Cattle manure and straw have contrasting effects on organic nitrogen mineralization pathways in a subtropical paddy soil

Abstract

Organic materials are widely recommended for the maintenance and/or accumulation of organic carbon and total nitrogen in agricultural soils. However, the relative effectiveness of different organic materials on gross N transformation and inorganic-N supply is not known. Here, a 15N tracing incubation study was conducted to investigate the rates of gross N mineralization, nitrification, and microbial immobilization of and in a paddy soil managed using different organic materials. Soil samples were collected from a rice field that has been under long-term study (30 years) and is receiving four different fertilizer treatments: no fertilizer (CK), chemical fertilizer (NPK), chemical fertilizer plus cattle manure (NPKM), and chemical fertilizer plus straw (NPKS). The samples were incubated with 15NH4NO3 or NH415NO3, and the results were calculated based on a 15N tracing model. The analysis showed that mineralization of labile and recalcitrant organic-N pools was significantly stimulated by NPKM and NPKS treatments, respectively, but not by NPK alone. Heterotrophic nitrification was negligible in all treatments. Therefore, the enhanced inorganic-N supply rates (total mineralization + heterotrophic nitrification) due to NPKM and NPKS application could be attributed to the increasing mineralization of the labile organic-N pool and recalcitrant organic-N pool, respectively. The rate of autotrophic nitrification rate was significantly increased by NPK and NPKM application, but not by NPKS application. However, both consumption rates (immobilization of and dissimilatory reduction to ) were low and unaffected by fertilizer application. Our results suggest that NPKM stimulated the processes of mineralization of the labile organic-N pool and autotrophic nitrification to increase accumulation in soil. In contrast, application of NPKS increased the rate of mineralization of recalcitrant organic-N pool but did not affect the autotrophic nitrification rate.