The Effects Of Plant Decomposition On Soil Carbon Pool In Poyang Lake Wetlands Driven By Water Conditions And The Soil Bacterial Response

At present,due to the impact of global climate change and water conservancy projects,the dry season of lake wetlands in the middle and lower reaches of the Yangtze River tends to be advanced and prolonged,and the flooding season is shortened.The change of wetland hydrological rhythm will affect the distribution and decomposition state of wetland plants,and then affect the storage of soil carbon pool.However,little is known about how decomposition of wetland plants driven by water conditions affects the composition and stability of wetland soil carbon pools,and thus affects soil carbon sequestration and regional or global climate.As well as little is known about how soil bacterial communities respond to changes in water conditions and determine the fate of organic carbon.In order to answer the above scientific questions,Poyang Lake Wetlands,a typical lake wetland that connects the Yangtze River in China,was selected as the research object,and selected Carex cinerascens(high-quality),Triarrhena lutarioriparia(low-quality),to carry out these three aspects of research:(1)the effects of high-vs.low-quality litter decomposition on soil carbon pool and the response of microorganisms under simulated natural wetland conditions;2)the effects of flooding on different quality litter decomposition process and soil carbon pool,and the response of microorganisms;3)effects of decomposition of fresh plants and litter on soil carbon pool and responses of microorganisms under flooding conditions.During the experiment,we used environmental chemistry technology to analyze the composition of soil carbon pool,used the uv-vis spectroscopy and three-dimensional fluorescence spectroscopy to analyze the composition of soil soluble organic carbon(DOC),and used 16 S r RNA gene high-throughput sequencing technology to analyze the changes of bacterial community composition.The main results of this study were as follows:1.The effects of high-vs.low-quality litter decomposition on soil carbon pool composition and soil bacterial community were significantly different.The dry mass loss of high-quality litters(39.83%)was significantly higher than that of low quality litters(24.64%),and its loss rate of hemicellulose and cellulose was faster,while the loss rate of lignin was slower.The decomposition of high-quality litter increased soil labile carbon pool(LCP)and labile index(LI),but decreased soil non-labile carbon pool(NLCP)and soil carbon storage.However,decomposition of low-quality litters significantly increased soil NLCP by nearly two times,thus increasing soil carbon storage and stability.This study also found that the decomposition of high-quality litter decreased the molecular weight,hydrophobic components and humification degree of soil DOC,thus enhancing the bioavailability and degradability of DOC,and promoting the increase of bacterial endogenous DOC.However,the decomposition of low-quality litters made the spectral characteristics of soil DOC present an opposite law,which made the structure of soil DOC more complex.In addition,the decomposition of lowquality litters increased the richness and diversity of soil bacterial community more significantly than that of high-quality litters.Soil bacteria played a direct regulatory role in the dynamic change of soil carbon pool.Proteobacteria and Chloroflexi were the main bacteria groups associated with the decomposition of high-quality litter.The bacteria associated with the decomposition of low-quality litters mainly belonged to Acidobacteria,Actinobacteria and Verrucomicrobia.2.Compared to wet condition,the decomposition rate and the loss of organic matter(cellulose,cellulose and lignin)of litters were significantly increased by flooding.Moreover,the decomposition of high-quality litters made the soil carbon pool more labile under flooding condition,while the decomposition of low-quality litters made the soil carbon pool more stable.The decomposition of high-quality litter was more beneficial to reduce the aromaticity,molecular weight and humification degree of soil DOC under flooding condition.Low-quality litter had the opposite effect.Flooding significantly increased the richness and diversity of soil bacterial community and had a more significant direct effect on the dynamic change of soil carbon pool composition.Among them,the bacteria associated with the decomposition of highquality litters mainly belonged to Proteobacteria,Chloroflexi and Bacteroidetes,while the bacteria associated with the decomposition of low-quality litters mainly belonged to Acidobacteria,Firmicutes and Gemmatimonadetes.These results indicate that flooding does not change the effect of litter quality on soil carbon pool and the response of bacterial community,but enhanced this effect.3.Compared to litter decomposition,decomposition of fresh plants enhanced the effect of plant quality on soil carbon pool and microorganisms.The degradation amount and the loss of organic matter(hemicellulose,cellulose and lignin)of fresh plants were greater than that of litters.Additionally,compared to high-quality litters,decomposition of fresh high-quality plants was more beneficial to increase soil Li and LCP but decrease soil NLCP.Moreover,its decomposition reduced the humification degree of soil DOC and enhanced the bioavailability of DOC.Compared to low-quality litters,decomposition of fresh low-quality plants was more beneficial to increase soil NLCP but decrease LCP and LI,but it had no significant effect on soil DOC composition.Moreover,fresh plant decomposition made the soil bacterial community more active in regulating the soil carbon cycle.Proteobacteria,Chloroflexi and Bacteroidetes were mainly associated with the decomposition of fresh high-quality plants,while Acidobacteria,Actinobacteria and Firmicutes were mainly associated with the decomposition of fresh low-quality plants.These results indicate that the decomposition of a large number of fresh high-quality plants is not conducive to soil carbon sequestration,while the decomposition of fresh low-quality plants is more conducive to soil carbon sequestration under the condition of continuous high water level in the wetland.In conclusion,under the background of climate change and wetland water situation change,with the shorting of flooding season,prolonging of dry season and expansion of vegetation area,the decomposition pattern of wetland plants in Poyang Lake will be dominated by litter decomposition under wet condition.Combined with the results of our study,we can predict that the carbon sink of Poyang Lake Wetlands will become more prominent,which could play a positive role in mitigating local climate warming.In addition,soil bacteria directly control the formation and stability of soil carbon pool during litter decomposition.This study provides good predictions for assessing the effects of plant decomposition on wetland soil carbon budget under change of hydrological regime,and provides scientific supports for exploring the role of global lake wetland carbon sources and sinks.It also provides scientific bases for exploring the impact of wetland ecosystem on global carbon cycle and climate change.