Variation Characteristics Of Soil And Aggregate-associated Microbial Residual Carbon Following Vegetation Restoration On Farmland In The Loess Plateau

Since the increase of soil organic carbon sequestration is closely related to the improvement of ecological environment,the soil organic carbon sequestration following vegetation restoration on farmland has become an urgent scientific problem to be recognized in the construction of ecological environment in the Loess Plateau.Soil aggregates were sensitive to land use change and soil organic carbon accumulation was affected by the change of aggregate distribution.Microbial residues is an important component of soil organic carbon.The quantity and composition of microbial residues are closely related to the accumulation of soil organic carbon.However,the response of microbial residues in bulk soils and aggregates to vegetation restoration is not clear,which hinders our understanding of the role of microorganisms in soil organic carbon accumulation.In this study,we collected 0-10 and 10-20 cm soil samples from farmland,woodland and grassland along climate and texture gradients in the Loess Plateau.We determined the proportion of < 0.25,0.25-1,1-2 and > 2mm aggregates by dry-sieving method,simultaneously determined organic carbon and amino sugars in bulk soils and aggregates.The content of microbial residual carbon was calculated with amino sugars.The change characteristics of microbial carbon and its contribution to organic carbon in soil and aggregates were studied by using multivariate ANOVA.Linear regression analysis was used to explore the driving factors of soil microbial carbon content and contribution of microbial carbon to organic carbon following woodland and grassland restoration.The main results are as follows:(1)Woodland and grassland restoration significantly increased the contents of soil organic carbon and microbial residual carbon,the ratio of fungi to bacterial residual carbon and the contribution of fungal residual carbon to organic carbon,but significantly decreased the contribution of soil bacterial residual carbon to organic carbon.(2)The content of bacterial residual carbon increased in the north and decreased in the south following woodland restoration.Grassland restoration increased the content of bacterial residual carbon in the north and south,and the increase in the north was higher than that in the south.After woodland and grassland restoration,the contribution of soil bacterial residual carbon to organic carbon remained unchanged in the north and decreased in the south.(3)After woodland restoration,the average contents of soil organic carbon,bacterial,fungal and microbial residual carbon changed by 1.64-8.54,-0.58-0.54,0.62-2.99 and 0.72-3.25 g kg-1,respectively.The average contribution of soil bacterial and fungal residual carbon to organic carbon changed by-18.19-1.93 and-1.47-9.12 %,respectively.After grassland restoration,the average contents of soil organic carbon,bacterial,fungal and microbial residual carbon changed by 0.94-11.30,-0.33-0.59,0.47-4.83 and 0.13-5.31 g kg-1,respectively,and the average contribution of soil bacterial and fungal residual carbon to organic carbon changed by-10.10-3.47 and-1.38-10.23 %,respectively.(4)Soil microbial residual carbon content and its contribution to organic carbon are affected by soil texture,climate,soil organic carbon content,soil total nitrogen content,and soil aggregates.(5)Woodland and grassland restoration significantly increased the contents of aggregateassociated organic carbon,fungal and microbial residual carbon,increased the contribution of fungal residual carbon to organic carbon,but significantly decreased the contribution of bacterial residual carbon to organic carbon.Woodland restoration significantly increased The content of bacterial residual carbon in < 0.25 mm and 0.25-1 mm aggregates,and grassland restoration significantly increased the content of bacterial residual carbon in < 0.25 mm,0.25-1 and > 2 mm aggregates.(6)After woodland and grassland restoration,the changes of fungal and microbial residual carbon content in < 0.25 mm aggregates,bacterial residual carbon content,and contributions of bacterial and fungal residual carbon to organic carbon in each aggregates showed significant zonal characteristics,and the zonal characteristics were consistent with those of bulk soil.(7)The contents of bacterial residual carbon and contribution of bacterial residual carbon to organic carbon in 0.25-1 mm and > 2 mm aggregates were significantly lower in woodland than in grassland,while the contents of organic carbon and total nitrogen,fungal and microbial residual carbon in 0.25-1 and 1-2 mm aggregates were significantly higher in woodand than in grassland.In conclusion,this study emphasizes the important role of microbial residual carbon in soil organic carbon accumulation after farmland conversion,and further deepens our understanding of soil organic carbon sequestration after farmland conversion.This study indicates that after farmland conversion soil and aggregate microbial residual carbon content change significant,bacterial and fungal residual carbon contribution to organic carbon changes significant,the increase of microbial residual carbon content is an important way for soil organic carbon and total nitrogen accumulation,the increase of the contribution of fungal residual carbon to organic carbon is beneficial to the accumulation of soil organic carbon and total nitrogen content,The increase of microbial residual carbon content was an important reason for the enhancement of soil agglomeration.In addition,the changes of microbial residual carbon content and the contribution of bacterial and fungal residual carbon to organic carbon in soil and aggregates showed significant zonal characteristics.In addition,soil microbial residual carbon content,the contribution bacterial and fungal residual carbon to organic carbon were affected by climate and soil texture,soil organic carbon and total nitrogen content,and physical protection of aggregates.