Changes of soil properties and carbon fractions after long-term application of organic amendments in Mollisols

Land degradation threatens soil fertility as well as environmental security. To assess the extent of degradation of black Mollisols in northeast China, chemical and microbial indicators of soil quality were determined after long-term application of organic amendments combined with chemical fertilizers. The application of high amounts of organic manure plus chemical fertilizer (NPK) significantly increased soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), and mineral nitrogen (Nmin), as well as microbial biomass carbon (MBC), extractable dissolved organic carbon (DOC), microbial quotient (qMIC), carbon (C) in free light fraction (fLF) and its percentage of total soil C. Soil C/N ratio and C portion in heavy fraction (HF) both decreased significantly, which might have resulted from accelerated mineralization of stable C. Application of straw at a low rate plus NPK significantly increased Nmin and MBC concentrations and qMIC. Compared with the unamended control, no negligible differences were found for most of the selected parameters under the treatments NPK alone, high-rate straw plus NPK, and low-rate manure plus NPK. These results indicated that NPK along with either a high rate of manure or a low rate of straw profoundly changed certain key soil chemical and microbial properties. Despite these apparent benefits in soil properties, however, high rates of manure application is likely contributing little to soil C stabilization. Low rates of organic amendments, especially crop straw, are therefore recommended, as this practice can improve or at least maintain soil properties in degraded Mollisols, while minimizing the adverse impacts of more intense applications. •Low rates of straw plus NPK increased Nmin and MBC concentrations and qMIC.•High amounts of organic manure plus NPK increased most of the selected parameters.•High rates of manure application accelerated carbon mineralization.•Low rates of organic amendments are recommended to improve soil quality.