Study On The Promotional Effects And Driving Factors Of Exogenous Organic Carbon Substrates On The Formation Of Macroaggregates In Coastal Saline-alkali Soil

Coastal saline-alkali land is an important reserve cultivated land resource.However,special soil physicochemical properties,such as high salilinity,high pH,low carbon,and poor aggregation status,which seriously limit the agricultural sustainable development of coastal saline alkali soil.Existing studies have shown that the application of exogenous organic carbon can improve soil organic carbon retention in coastal saline-alkali land,facilitate the formation of large aggregates(>0.25mm),and further promote soil salinity and pH reduction,which is an important agricultural practice in the amelioration of coastal saline-alkali land.However,underlying mechanism of the promotional effects and driving factors of exogenous organic carbon substrates on the formation and stabilization of soil aggregates in coastal saline-alkali land still needs to be further studied and discussed.In current study,typical coastal salinealkali soils were selected as the research object,and influences of different exogenous organic carbon(types:sewage sludge,sludge vermicompost,vinegar residue;application amount:0,25,50,100,200 t ha-1)on soil macroaggregates(0.25-2,>2 mm),soil physicochemical properties,enzymatic activities,compositional and structural diversities of microbial communities of coastal saline-alkali land were studied.Furthermore,driving factors of exogenous organic carbon substrates on the formation of macroaggregates in coastal salinealkali land were discussed.Main results are as follows:(1)Compared with the soil treated without organic carbon substrates,proportions of aggregates with size lager than 2 mm was higher than those of aggregates with size of 0.25-2 mm after the application of domestic sludge,earthworm manure and vinegar residue.Moreover,weight mean diameter(MWD),and geometric mean diameter(GMD)were significantly higher in exogenous organic carbon substrates amended soils as compared to those in the control group.The application of exogenous organic carbon improved the abundances of active(eg.,alcohols,ethers,carboxyl groups,hydroxyl groups and methyl groups)and stable(eg.,aromatic and olefin groups)functional groups in those macroaggregates in the proportion and stability of macroaggregates increased with the increase of the application amount of exogenous organic carbon.Particularly,the most obvious promotional effect of exogenous organic carbon on the formation of large aggregates as well as the abundances of functional groups were observed in soils amended by ameliorants at the application rate of 200 t ha-1 as compared to other treatments.(2)Compared with the soil treated without organic carbon,the pH,EC and salt content of soil macroaggregates decreased significantly when domestic sludge,earthworm manure and vinegar residue were applied.The nutrient content and enzyme activity of large aggregates were also significantly increased.After applying domestic sludge,earthworm droppings and vinegar residue,the pH of 0.25-2 mm aggregates was lower than that of>2 mm aggregates.The salt content of>2 mm aggregates was lower than that of 0.25-2 mm aggregates.The contents of nutrient and enzyme activities in 0.25-2 mm aggregates of domestic sludge and earthworm manure were significantly higher than those in>2 mm aggregates.With the increase of the application amount of exogenous organic carbon,the pH,EC and salt content of large aggregates showed a decreasing trend,while the nutrient content and enzyme activity showed an increasing trend.When the dosage was 200 t ha-1,it had the most obvious effect on reducing salt and pH,increasing nutrient content and enzyme activity of large aggregates.(3)Compared with the soil treated without organic carbon,the fungal community composition diversity(Shannon index)in aggregates of 0.25-2 mm was decreased by the application of domestic sludge,earthworm manure and vinegar residue.The composition of microbial community in large aggregates changed.The Shannon index of the fungi community was lower than that of the control group when domestic sludge was applied,and that of the fungi community in>2 mm aggregates was higher than that in 0.25-2 mm aggregates.The Shannon index in the macroaggregates of fungi and bacteria in all treatment groups was lower than that in the control group.Application of vinegar residue decreased the Shannon index of fungal community in 0.25-2 mm aggregates.Among treatment groups with different application amounts of organic carbon:With the increase of application amounts,the influence of Shannon index of bacteria and fungi showed a trend of first decreasing and then increasing.The Shannon index of bacteria at 200 t ha-1 was significantly higher than that in the control group.The Shannon index of bacteria and fungi was lower than that of the control group.(4)The Structural Equation Model(SEM)results showed that the organic carbon,total nitrogen and the diversity of fine/fungal community structure in the coastal saline-alkali soil significantly promoted the formation of soil macroaggregates after the application of exogenous organic carbon,and pH,EC and salt content were negatively correlated with the formation of macroaggregates.In conclusion,organic carbon,total nitrogen,FDA enzyme,urease,bacterial community structure diversity and fungal community structure diversity were the main factors promoting the formation of soil macroaggregates after application of different exogenous organic carbon.