Distribution Characteristics Of Different Soil Carbon Components In Yulin Estuarine Reservoir Wetland

In the semi-arid region of Northwest China,special climatic,hydrological,and geomorphological conditions play a unique role in regulating the storage,transformation,and release of wetland carbon.The vegetation is mostly grass,which limits the distribution of wetland.In order to reveal the distribution characteristics and influencing mechanisms of soil carbon components in the semi-arid region,the content and distribution characteristics of different soil carbon components at 0～40 cm in the Yulin Estuary Reservoir Wetland were studied,the correlation between carbon components and soil physical and chemical factors was analysed,and the structural equation was constructed.The species and abundance of soil microorganisms in representative sampling sites of field experiments were detected by molecular biological techniques,and the relationship between species abundance distribution in dominant communities and environmental factors was analysed.By continuously measuring the concentrations of CO₂,CH₄,andδ¹³C-CO₂ in Hekou Reservoir,the spatial distribution characteristics of these three factors were analysed.In addition,in order to study the effects of plant residue decomposition on soil carbon content,the decomposition characteristics of plant residues were analysed through laboratory simulation experiments,and the trend of soil carbon components during the decomposition process was analysed and discussed.At the same time,the effects of temperature and plant residue quality on decomposition rate were investigated,and a plant residue decomposition model was constructed based on the Double exponential model.The main findings are as follows:（1）The spatial distribution characteristics of soil physical and chemical factors and carbon content were studied in the horizontal and vertical directions of Hekou Reservoir Wetland.In the horizontal direction,soil physical and chemical factors and carbon content have strong spatial heterogeneity.In the vertical direction,total organic carbon（TOC）,total inorganic carbon（TIC）,dissolved organic carbon（DOC）,microbial biomass carbon（MBC）,and light fraction organic carbon（LFOC）in the topsoil were higher than those in the lower soil.According to the condition of water accumulation,the distribution of TOC,TIC,DOC,and LFOC basically belongs to the condition of water accumulation areas with more than no water accumulation areas.However,due to the unique hydrological conditions of the estuarine wetland soil,the DOC content of the lower layer of the soil is higher in the non-water area than in the water area.As the organic carbon of reactive microbial biomass,the distribution of MBC is consistent with a suitable environment（a sufficient carbon source and good aeration）.In different sampling sites,the top ten species of soil microbial relative abundance were basically the same;the dominant bacteria were proteobacteria,firmicutes,bacteroidetes,etc.（2）The correlation between soil physical and chemical factors and carbon components was revealed,and the influence mechanism of soil physical and chemical characteristics on the distribution of different carbon components was explained by constructing a structural equation model.There were significant positive correlations between TOC and water content,total nitrogen（TN）,TIC,DOC,and LFOC,and negative correlations between TOC and bulk density and pH.Tic was positively correlated with water content（TC/TN）,MBC,TN,DOC,and LFOC.DOC was positively correlated with TN,MBC,and LFOC.LFOC was positively correlated with water content,TN,and MBC（P<0.01）and negatively correlated with bulk density（p<0.05）.According to the correlation between soil physical and chemical factors and different soil carbon components,the structural equation model showed that the explanation of soil physical and chemical factors to soil TOC,TIC,DOC,and LFOC were 92%,69%,50%,and63%,respectively.（3）The distribution characteristics of CO₂,CH₄,andδ¹³C-CO₂ in the shallow water of Hekou Reservoir wetland were studied.The concentration of CO₂ ranged from 7.09 to 13.86μmol·L^-1,and the average concentration was 10.1296μmol·L^-1.From upstream to downstream,the CO₂ concentration in surface water decreased first and then increased.The concentration of CH₄ ranged from 0.23 to 1.54μmol·L^-1 and the average concentration was 0.8103μmol·L^-1.Theδ¹³C-CO₂ values ranged from-5.59 to-2.51‰,with an average value of-4.12‰.From upstream to downstream,the trend ofδ¹³C-CO₂ in surface waters increased first and then decreased.（4）The decomposition characteristics of plant residues were investigated,and the effects of temperature and mass of plant residues on decomposition rate were analysed.In the indoor simulation experiment,the mass of plant residue decreases gradually with time,and its decomposition rate becomes slower and faster.Because of the different chemical composition of leaves and stems,their decomposition rates are also different.The relationship between influencing factors and decomposition rate:there was a significant positive correlation between leaf decomposition rate and stem residue mass（p<0.01）,and there was a significant positive correlation between stem decomposition rate and air temperature（p<0.05）.According to the Double exponential model,the decomposition model of plant residues was constructed.The decomposition constants of leaves and stems decreased from 0.021 to 0.005 and from 0.003 to0.001 at 0～80 day and 80～260 day,respectively.（5）The trend of changes in soil carbon components during the decomposition of plant residues was analysed and discussed.During the decomposition of plant residues,the change trend of soil TOC and DOC content was increased rapidly（0～7 days）-decreased rapidly（7～21 days）-decreased steadily（21～260 days）,while the content of TIC increased rapidly（0～12days）,remained stable,and increased slightly（12～260 days）.There was a significant positive correlation between TOC and DOC（p<0.01）,but there was no significant correlation between TIC and TOC and DOC.