title: Controls over carbon and nutrient dynamics in wetland soils: an ecological stoichiometry perspective
Creators: name: Yao, Lu
Publisher: name: Griffith University
Year: 2020
Subjects: subject: wetlands
subject: soils
subject: carbon
subject: biogeochemistry
subject: nutrient retention
subject: stoichiometry

description: Wetland ecosystems can store large amount of carbon (C) and serve as a biogeochemical ‘hotspot’ for nutrient retention. According to Ramsar Convention, wetlands are defined as areas permanent or temporary with fresh, brackish or saltwater, including areas of marine water the depth of which at low tide does not exceed 6 metres. The C and nutrient dynamics in soil depends on an array of biotic and abiotic factors as well as their interactions. Soil microbial demand of C and nutrients is regulated by the elemental stoichiometry of microbial biomass and resource availability in the environment. Globally, human land use induces C loss and the imbalance of nitrogen (N) and phosphorous (P) in wetland ecosystems and alters soil biogeochemical processes. The aim of my PhD research was to investigate the key factors and processes controlling C and nutrient dynamics in wetland soils with respect to soil physicochemical properties, concentrations and stoichiometric ratios of C, N and P in total, labile, microbial biomass and soil enzymes pools, as well as microbial functional genes, in order to improve understanding of stoichiometric controls C and nutrient dynamic in wetland ecosystems under environmental changes. To achieve this aim, four major experiments were conducted, including: a) Sediment N remove processes (i.e., potential denitrification rate, unamended denitrification rate and net N2O production rate) and their relationships to water quality and sediment characteristics were investigated in 7 sub-lakes of the Lake Donghu, the largest urban lake in China; b) Organic C stock of wetland in Queensland was estimated as a function of soil depth, transect sampling (wetland soils to transition zone to non-wetland sites), and climatic regions, and the key factors that control organic C stock in wetland soils were determined using ecological stoichiometry approach. This study was motivated by the Department of Natural Resources and Water under the Queensland Wetlands Programme (QWP); c) Concentrations and stoichiometry of C and nutrients in total, labile, biomass pools were compared in upland soil, riparian wetland and sediment along two distinct transects, each transect was covered by sugarcane and forest, respectively; d) Stoichiometric ratios of C, N and P in soil total and microbial biomass pools, and associated properties were compared in soils under different vegetation types and soil management practices in wetlands located in tropical Queensland, and in turn to investigate the stoichiometric response of soil microbes under changed environment condition. It was hypothesized that: a) The sediment denitrification parameters would be controlled by both water quality and sediment characteristics, such as concentrations and stoichiometric ratios of N and P; b) Wetland in tropical and subtropical areas would have higher C stocks than in semi-arid and arid areas. Climate, soil properties and C to nutrient ratios may exert a strong influence over OC stock; c) Cultivation in upland would result in the lower stoichiometric ratios of resource C, N and P and microbial biomass C, N and P through decrease C concentration but increase nutrient availability for microbial utilization in downstream wetlands. Meanwhile, due to the stoichiometric flexibility of microbes, riparian zone might have a ‘dilution’ effect on nutrient retention process along a land-use gradient from upland to downstream aquatic ecosystem; d) Vegetation type and soil management practice would modify the stoichiometric ratios of resource, and thus regulate the biogeochemical dynamics and cycling of C and nutrient via changing soil enzymes activity. Excessive N in lakes may lead to eutrophication and many accompanying environmental problems such as water quality decline and loss of aquatic biodiversity. Denitrification in lake sediments can alleviate the effects of eutrophication through removal of N to the atmosphere as N2O and N2. However, N2O contributes to the greenhouse effect and global warming. From the first experiment (Chapter 3), we measured three denitrification parameters (i.e., potential denitrification rate, unamended denitrification rate and net N2O production rate) in surface sediments which were collected from 7 sub-lakes of the Lake Donghu (30.5667N, 114.4167E), one of the most eutrophic lakes in China. The results showed that a range of water quality and sediment characteristics (e.g., total N and total phosphorus) varied significantly among sub-lakes. The unamended denitrification rate varied between 0.51 and 26.0 ng N g-1 h-1, while the N2O production rate ranged from less than 0 to 1.68 ng N g-1 h-1. However, there was no significant difference among the sub-lakes in these denitrification parameters. The unamended denitrification rate was positively related to the water NO3– concentration, sediment moisture and bulk density. The findings of the present study suggest that sediments in eutrophic lakes can remove large quantities of N through denitrification and may become a significant source of N2O if the N input is maintained or to increase. In addition, it has been proposed that about 1.25 mol of C is required by denitrifiers for the complete denitrification of 1 mol of NO3–, indicating that denitrification is regulated by stoichiometry of C and N. Thus, it is a necessity to include the stoichiometry of C and nutrient when assessing biogeochemical processes in wetlands under environmental change. Wetlands have disproportional potential to store C in respect of occupying small area of landscape. Meanwhile, wetlands in different climatic zones are not equally effective in sequestering C. A large-scale study of wetland soil C stocks and controlling factors were conducted in the state of Queensland in north-eastern Australia. As part of this PhD study (Chapter 4), adjacent wetland soil, transition zone (between wetland and non-wetland soil sites) soils and non-wetland soil samples were collected across Queensland. From the analysis of organic C (OC) stock to a soil depth of 1 m for adjacent wetlands, transition and non-wetlands sites, it has been found that wetlands had the highest OC stocks (19.2 ± 22.1 kg m-2), compared to transition zones (12.9 ± 16.0 kg m-2) and non-wetland soils (12.1 ± 14.8 kg m-2). Subtropical wetlands (44.7 ± 18.5 kg m-2) sequestered more OC than wetlands in tropical areas (39.9 ± 22.9 kg m-2), 8.0 times more than in semi-arid areas (5.6 ± 14 kg m-2) and 15.4 times more than in arid areas (2.9 ± 1. 0 kg m-2). Based on the estimated area of wetlands in Queensland (68,423 km2), it is estimated that, to a depth of 1.0 m, the average stock of organic C is approximately 1,081,083,400 Mg C in soil from permanently or seasonally inundated wetlands in Queensland. Overall, our results revealed that the capacity for C storage was inherently variable among wetlands. A comparatively high variation in OC stock between wetlands in different climatic zones indicated that the climate, soil properties, C and nutrient ratios may exert a strong influence over OC stock. Stepwise regression analyses further revealed the relative importance of climate, soil properties, stoichiometry of C and nutrient and their complex interconnections in regulating OC stock across climatic regions in Queensland. To reduce the uncertainty of sub-regional and regional (e.g. Queensland) effects, insight into local scale is required to sharpen regional prediction of how soil C and nutrient dynamics changed in response to environmental changes. Agricultural land use within a local scale, such as the Great Barrier Reef catchment, has proved to have strong influences on soil biogeochemical properties, which in turn have significant impacts on C and nutrient availability in downstream wetland systems. We examined concentrations and stoichiometry of C and nutrients in total, labile, biomass pools in upland soil, riparian wetland and sediment along two distinct transects (sugarcane versus forest) in Maryborough sugarcane farmland (25.6494N, 152.6739E), which is located in the Fraser Coast Region, Queensland, Australia. Sugarcane cultivation significantly reduced total C, nitrogen (N), labile C and N in riparian soils by 69%, 62%, 33% and 45%, respectively, but increased NO3--N and labile P by 88% and 99% in riparian areas and 50% and 73% in downstream sediment. The presence of native forest resulted in significantly higher NH4+-N concentrations in downstream wetlands. Concentrations of microbial biomass C and N were generally lower, but the abundance of genes associated with nitrifiers (ammonia oxidizing bacteria and archaea) was higher in the sugarcane transect than in the forest transect. These differences between two transects could be attributed to different organic inputs and biogeochemical processes associated with the different vegetation types and management practices in the upland systems. Difference in d13C signature from the two transects further confirmed the significant influence of vegetation type on downstream wetlands. Sugarcane cultivation led to a consistent stoichiometric shift in both resource and microbial biomass towards lower C:P and N:P ratios across upland soils, wetlands and sediment, compared with the forest transect. The average total and microbial biomass C:N:P ratios in soil under sugarcane were 136:9:1 and 180:33:1, respectively. The average total and microbial biomass C:N:P ratios in soil under forest were 410:22:1 and 594:76:1, respectively. It is concluded that the shifts in resource stoichiometry induced by upland land use change regulated the changes in the stoichiometry of microbial biomass C, N and P in wetland ecosystems. Vegetation type and land cover management induce the amount, form or proportion of the organic inputs and nutrient availability. Subsequently microbes can adjust C and nutrient acquisition efficiency by altering the potential activity and proportions of enzyme activities. The effects of vegetation type and associated management practices were further investigated through in this study. Soil cores (0-30 cm) under mangroves, marsh, Melaleuca and sugarcane (18.8957N, 146.2594E) from the Great Barrier Reef Catchment in northern Queensland were collected to investigated the effect of vegetation type and management practices on soil properties, such as C stock, total C, N and P, availability of N and P, and microbial biomass C, N and P, C-, N- and P- acquiring enzyme activity, as well as stoichiometric ratios of C, N and P in different pools. Soil from the seaward mangroves and marsh had higher EC, moisture, Olsen P, but lower NO3--N concentration. Melaleuca soil had higher C storage, total C, N and P, NO3--N concentrations, C-, N- and P- acquiring enzyme activities, but lower pH. The moisture, C stock and Olsen P were lower, and bulk density was higher in the sugarcane soil, resulting from intensive soil management practice. Mangroves soil had higher total C:N ratio, while sugarcane soil had higher ratios of inorganic N to Olsen P concentrations. These could be resulted from different organic inputs from vegetation types, as well as different availability of nutrient caused by soil management practices. The principal component analysis (PCA) further confirmed the influence of cultivation practices on soil properties, while the microbial community is stoichiometrically constrained by resource availability indicated by the positive association between microbial biomass N:P ratio and total N:P ratio. It confirmed that microbial demand of nutrients was governed by the conservative stoichiometry between microbial biomass and the elemental heterogeneity of the environment, and under nutrient limited condition, microbial demand of nutrients can be compensated by an upregulation of specific enzyme synthesis. The stoichiometric flexibility of systems is able to link the biogeochemical processes across scales. The stoichiometric homeostasis of organisms (plants and microbes) is able to deal with different C and nutrient conditions provides important information in understand their behaviour under global environmental change, such as urbanization, land use change from wetland to agricultural land, as well as sea level rise. Therefore, in the future work, the stoichiometric flexibility and stoichiometric homeostasis of ecosystems should be integrated into Earth System Models, the results can provide better management for wetland ecosystems conservation and restoration.