Responses of soil microbial biomass nitrogen to organic fertilizer with different degrees of maturity and regu-lation to soil mineral nitrogen.

By combining the composting process with soil culture experiment, we conducted an experiment with four treatments, including conventional chemical fertilizer (CK), chemical fertilizer + compost maturity reaching 50% germination index (GI, the same below) organic fertilizer (CO1), chemical fertilizer + compost maturity reaching 80% GI organic fertilizer (CO2), chemical fertilizer + compost maturity reaching 100% GI organic ferti-lizer (CO3). We measured soil microbial biomass nitrogen (MBN), mineral nitrogen (NH-N, NO-N), net nitrification rate, microbial biomass carbon (MBC), dissolved organic carbon (DOC), soil urease and soil protease, aiming to reveal the regulatory effect of soil MBN on mineral nitrogen. The results showed that organic fertilizer application significantly increased MBN and NH-N concentrations by 50.1%-62.4% and 109.9%-147.1%, reduced NO-N concentration and net nitrification rate by 23.3%-46.8%, and 26.2%-51.5%, and enhanced MBC, DOC, urease and protease activities by 33.8%-69.6%, 7.4%-20.8%, 11.2%-69.0% and 9.4%-25.1%, respectively. The change ranges of CO2 and CO3 were significantly higher than CO1. Redundancy analysis (RDA) and structural equation model (SEM) results showed that the application of organic fertilizer with higher degree of maturity (GI≥80%) positively regulated soil MBC, MBN, NH-N, and the activities of urease and protease, but had a negative effect on soil net nitrification rate. The combined application of chemical fertilizers and high decomposed organic fertilizers could significantly increase soil MBN and NH-N contents, as well as soil urease and protease activities, but reduce soil net nitrification rate. To efficiently utilize organic solid wastes, it is recommended to use chemical fertilizer in combination of organic fertilizers with 80% decomposing degree in practical production to reduce the cost in both economy and time.