Multidimensional Analysis Of Tidal C And N Input Affecting Soil Carbon Turnover In Wetland Of The Yangtze Estuary And Its Microbial Mechanism

Wetland is the kidney of the earth and has powerful pollution purification function.At the same time,due to its high primary productivity and the low temperature and anaerobic environment caused by long-term flooding,it is difficult for plant litter to be mineralized into carbon dioxide（CO₂）by microorganisms after returning to the field.Wetland is also considered as a typical"carbon sink",which plays an important role in alleviating global warming caused by the rise of atmospheric CO₂concentration.Estuarine wetlands have extremely high primary productivity and carbon accumulation rate,but they are also more disturbed by human factors than inland alpine wetlands,especially siltation promotion/reclamation,alien invasion,exogenous input of pollutants and nutrients,etc.Various human activities may affect the carbon"sink"function of estuarine wetlands.The Yangtze River estuary wetland has a special strategic position in Shanghai’s economic,social and ecological environment construction.It is the most important ecological barrier in Shanghai,but it is also inevitably facing the interference and threat from human activities.Among them,water pollution and eutrophication are an important environmental problem.Theoretically,the input of organic carbon and nutrients in the tide may cause the changes of physical and chemical properties of wetland soil,induce the changes of soil microbial structure,and then affect the whole process of wetland soil carbon turnover.However,the stability of tidal input organic carbon and how it affects the transformation process of wetland soil organic carbon,and then affects soil respiration.At present,there is a lack of in-depth research and specific evidence,and the microbial mechanism of its impact is not very clear.In this paper,Jiuduansha wetland in the Yangtze River estuary was selected as the research object.Based on the analysis of the correlation between tidal water properties and wetland SR and SOC,by analyzing the occurrence relationship of typical markers in wetland soil and water environment and plants,combined with the microcosm simulation of isotope labeled plants,from the aspects of the stability and retention of tidal and plant organic carbon in the wetland soil environment of the Yangtze Estuary,and the effects of tidal carbon（C）,Nitrogen（N）input on wetland SR and carbon output,the effects of tidal C,N input on wetland soil carbon turnover were studied,and the microbiological mechanism of tidal C,N input on wetland soil carbon turnover was explored.The research results will further enrich the understanding of the carbon sequestration process and its influencing factors of estuarine wetland,and play a role in throwing bricks and welcoming jade for exploring the correlation between pollution purification function and carbon sink function of estuarine wetland.The main research contents and results are as follows:1.Correlation between tidal C,N input and soil respiration and organic carbon content in the Yangtze River Estuary wetlandsThe results showed that the concentrations of total organic carbon（TOC）,total nitrogen and nitrate nitrogen decreased gradually from the upstream waters of the wetland to the downstream waters,from 2.69±0.6,1.97±0.27 and 1.63±0.32 mg·L^-1to2.07±0.19,1.41±0.39 and 1.23±0.39 mg·L^-1respectively.The results of correlation analysis showed that in addition to the content of organic carbon,the main physical and chemical properties of the wetland soils in the upstream and downstream were positively correlated with the tidal properties,and the soil properties of low tidal flat were more significantly affected by tide.The organic matter content in the upstream was high,but the SOC content of low tide flat in Jiangya Nansha was lower than that of low tide flat in Xiasha.The SR of low tide flat in Jiangya Nansha was significantly higher than that of low tide flats in Shangsha and Xiasha,indicating that the SR strength may be the main reason for its low SOC content.The results of statistical analysis showed that the organic carbon content,total nitrogen and nitrate nitrogen in the tide were significantly positively correlated with SR in the low tide flat.The above research results seem to show that the organic carbon input by tide was easier to decompose into CO₂,which was not conducive to preservation in wetland soil,while the higher N nutrient input by upstream tide may further promote the decomposition of SOC.2.Stability study of tidal input organic carbon in wetland soil based on marker analysisBy analyzing the occurrence relationship of markers such as n-alkanes and lignin in soil,water and plant environment,the source and retention capacity of SOC in Jiuduansha wetland soil were tracked from the perspective of carbon input.The dimensionless ratios of the input of phytoplankton n-alkanes to the retention of phytoplankton N-alkanes in mudflat soil of Nansha,Shangsha and Xiasha tides were1.259,1.642 and 2.148 respectively.The relative retention ratios of plant lignin carbon input from upstream to downstream to the retention of plant lignin carbon in low tide flat soil（10 cm depth）were 0.92,0.704 and 0.615,respectively.According to the above proportional relationship between input and retention,it can be inferred that the upstream tidal source organic carbon input was more and had high degradability,while the downstream tidal source organic carbon input was less and had low degradability,which was an important reason for the strong SR of the upstream wetland.From Xiasha high tide flat to low tide flat,the input of plant-sourced lignin carbon gradually decreased,while the input of tidal source organic carbon increased,but the relative retention ratio of plant-sourced lignin carbon was the highest in low tide flat soil,which is 0.615,while that of middle tide flat and high tide flat were 0.16and 0.148 respectively,indicating that tidal source organic carbon was easier to decompose than plant-sourced carbon,As a result,a higher proportion of plant-sourced organic carbon was retained in the soil in the low tide flat.3.Microcosm verification of the contribution of plant/tidal source organic carbon to wetland soil respiration based on isotope labelingThrough isotope labeled wetland plants and combined with the constructed Jiuduansha wetland microcosm system,the contribution of plant and tidal organic carbon input to wetland SR was explored from the perspective of carbon output.The experimental results showed that in the microcosm system,with the gradual increase of tidal organic carbon and N nutrients input intensity,SR increased continuously,and the released CO₂was mainly from the decomposition of plant soil mixture SOC（81.35%）to the decomposition of tidal organic carbon（91.18%）.It showed that with the increase of tidal organic carbon input intensity,wetland soil will preferentially decompose tidal organic carbon,and the decomposition of plant-sourced organic carbon in soil will be inhibited.This further verifies that tidal organic carbon was easier to decompose into CO₂and release than plant organic carbon.4.Microbial ecological mechanism of tidal C,N input stimulating wetland soil respiration intensityBy detecting the soil microbial activity and microbial community structure of Jiuduansha wetland,combined with the microcosm system simulation experiment,the microbial mechanism of tidal input stimulating wetland SR was studied.The results showed that due to the high input of organic carbon and N nutrients,the abundance of Gammaproteobacteria,Vicinamibacteria and Alphaproteobacteria with strong mineralization ability was induced in the low tidal flat of Jiangya Nansha of Jiuduansha wetland,resulting in strong soil microbial heterotrophic activity and SOC mineralization,which was finally reflected in higher SR and lower SOC content.In the microcosm system,with the increasing tidal input,the dominant flora in the soil changed significantly.For example,the Proteobacteria flora（mainly including mineralizing bacteria Gammaproteobacteria and Alphaproteobacteria）increased and the Firmicutes flora involved in complex organic carbon metabolism decreased.The SR intensity also increased,and the microbial substrate also changed from plant-sourced organic carbon with complex structure to tidal organic carbon.This showed that the increase of tidal organic carbon input intensity will significantly affect the microbial structure and properties,and then affect the catabolism of the corresponding substrate.Therefore,the change of soil microbial activity and structure may be the pioneer index of the change of soil carbon metabolism model in wetland.Overall,the multi-dimensional analysis and verification results showed that the tidal source organic carbon in the Yangtze River estuary was easier to degrade than the plant source organic carbon,the upstream was easier to decompose than the downstream,and The CO₂released by SR mainly came from the decomposition of organic carbon from tidal source.With the increase of the input intensity of organic carbon from tidal sources,the number and activity of heterotrophic microorganisms in wetland soil will increase,which will stimulate the SR intensity.The input of N nutrients in tides will aggravate this process.Since most of the organic carbon from tidal sources did not come from the CO₂fixed by photosynthesis,it only slightly increased the carbon input of the wetland.However,it was easier to decompose into CO₂than plant-sourced carbon,thus significantly enhancing the carbon output of wetland soil.Therefore,the absorption and interception function of estuarine wetland to organic pollutants and N nutrients in the tide may weaken its carbon sink function.