Dissimilatory nitrate reduction to ammonium dominates nitrate reduction in long-term low nitrogen fertilized rice paddies

Abstract

Abstract Dissimilatory nitrate reduction to ammonium (DNRA) and diazotrophic N2 fixation contribute to nitrogen (N) supply in rice paddies, whereas denitrification contributes to N loss. Continuous N fertilization in rice paddies is known to increase denitrification and reduce N2 fixation, however little is known about its effect on DNRA and the NO3− partitioning between DNRA and denitrification. Here, we investigated the rates of DNRA, denitrification and N2 fixation, and their relevant microbial gene abundances, in long-term high and low N fertilized rice paddies using a 15NO3− tracer, an acetylene reduction assay and quantitative PCR analysis, in laboratory incubation studies. We observed that DNRA exceeded denitrification by a factor of eight in low N fertilized rice paddies, while DNRA was almost half of the denitrification rate in high N fertilized rice paddies. The nrfA gene abundance, related to DNRA, was significantly higher in the low N fertilized rice paddies and was positively correlated with DNRA rates. However, no clear difference in denitrifying gene (narG, nirK and nosZ) abundances was observed between the N fertilization regimes. The proportion of total NO3− reduced by DNRA had a significantly positive correlation with the soil organic carbon-to-NO3- ratio and negative correlation with the soil NO3− concentration. N2 fixation added ten times more N in the low N input than in the high N input paddies. Our findings highlight the self-regulated microbial N cycling in low N input paddy systems which maintain long-term paddy soil N nutrition.