Internal transformation of fertilizer-derived organic nitrogen fractions in an arable soil and the regulation of maize residue mulching

• Maize residue mulching enhanced fertilizer N retention in soil by 9.2–10.9%. • Hydrolysable SON fractions circulate internally to perform complementary functions. • Fertilizer derived hydrolysable ammonium N and unknown N act as fast turnover pools. • Microbial fractions contribute to long-term stabilization of fertilizer N. • Maize residue enhanced specific functions of SON fractions via internal cycling. Soil organic nitrogen (SON) is a major residual fertilizer nitrogen (N) reservoir in soil and affects soil N retention and supply capacity. However, owing to the diverse origins and compositions of SON fractions, the internal transformation of residual fertilizer N associated with specific SON fractions and the influence of maize residue mulching remain unclear. Thus, via the 15 N-labelling technique and SON fraction analysis, we tracked fertilizer N allocation into SON fractions over a 9-year trial, with and without maize residue mulching. Fertilizer N was initially retained more in active hydrolysable N (HN) fractions (24.7–26.6 % of applied N) than in acid insoluble N (NHN) fractions (1.2–1.9 % of applied N), and decreased more rapidly in HN by the 9 th year (5.5–8.7 % of applied N). Therefore, the HN served as a reservoir with residual effects of fertilizer N and both the HN and NHN fractions contributed to long-term N retention in the soil. Among the HN fractions, the high enrichment and rapid release of fertilizer N in hydrolysable ammonium N (HAN) and hydrolysable unknown N (HUN) indicated that HAN and HUN were the two fast turnover pools. However, microbial fractions, such as amino sugar N (ASN) and amino acid N (AAN), were crucial for fertilizer N stabilization. Our study revealed internal cycling among the different hydrolysable SON fractions, and hydrolysable SON fractions primarily achieved functional complementarity through HUN. Maize residue mulching could enhance the specific functions of SON fractions (ASN, AAN, HAN, HUN and NHN) from the perspective of long-term fertilizer N transformation, which eventually improved fertilizer N retention in the soil (from 6.9 % to 9.2–10.9 % of applied fertilizer N). This investigation provides an innovative perspective on N nutrient management through fertilizer N transformation in SON fractions and the response to maize residue mulching.