Rice planting reduced N2O emissions from rice-growing seasons due to increased nosZ gene abundance under a rice-wheat rotation system

Abstract

Paddy fields experience more frequent events because of redox-induced changes during their growth period, which further govern the processes that influence soil N2O emissions. However, studies on the effects of rice planting on soil N2O emissions and the relevant mechanisms are scarce. Thus, a field employing rice-wheat rotation (RW) in central China, including treatments with rice planting (RW-CC) and without rice planting (RW-NC), was selected to investigate the effects of rice planting on N2O emissions. Several key soil variables and functional genes (nirS, nirK, and nosZ) related to N2O-production and consumption were selected to determine the mechanisms underlying the effects of rice planting on N2O emissions. The results showed that seasonal N2O emissions from RW-NC treatment were 0.88 ± 0.08 and 1.69 ± 0.24 kg N ha−1 for the first and subsequent rice cultivation seasons, respectively, and were significantly higher than those from RW-CC treatment (0.51 ± 0.10 and 1.04 ± 0.18 kg N ha−1, respectively). This indicated the mitigation effect of rice planting on field N2O emissions. For both the RW-CC and RW-NC treatments, N2O fluxes were positively correlated with soil ammonium (NH4+), microbial biomass carbon (MBC), and nirK genes, and negatively correlated with the nosZ gene, indicating that the production- and consumption-related functional genes of soil available C, N, and N2O are the key factors controlling N2O emissions. Compared to RW-NC treatment, a higher abundance of the nosZ gene and lower (nirS+nirK)/nosZ ratios for RW-CC treatment may contribute to a greater reduction of N2O to N2, thus resulting in decreased N2O emissions. Structural equation model (SEM) analysis showed that soil moisture (flood depth), NH4+, MBC, and functional genes together accounted for more than 85 % of the explanatory effects on N2O emissions for both treatments. In general, our research indicates that promoting rice planting could reduce soil N2O emissions during the rice-growing season, thereby reducing the exposure of soil to N fertilization when adopting field agronomic practices; our results also suggest that crop rotation and intercropping should be encouraged.