Effect Of Wetlands Soil Organic Carbon Sink And Bacterial Community Structure Respond To Hematite And Lignite Addition

Wetland's hydrological function degradation will cause a large area of wetland soil to be exposed to the air;increase the decomposition rate of soil organic carbon(SOC)and the rate of greenhouse gas emissions,and reduce the SOC content.The biogeochemical cycle of iron has an important impact on the SOC carbon storage and molecular structure stability.On one hand,iron-bound organic carbon(OC-Fe)is formed through adsorption,co-precipitation,and complexation,preventing OC decomposition by microorganisms;on the other hand,when OC-Fe is in an anaerobic environment,iron-reducing bacteria can cause the organic carbon(OC)retained by iron oxides to be released again via dissimilatory iron reduction.Therefore,iron oxides play a“double-edged sword”role in the C cycle.However,past research did not consider these two processes at the same time,making the net effect of iron oxide on the carbon cycle in the soil unclear.In order to explore whether iron oxides can exert their carbon sequestration potential to improve wetland soil carbon sink function and help wetland restoration,this study selected freshwater wetland soils in the Sanjiang Plain Honghe Nature Reserve and saline-alkaline wetland soil in the Songnen Plain Momoge Nature Reserve.The method of laboratory anaerobic incubation experiment was used to explore the three treatments of exogenous hematite addition(Fe),lignite addition(C),and co-addition(FeC)on wetland soil greenhouse gas emissions and sodium pyrophosphate(PP)extracted complex iron-bound carbon(OC_PP),hydrochloric acid-hydroxylamine hydrochloride(HH)extracted amorphous iron-bound carbon(OC_HH)and hydrochloric acid-sodium dithionite(DH)extracted crystalline iron-bound carbon(OC_DH).We analyzed SOC chemical composition variation by pyrolysis-gas chromatography-mass spectrometry,and evaluated the bacterial community structure,diversity,network and function by high-throughput sequencing methods.The main conclusions are as follows:(1)The addition of exogenous Fe and FeC significantly decreased the dissimilatory iron reduction in freshwater wetland soils,and slightly increased in saline-alkaline wetland soils;the addition of exogenous C had no significant effect on the reduction process of dissimilatory iron reduction in soils of two types of wetlands.Fe addition significantly increased the net mineralization of soil organic carbon in freshwater wetlands,but C addition significantly increased the net mineralization of soil organic carbon in saline-alkaline wetlands.(2)Fe addition significantly decreased the OC_PP content in freshwater wetlands and saline-alkaline wetlands,and increased the OC_HH content.The addition of exogenous C and FeC significantly increased the OC_PP content in freshwater wetlands and saline-alkaline wetlands.OC_PP accounts for 50.91%?74.79%of soil organic carbon,indicating that organic-mineral complexes were the main mechanism of soil organic carbon accumulation in freshwater wetlands and saline-alkaline wetlands.(3)The results of the molar ratios of OC_HH/Fe _HH and OC_DH/Fe _DH showed that Fe _HH and Fe _DH protect organic carbon mainly by adsorption or co-precipitation.The OC_PP/Fe _PP molar ratio was between 10-60,indicating that there were other potential mechanisms.(4)The proportions of aromatic,phenol,and aliphatic compounds in SOC of freshwater wetlands and saline-alkaline wetlands were consistent,and the order of their proportions as following:aromatic compounds>phenolic compounds>aliphatic.The influence of FeC addition on the chemical composition of soil organic carbon in freshwater wetlands was dominated by Fe addition,while in saline-alkaline wetlands,C addition was dominated.(5)Fe addition increased the mineralization of soil organic carbon and accelerate greenhouse gas emissions,while C addition increased the effect of iron oxide on soil organic carbon retention(freshwater wetland:5.9±3.6 mg g^-1;saline-alkaline wetland:8.3±3.2 mg g^-1)to promote the accumulation of organic carbon.Therefore,lignite can be used as a wetland soil conditioner to help the wetland restoration.(6)The addition of exogenous Fe and C significantly changed the composition of soil bacterial communities in freshwater wetlands and saline-alkaline wetlands,and significantly increased bacterial diversity and abundance.DOC and Eh were important environmental factors that affect the soil bacterial community structure in freshwater wetlands and saline-alkaline wetlands.C addition increased the complexity of the soil bacterial network in freshwater wetlands;however,the addition of Fe and FeC both resulted in a decrease in the complexity of the network.The three treatments all increased the complexity of the soil bacterial network in saline-alkaline wetland.(7)The addition of exogenous Fe and FeC significantly decreased the ability of freshwater wetland soil to dissimilate iron reduction,and increased the ability of methane oxidation,sulfate respiration,sulfide respiration,fermentation,decomposition of aromatic compounds,and decomposition of hydrocarbons.All treatments have significantly decreased the capacity of methane oxidation in saline-alkaline wetland soil,sulfide respiration,decomposition of aromatic compounds and hydrocarbons.Sulfate respiration of saline-alkaline wetland is significantly higher than that of freshwater wetland,and denitrification of freshwater wetland soil is significantly higher than that of saline-alkaline wetland.