GEP, ER, and NEP in Northern Hemisphere grassland ecosystems; (2) How do the magnitude and duration of the experiment affect the GEP, ER, and NEP in Northern Hemisphere grassland ecosystems; and (3) How do the local climate conditions affect the response of GEP, ER, and NEP to elevated temperature and increased precipitation in Northern Hemisphere grassland ecosystems. To comprehensively address these issues, we analyzed data from 50 studies, each of which involved the responses of GEP, ER, and NEP to elevated temperature and increased precipitation in grassland ecosystems in the Northern Hemisphere (Fig. 1).

## 2. Materials and methods

### 2.1. Data collection

Based on the Web of Science (www.webofscience.com) and Google Scholar (scholar.google.com), a literature search was carried out to identify peer-reviewed publications published between 1990 and 2020, which investigated the impacts of increased precipitation and elevated temperature manipulation on carbon fluxes in Northern Hemisphere grassland ecosystems.

We employed the following keyword combinations to investigate GEP, ER, and NEP in response to increased precipitation and elevated temperature: (a) rainfall OR precipitation OR experimental warming (OR climate change OR elevated temperature) AND (b) carbon fluxes (To determine whether an ecosystem is a carbon sink or source, gross ecosystem productivity (GEP), ecosystem respiration (ER), and net ecosystem productivity (NEP) are commonly used. These parameters are employed in our study) AND (c) grassland.

Only include studies that meet the following criteria will be considered: (1) Experiments that examine the impact of increased precipitation and elevated temperature on a natural grassland ecosystem (There is no strong human interference) must employ a paired group design, comprising a control group and a treatment group, with each group replicated at least 3 times. (2) Furthermore, the experiment site should be in a natural grassland ecosystem where at least one selected variable is related to the carbon fluxes measured by the control group and treatment group; (3) It is necessary to maintain consistent environmental conditions between the control group and treatment group before starting precipitation and temperature treatments; (4) For the control and treatment groups in the study, the mean, standard deviation, or standard error of the selected variables should be reported, as well as duplicate data; (5) For the duration of the carbon flux variables measurement, a minimum of one growing season is required, meanwhile clearly describing the relative changes or absolute in precipitation as well as temperature; (6) Multi factor experiments involving changes in precipitation and temperature should only use data from the control group and data from precipitation and temperature treatments; (7) The experiment was located in the grassland ecosystem of the Northern Hemisphere. If studies involving multiple sites or different conditions, such as different biological communities, different years, different levels of precipitation and temperature fluctuations, each study is considered an independent observation result. Based on these criteria, a total of 308 paired observations were identified from 50 published studies, of which 254 were for increased precipitation treatment and 54 were for elevated temperature treatment, for comparison between the control and treatment groups.

### 2.2. Data processing

Basic information such as mean, number of repetitions, and standard deviation were obtained from each study. The data was directly extracted from the original publication. Graphic data was digitized using a drawing digitizer version 2.6.8. In addition, parameters such as precipitation and temperature changes compared to the control group were recorded, as well as treatment duration in years. We also obtained both latitude and longitude (in degrees), mean annual temperature (MAT) (in °C), and mean annual precipitation (MAP) (in mm/yr). In the absence of MAT or MAP data, we conducted comprehensive searches on other sources.

The data on carbon fluxes of the North Hemisphere grassland ecosystems were mainly collected from both China and North America. There are 14 sites were collected for precipitation experiments while 17 sites were collected for warming experiments in total
(Fig. 1). The studies that met our requirements ranged in location between 30.77 °N and 44.55 °N, while MAT ranged from -3.8-17 °C and MAP from 160 to 914 mm (Table 1). The treatment period varied from one year to nine years, with the increased precipitation treatment's magnitude ranging from 0.08 to 489 mm (Table 1). Warming treatments ranged in magnitude from 0.4 to 2.8 °C, with treatment duration spanning one to thirteen years (Table 1).

## 2.3. Data analysis

All statistical analyses by R v.3.5.2 (R Core Team, 2018). Firstly, a linear mixed-effects model were used to assess carbon fluxes responses to changes in elevated temperature and increased precipitation treatment:

$$ \ln RR = \beta_0 + \beta_1 \times treatment + \pi_{study} + \varepsilon $$

where $\beta_0$ and $\beta_1$ represent the estimated coefficients; the random effect of "study" represented by $\pi_{study}$, explains the autocorrelation among results within each study; the sampling error, denoted by $\varepsilon$; and the effect size of increased precipitation and temperature on carbon fluxes from each observation, as reported by Hedges et al. (1999) are indicated by lnRR::

$$ \ln RR = \ln X^{-}_{treatment} - \ln X^{-}_{control} $$

where the variables for distinct carbon flux mean values are indicated by $X^{-}_{treatment}$ and $X^{-}_{control}$ in the treatment group and control group, respectively. Previous studies have commonly utilized lnRR (Peng and Chen, 2021). Specifically, a positive lnRR suggests that increased precipitation and elevated temperature have increased carbon fluxes, while a negative lnRR suggests they decreased carbon fluxes. It is crucial to take into account individual observations when estimating effect size. The corresponding variance for each ln RR (v) was calculated as:

$$ v = \frac{Se^2}{n_e X^{-}_{treatment}} + \frac{Sc^2}{n_e X^{-}_{control}} $$

with $n_e$ and $n_c$, $Se$ and $Sc$ denoting the sample size and standard deviation of the increased precipitation and elevated temperature and control groups, respectively. Subsequently, we evaluated whether the responses of carbon fluxes to treatments with higher temperatures and more precipitation differed noticeably from zero using a mixed-effects weighted-estimation model. These mixed-effects models were fitted using the lme4 software package by restricted maximum likelihood estimation, where v represents the weight of each matching lnRR observation (Bates et al., 2015). Moreover, we evaluated the normality assumption of each model using the Shapiro-Wilk test, and in cases where this assumption was violated, and we also employed 1000 iterations with the boot package to bootstrap the fitted coefficients (Chen et al., 2021a). To evaluate the influence of magnitude, duration, and local climate, the weighted effect size and its 95 % confidence interval (CI) were computed for every variable of carbon fluxes. This was accomplished using a multi-level meta-analysis approach outlined by previous study (White et al., 2012).

# 3. Results

## 3.1. The overall effect of increased precipitation and elevated temperature on carbon fluxes in North Hemisphere grassland ecosystems

Generally, the effects of increased precipitation and elevated temperature on GEP, ER, and NEP exhibited variability. Increased precipitation dramatically raised GEP and ER of grassland ecosystems in the Northern Hemisphere in comparison to the control group, with an average increase of 9.8 % and 12.5 %, respectively (Fig. 2a, Table S1). However, no significant effect was found with NEP by increased precipitation. Different with increased precipitation, warming had no significant impact on GEP, ER, and NEP of the Northern Hemisphere grassland ecosystems, respectively (Fig. 2b, Table S1).

**Table 1**

Summary of site characteristics, magnitude (%) and duration (year) of precipitation treatments for the whole database.

<table><thead><tr><th></th><th>Parameter</th><th>Unit</th><th>Mean (SD)</th><th>Range</th><th>N</th></tr></thead><tbody><tr><td rowspan="5">IP</td><td>Latitude</td><td>°</td><td>40.90 (4.49)</td><td>30.77-44.55</td><td>254</td></tr><tr><td>Longitude</td><td>°</td><td>106.85 (36.24)</td><td>-117.7-123.52</td><td>254</td></tr><tr><td>MAT</td><td>°C</td><td>2.11 (2.90)</td><td>-1.2-17</td><td>254</td></tr><tr><td>MAP</td><td>mm</td><td>374.48 (92.73)</td><td>160-747</td><td>254</td></tr><tr><td>Rate</td><td>%</td><td>129.35 (78.86)</td><td>0.08-489</td><td>254</td></tr><tr><td rowspan="5">Warming</td><td>Duration</td><td>year</td><td>3.32 (2.14)</td><td>1-9</td><td>254</td></tr><tr><td>Latitude</td><td>°</td><td>38.09 (4.82)</td><td>31.39-44.50</td><td>54</td></tr><tr><td>Longitude</td><td>°</td><td>91.80 (55.19)</td><td>-97.52-123.52</td><td>54</td></tr><tr><td>MAT</td><td>°C</td><td>2.34 (4.53)</td><td>-3.8-16.3</td><td>54</td></tr><tr><td>MAP</td><td>mm</td><td>439.63 (150.64)</td><td>162.5-914</td><td>54</td></tr><tr><td></td><td>Rate</td><td>%</td><td>1.27 (0.71)</td><td>0.4-2.8</td><td>54</td></tr><tr><td></td><td>Duration</td><td>year</td><td>4.14 (2.69)</td><td>1-13</td><td>54</td></tr></tbody></table>

SD indicates standard deviation.
<figure><img src="image_7.png" /><figcaption>**Fig. 2.** The responses of carbon fluxes to increased precipitation (a) and warming treatments (b) were assessed by calculating weighted effect sizes along with their corresponding 95 % confidence intervals. These values indicate the effect of the magnitude of increased precipitation and warming on carbon fluxes relative to the control group. GEP, gross ecosystem productivity; NEP, net ecosystem productivity; ER, ecosystem respiration. The purple color denotes a significant positive effect of increased precipitation and warming, orange color represents an insignificant effect, pink denotes significant positive effect of increased precipitation and warming, respectively. To provide a more transparent representation of the results, significant effects are further denoted by "*" for p<0.05 and "." for p<0.1, while "ns" indicates insignificant effects.</figcaption></figure>

## 3.2. *The relationship between carbon fluxes, treatment rate and duration in North Hemisphere grassland ecosystems*

Different with carbon fluxes response to increased precipitation, the effect sizes of GEP in the Northern Hemisphere grassland ecosystems significantly decreased with treatment duration and rate of increased precipitation, respectively (Fig. 3a, Table S2). Interestingly, GEP was significantly increased with treatment rate*duration of increased precipitation (Fig. 3a, Table S2). Similiar with carbon fluxes response to warming, neither treatment rate nor duration demonstrated a significant effect on GEP, ER, and NEP in warming experiments (Figs. 3, 4, 5; Tables S2, S3, S4).

<figure><img src="image_8.png" /><figcaption>**Fig. 3.** Effects of treatment rate, duration, and local MAT and MAP on GEP were assessed through increased precipitation and warming manipulation. MAT, mean annual temperature; MAP, mean annual precipitation; GEP, gross ecosystem productivity. Treatment magnitude refers to the relative alteration in precipitation and temperature compared to the control group, while duration represents the experimental period in years. The purple color denotes a significant positive effect of increased precipitation and warming, orange color represents an insignificant effect, pink denotes significant positive effect of increased precipitation and warming, respectively. To provide a more transparent representation of the results, significant effects are further denoted by "*" for p<0.05 and "." for p<0.1, while "ns" indicates insignificant effects.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 4.** Effects of treatment rate, duration, and local climate MAT and MAP on ER were assessed through manipulation of increased precipitation and warming. MAT, mean annual temperature; MAP, mean annual precipitation; NEP, net ecosystem productivity. Treatment magnitude refers to the relative alteration in precipitation and temperature compared to the control group, while duration represents the experimental period in years. The purple color denotes a significant positive effect of increased precipitation and warming, orange color represents an insignificant effect, pink denotes significant positive effect of increased precipitation and warming, respectively. To provide a more transparent representation of the results, significant effects are further denoted by "*" for p<0.05 and "." for p<0.1, while "ns" indicates insignificant effects.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 5.** Effects of treatment rate, duration, and local climate MAT and MAP on NEP. MAT, mean annual temperature; MAP, mean annual pre-cipitation; NEP, net ecosystem productivity. Treatment magnitude refers to the relative alteration of precipitation and temperature compared to the control group, while duration represents the experimental period in years. The purple color denotes a significant positive effect of increased pre-cipitation and warming, orange color represents an insignificant effect, pink denotes significant positive effect of increased precipitation and warming, respectively. To provide a more transparent representation of the results, significant effects are further denoted by "*" for p<0.05 and "." for p<0.1, while "ns" indicates insignificant effects.</figcaption></figure>

### 3.3. The relationship between carbon fluxes and local climate conditions in North Hemisphere grassland ecosystems

The response of local climate to GEP with warming was more responsive than that of increased precipitation. The response of increased precipitation for GEP exhibited a significant increase with MAT (Fig. 3a, Table S5), while the response of warming on carbon fluxes including GEP, ER, and NEP demonstrated a significant increase with MAP (Figs. 3, 4, 5, Tables S5, S7, S8). Meantime, the impacts of increased precipitation on ER and NEP were not significantly affected by MAT or MAP. MAT also did not exert a significant impact on GEP, ER, or NEP under warming conditions (Figs. 3, 4, 5, Tables S5, S6, S7).
# 4. Discussion

## 4.1. Increased precipitation enhanced GEP and ER in North Hemisphere grassland ecosystems

Increased precipitation led to an enhancement in GEP and ER, while it did not have a significant impact on NEP (Fig. 2a). The inconsistency results between our results and previous individual experiments indicating considerable uncertainty regarding the response of carbon fluxes to increased precipitation. These uncertainties can be attributed to several factors: (1) the magnitude of increased rainfall (Post and Knapp, 2021); (2) the lasting time of increased rainfall (Felton et al., 2021); and (3) the regional climate conditions, such as MAT and MAP (Avolio et al., 2020).

In arid grassland ecosystems, water availability is regarded as the main driving factor for carbon fluxes (Jiang et al., 2012). Previous study has reported that drought caused by warming can reduce both GEP and ER (Maestre et al., 2015), this is because the reduced photosynthetic carbon absorption and transpiration through stomatal closure under water deficit conditions, thereby inhibiting GEP (Yang et al., 2011). A field experiment in Mediterranean C3/C4 grassland also demonstrated that drought can simultaneously reduce GEP and ER, it also significantly reduced NEP through a stronger inhibitory effect on GEP than ER (Aires et al., 2008). In contrast to the reduced water availability by drought, increased precipitation can positively affect both GEP and ER simultaneously, but there is currently no consensus on its impact on NEP (Zhang et al., 2017). Some studies have shown that increased precipitation does not have a significant impact on NEP due to the significant uncertainty of carbon sinks in grassland ecosystems (Ahlström et al., 2015; Liu et al., 2023b). In accordance with these results, our study also showed a increased GEP and ER but no significant impact on NEP with more precipitation (Fig. 2a). This may attribute to increased capacity of photosynthesis and transpiration with alleviated water constraints, however, the improvement in GEP and in NEP are similar with more precipitation, which may explain why no significant effect in NEP was found. When experiments was carried out on a longer time scale, a decreasing trend of GEP was observed (Fig. 3a). Therefore, it is necessary to carefully explain the previous studies on the short-term impact of grassland productivity, emphasizing the important understanding of the feedback mechanism between the ecosystems' long-term response and climate change in North Hemisphere grassland. Two primary mechanisms can be used to elucidate the long-term weakening of the impact of increased precipitation on plant productivity. Firstly, as the duration of the experiment increases, other factors including light availability and soil nutrient availability may restrict plant development as precipitation increases (Huxman et al., 2004). Secondly, modifications in the functional composition of plants can increase an ecosystem's resilience to withstand sustained fluctuations in precipitation patterns (Griffin-Nolan et al., 2019). With increased precipitation, our results also suggest that the response of local MAT was positively correlated with GEP. Considering that grassland ecosystems are mainly distributed in arid and semi-arid regions, elevated temperatures can exacerbate water stress conditions through more soil evaporation (Ahlström et al., 2015). Consequently, the increase in local temperature amplifies the enhancement effect of increased precipitation on GEP in grassland ecosystems.

## 4.2. Warming did not significantly affect the carbon fluxes of North Hemisphere grassland ecosystems

In contrast to the influence of elevated precipitation on carbon fluxes, our analysis revealed that warming did not exert a notable

<figure><img src="image_11.png" /><figcaption>**Fig. 6.** Illustration of increased precipitation and warming (including treatment rate, duration, and local climate factors) effects on grassland carbon fluxes (including gross ecosystem productivity, net ecosystem productivity and ecosystem respiration) in the Northern Hemisphere. The red and yellow arrows indicate significant increases and decreases, respectively, in a response variable, whereas the horizontal line and "ns" indicates nonsignificant changes. GEP, gross ecosystem productivity; NEP, net ecosystem productivity; ER, ecosystem respiration; MAT, mean annual temperature; MAP, mean annual precipitation.</figcaption></figure>
impact on carbon fluxes including the GEP, ER, and NEP in North Hemisphere grassland ecosystems (Fig. 2b). Similarly, a number of previous studies also have shown that elevated temperature may not have an impact on carbon fluxes in grassland ecosystems (Lin et al., 2010; Pendall et al.; Rustad, 2001; Wu et al., 2011). This may be attributed to several factors, including (a) differences in initial climatic conditions; (b) the temperature sensitivity of different carbon cycling processes varies; (c) the difference in duration of warming treatment and (d) the difference in experimental methods of warming (Wang et al., 2019).

Due to the limitations of temperature and moisture conditions on carbon cycling processes, local climate conditions usually strongly affect the response of grassland ecosystems to warming. For example, grassland ecosystems in cold regions may be more sensitive to warming because they typically have larger soil organic carbon storage, which may be easily decomposed (Crowther et al., 2016), and decomposition typically increases with increasing temperature (Davidson and Janssens, 2006). Concurrently, the impact of warming on carbon fluxes in different grassland ecosystems varies. In Tibetan Plateau, experimental warming increased GEP of meadow grassland ecosystem, but did not increase ER, thus increasing NEP (Chen et al., 2017). Warming simultaneously suppressed GPP and ER in the Oklahoma short grass grasslands of US, but did not have a significant impact on NEP (Xu et al., 2016). The promoting effect of warming on carbon fluxes in grassland ecosystems may only be temporary, and the initial response to warming can be minimized through nutrient limitation in a longer time scale (Sebastia, 2007). The different response of GEP, ER and NEP in different grassland may offset their response in the whole North Hemisphere grassland ecosystems.

Our studies also suggest that alterations in GEP, ER, and NEP induced by elevated temperature are positively correlated with MAT and MAP. Therefore, it can be inferred that grassland ecosystems in cold and arid environments exhibit higher sensitivity to changes in carbon fluxes caused by elevated temperature. Moreover, as the water supply increases, the sensitivity of carbon fluxes to elevated temperature will become more pronounced under higher humidity conditions, and it may even reach a saturation point in excessively humid environments. On the contrary, soil moisture is the primary factor limiting vegetation and microbe activities in arid and semi-arid regions globally (Li et al., 2022). Our results also showed that the rate and duration of both increased precipitation and elevated temperature can significantly affect carbon fluxes in grassland ecosystems. The local climate (MAT and MAP) conditions also play a crucial role in influencing the carbon cycling process (Fig. 6). In future research, it is imperative to strengthen the comprehensive understanding of how different factors interact with each other to mechanically form carbon fluxes.

### 4.3. Limitations of this study

The present study revealed the significance of the magnitude, duration, and local climate conditions of the experiment on North Hemisphere grassland carbon fluxes responses. Nevertheless, a comprehensive understanding of the consequences of increased precipitation and warming at longer time-scales remains elusive, largely due to the relatively short experimental periods used for the majority of carbon cycling processes. Consequently, the necessity for further long-term field data has been identified, in order to accurately assess the influence of increased precipitation or warming on C cycling processes over an extended period of time (over 10 years). It also should be noted that most of sampling sites are located at east Asia and middle Asia (especially China), while only 2 sites are located at North America and no sites are located in Europe, which can limit the accuracy of results in reflecting the North Hemisphere grassland ecosystems response. There is a strong need for more study sites (especially in Europe and North America) in future experiments. Meanwhile, the majority of precipitation and warming experiments included in the meta-analysis have been conducted in grasslands, and there is a lack of data from other biomes, including deserts, forests, wetlands, tundra and savannah. This limits our ability to identify significant differences between biomes. Furthermore, precipitation and warming experiments could affect terrestrial C cycling jointly with multiple climate change factors, a multifactorial experimental approach combine with advanced modeling will prove invaluable for the study of ecosystem responses in the future.

## 5. Conclusion

In most cases, the intricate relationship between the duration and magnitude of increased precipitation and elevated temperature, as well as local MAT and MAP, has not been fully explored in the meta-analysis when determining carbon fluxes in grassland ecosystems. Our study indicates that the response of carbon fluxes in North Hemisphere grassland ecosystems largely depends on the magnitude and duration of increased precipitation and elevated temperature treatments, as well as the local climate. This highlights the profound impact of constantly changing temperature and precipitation patterns on controlling whether ecosystems function as net carbon sinks or sources.

### CRediT authorship contribution statement

Weikang Zhang, Zhaogang Liu, Ming Zhao, and Congcong Zheng conceived the manuscript. Weikang Zhang, Zhaogang Liu and Ming Zhao performed the statistical analyses. Weikang Zhang, Zhaogang Liu, Ming Zhao, Zhaoxia Jiang and Congcong Zheng drafted the manuscript. All the authors contributed to the manuscript and approved the submitted version.

### Ethical statement for solid state ionics

1) This material is the authors' own original work, which has not been previously published elsewhere.

2) The paper is not currently being considered for publication elsewhere.

3) The paper reflects the authors' own research and analysis in a truthful and complete manner.
4) The paper properly credits the meaningful contributions of co-authors and co-researchers.

5) The results are appropriately placed in the context of prior and existing research.

6) All sources used are properly disclosed (correct citation). Literally copying of text must be indicated as such by using quotation marks and giving proper reference.

7) All authors have been personally and actively involved in substantial work leading to the paper, and will take public responsibility for its content.

## Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

Weikang Zhang and Ming Zhao were supported by the National Natural Science Foundation of China (32201348,32301331), Congcong Zheng was supported by the Joachim Herz Foundation (850035).

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.gecco.2024.e03289.

## Data availability

Data will be made available on request.

## References

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