Changes in N supply pathways under different long-term fertilization regimes in Northeast China

Abstract

Different long-term fertilization regimes probably influence N supplying capacity and N cycling processes in soils. However, the effects of fertilization regimes on the pathways of N retention and supply and the impacts of these pathways on crop N uptake remain undocumented. In this study, the contributions of different pathways to N retention and supply were investigated on the basis of a long-term experiment that began in 1990, including six treatments: no fertilizer (CK), recycled manure (M), N and P fertilizers (NP), P and K fertilizers (PK), N, P and K fertilizers (NPK), and NPK fertilizers with recycled manure (NPKM). In the maize growth season, soil microbial mineralization–immobilization turnover (MIT) was the primary driving force of N transformation under treatments without synthetic N. Conversely, NH4+ fixation and release by soil clay minerals prominently regulate N retention and supply under treatments with synthetic N. In general, soil microbial biomass N (SMBN) mineralization and fixed NH4+ release accounted for 51 % of the variation in crop N uptake and the contribution of the latter to crop N uptake was 2-fold greater than that of the former. The synchrony between crop N uptake and fixed NH4+release was higher than that between crop N uptake and SMBN mineralization. Therefore, the fixed NH4+ pool was important in synthetic N regulation at a short or medium temporal scale, especially for soils with high 2:1 clay mineral content. By contrast, MIT dominated N cycling at a long temporal scale. Clarification of the pathways of N retention and supply can provide helpful implications for optimizing N fertilizer management and enhancing the synchrony between crop N demand and soil N supply.