Effects Of Substrate Quality On Organic Carbon Mineralization,microbial Community Composition And Functional Genes In Coastal Saline Soil

Soil respiration is the second largest carbon flux in the carbon cycle of terrestrial ecosystems,accounting for 60%-90%of the total carbon emissions of terrestrial ecosystems.Small changes in the process of soil organic carbon mineralization have a profound impact on environmental changes.Microorganisms are the direct participants in the process of soil material circulation and transformation,and their community composition and metabolic activity have a direct impact on soil organic carbon mineralization.The quality of exogenous substrate is considered to be an important factor affecting soil organic carbon mineralization,but the microbial participation strategy is still unclear.Coastal saline soil has a high carbon sequestration rate and a large carbon sequestration potential,and plays an important ecological mission under the background of double carbon.However,the analysis of the mechanism of organic matter cycle and transformation in saline soil is still in its infancy,and the research data are still lacking.In this study,six treatments of straw and glucose were added to three different degrees of saline soil（mild saline soil with glucose GL,moderate saline soil with glucose GM,severe saline soil with glucose GH,mild saline soil with straw SL,moderate saline soil with straw SM,severe saline soil with straw SH）.The effects of substrate quality on soil organic carbon mineralization,extracellular enzyme activity,microbial community and functional genes were explored,and the microbial strategies of substrate quality affecting organic carbon transformation in saline soil were discussed.The main results are as follows:（1）The contents of total nitrogen and organic carbon in the soil with straw addition were higher than those in the undisturbed soil at the end of culture,but the increase of total nitrogen and organic carbon content by adding straw substrate at the end of culture was greater than that by adding glucose.The organic carbon and total nitrogen of coastal saline soil added with straw increased by 2.24%and 23.36%respectively.（2）According to the correlation coefficient diagram,the soil extracellular enzyme activities added to the two substrates were negatively correlated with electrical conductivity.With the culture of soil extracellular enzyme activity decreased gradually.The addition of glucose had a great influence on the enzyme activity in the early stage of culture,but the addition of straw relatively reduced the effect of conductivity on some enzyme activities in the later stage of culture.（3）The cumulative release of CO₂ in glucose treatment was greater than that in straw treatment.According to the first-order kinetic equation,the potential mineralized carbon content of different saline soils under the addition of two substrates showed severe saline soil>mild saline soil>moderate saline soil.The potential mineralizable carbon contents of GL,GM and GH treatments with glucose addition were 1996.76 mg C kg^-1,1873.82 mg C kg^-1 and2592.19 mg C kg^-1,respectively.The potential mineralizable carbon contents of SL,SM and SH treatments with straw addition were 2745.50 mg C kg^-1,2636.80 mg C kg^-1 and 3218.41mg C kg^-1,respectively.（4）Metagenomic sequencing was performed on the 35th day of culture.It was observed that the abundance of microbial genera in severe saline soil was significantly higher than that in mild and moderate saline soil under the condition of substrate addition.Compared with the addition of glucose,the diversity of microbial genera in the addition of straw was higher.The addition of straw significantly increased the relative abundance of Proteobacteria,and glucose substrates were preferred by most microorganisms.Proteobacteria is involved in various processes of carbohydrate enzymes involved in the carbon cycle.Due to the relative abundance of microbial communities in the treatment of adding straw in severe saline soil is relatively large,and the respiration rate is higher than that of the other six groups on the 35th day,so the soil respiration rate and microbial abundance showed a positive correlation.（5）On the 35th day of cultivation,the relative abundance of carbohydrate enzymes was higher in severely saline soil（GH,SH）,and the six groups of related carbohydrate enzyme modules were positively correlated with electrical conductivity,with glycoside hydrolase having the highest correlation with electrical conductivity.Therefore,the relative abundance of carbohydrate enzymes is greatly affected by the degree of salinization compared to the addition of different types of substrates.