Carbon-Phosphorus Transfer At Wetland Soil-Water Interface Mediated By Microbial Ecology Subjected To Warming:Microcosmic Study

Global warming has exerted extensive impacts on worldwide wetlands, including carbon and phosphorus biogeochemical cycling at the soil-water interface pertain to water quality improvement. As a crucial "source" for carbon cycling, warming-induced carbon loss in wetlands impairs global carbon cycling, while in the other hand warming accelerates water eutrophication with phosphorus release from soils to water bodies as a key priming factor. Microbe drives biogenic element cycling, playing essential roles in carbon and phosphorus dynamics. In this study, a temperature controlled-microcosm system was developed to simulate climate change scenarios of an ambient temperature (control) and an elevated temperature (+5℃). Six wetlands with difference in carbon and phosphorus contents in soils were selected with soil samples been incubated under such novel microcosm platform. The main results obtained in this study were as follows:(1) Warmed accelerated the loss of recalcitrant organic carbon compared to labile organic carbon during2.5-year warming incubation. GeoChip4.0, a functional gene microarray, was used to determine microbial gene diversity and functional potential for the degradation of different organic carbon fractions (labile vs. recalcitrant). The results showed that the relative abundance of carbon-related genes responsible for the degradation of starch, hemicellulose, chitin and lignin have increased by8.6%,4.2%,6.8%and14.5%, indicating warming preferentially stimulated genes for degrading recalcitrant organic carbon (starch and hemicellulose) compared to labile organic carbon (chitin and lignin) in soil-microbe complex. Warming-enhanced polyphenol oxidase and peroxidase activities combined with increased (by27%) relative abundance in soil fungi were linked and coupled with these increased carbon-related genes, providing an underlying mechanism for carbon dynamics.(2) Dissolved organic matter (DOM) is the most bioavailable soil organic pool. Elevated temperature resulted in initially (2.5-year) higher concentrations in pore-water DOM concentrations in warmed samples, compared to the control. These effects diminished over longer time periods (4.5-year), which alleviated carbon loss in dissolved forms under sustained warming. Compared to the control, warming did not exert a significant effect on pore-water DOM released from bulk soils (p>0.05). For spectroscopy analysis, UV-visible spectral indices, including A254, SUVA254, and SUVA280were significantly higher in warmed samples. Of fluorescence spectral indices (FI, β:a, HIX, and IA/Ic), humification index (HIX) were significantly higher, while the signal intensity ratio of the fulvic-like to humic-like fluorescence peak (IA/Ic) was lower in warmed samples, compared to the control. Fluorescence regional integration (FRI) analysis suggested that warming consistently enhanced the contribution of humic-like substances to DOM composition for all tested wetlands. These spectral fingerprints implied a declined fraction of readily available substrates in DOM allocated to microbial utilization in response to4.5years of warming. As a negative feedback, decreased DOM biodegradability may have the potential to counteract initial DOM increases and alleviate carbon loss in water-saturated wetland soils.(3) Phosphorus transfer at the soil-water interface indicated warming promoted phosphorus release from soil to pore-water, then pore-water to overlying water. The release of phosphorus from soil to pore-water revealed a stronger and quicker response to experimental warming compared to the subsequent upward transfer into overlying water. A new model, potential phosphorus flux was constructed in this study, which showed that phosphorus was accumulated in the pore-water, and there was a higher phosphorus gradient between pore-water and overlying water under warming. As temperature increased in the summer, accumulated phosphorus was further released into the overlying water. The redistribution of potential mobile phosphorus fractions in soils by greater NH4Cl-P and lower BD-P (extracted by a bicarbonate buffered dithionite solution) accompanied by greater NaOH-P were also found under warming.(4) The soil environmental factors and microbial functional genes were further explored to bridge a linkage with phosphorus release. Warming decreased in-situ dissolved oxygen contents at the soil-water interface, the thickness of the oxidized soil layer and the associated soil redox potential. Warming enhanced the microbial functional diversity, and a variety of functional genes involved in oxidation-reduction reactions and phosphorus degradation were preferentially enriched under warming. The enriched genes included those encoding carbon monoxide dehydrogenase (CODH), acetyl-CoA carboxylase biotin carboxylase (ppc), and ribulose-1,5-bisphosphate carboxylase (rubisco) for carbon-fixation, nitrate reductases (narG) and nitrous oxide reductases (nosZ) for denitrification, cytochrome c involved in metal reduction (cytochrome c), and exopolyphosphatase (ppx) for polyphosphate degradation. Combined with functional genes, warming induced lower soil redox potential and led to higher contribution of anaerobic microbe to the metabolic processes. Redox potential was one of the most significant parameters linked to microbial functional gene structure and determining terminal electron accepting processes. Our work showed that warming can directly promoted phosphorus release through organic phosphorus mineralization and the roles of phosphatase excretion, or indirectly through modulating soil redox potential, thus exerting influence on phosphorus release.(5) Combined carbon loss and phosphorus release features at the soil-water interface discussed above, microbial stoichiometric homoeostasis was further investigated to explain the underlying mechanism behind these phenomena with the aspect of ecological indicators. The significant (p<0.05) regression fitting relationship between carbon, phosphorus and C:P in soil/water and carbon, phosphorus and C:P in microbial biomass for both control and warmed samples indicated homeostatic regulation coefficient (H) can be generally used as an important ecological indicator to reflect microbially-mediated ecological stability and capacity of resistance to environmental disturbance in ecosystems. The enhanced phosphorus release and carbon loss from wetland soils was tightly coupled with a decrease in H values under warming, indicating a diminished ecological storage stability in wetland soils. The established n-Hc:p index could fundamentally reflect the interactions or stoichiometric balance between biogenic elements, and can be a useful ecological tool for assessing potential phosphorus flux in ecosystems.