Straw return and low N addition modify the partitioning of dissimilatory nitrate reduction by increasing conversion to ammonium in paddy fields

Abstract

Dissimilatory nitrate reduction to ammonium (DNRA) contributes to nitrogen (N) conservation, whereas denitrification and anaerobic ammonium oxidation (anammox) lead to N loss in agroecosystems. However, their relative roles in nitrate (NO3−) partitioning for total dissimilatory NO3− reduction and the effects of long-term straw retention on such mechanisms remain largely unclear. In this study, after implementing straw return and N addition for seven consecutive years in a paddy field, we used 15N tracing combined with molecular biological techniques to investigate the rates of denitrification, anammox, DNRA, and their associated abundance of nosZ, hszB, and nrfA genes, respectively, as well as the characteristics of the DNRA community. The results showed that the rates of denitrification, anammox, and DNRA varied from 20.7 to 33.3, 0.12–0.34, and 0.88–3.20 nmol N g−1 h−1, respectively. Straw return significantly increased denitrification, anammox, DNRA activities, and the abundance of associated genes, except for the anammox hszB gene. With increasing N input rate, the denitrification rate increased (P < 0.05), the DNRA rate decreased (P < 0.05), whereas the abundance of the three functional genes did not change significantly. Compared with chronic high N addition under straw retention, low N amendment increased the DNRA-based NO3− partitioning by 61–111%, although denitrification dominated the dissimilatory NO3− reduction process. The significant increase in soil DOC:NO3− ratio (R2 = 0.89), and the reduction of soil pH (R2 = 0.87) and standing water NO3− concentration (R2 = 0.70), promoted N conservation through DNRA. The core subset of DNRA communities (R2 = 0.99), belonging to Bacteroidetes, Proteobacteria, Euryarchaeota, and Firmicutes phyla, were responsible for DNRA activities. This is the first study to examine all three dissimilatory NO3− reduction processes in response to long-term straw return and N addition. We propose that straw return with low N addition can largely favor DNRA partitioning among the three dissimilatory NO3− reduction processes through biotic and abiotic regulators.