The effect of upland crop planting on field N2O emission from rice-growing seasons: A case study comparing rice-wheat and rice-rapeseed rotations

Paddy fields, especially under flooded rice-upland crop rotation modes, undergo tremendous changes in soil redox conditions. The effects of different upland crop plantations on field N2O emissions from the rice-planting period and the underlying mechanisms are scarce. In this study, two types of flooded rice-upland cropping rotation modes, namely, rice-rapeseed rotation (RR) and rice-wheat rotation (RW), were selected to investigate the effects of different upland crop plantations on field N2O emissions during the rice-growing period, in central China. To confirm the effect of upland crop plantations on soil variables and associated functional genes, conventional rice planting (RR-CP and RW-CP) with a part of the non-rice area (RR-NP and RW-NP) during the rice season was implemented. Results revealed that seasonal N2O emissions from RR-NP and RW-NP treatments were 1.83 ± 0.18 and 1.43 ± 0.25, and 1.41 ± 0.09 and 0.88 ± 0.08 kg N ha−1, respectively, during two consecutive rice-plantation seasons, which were significantly higher than those from RR-CP treatment (1.38 ± 0.16 and 0.91 ± 0.15 kg N ha−1) and RW-CP treatment (0.95 ± 0.08 and 0.50 ± 0.10 kg N ha−1). Higher N2O emissions from RR rotation than from RW rotation, regardless of rice planting, indicated that upland cultivation affected soil N2O efflux. For rice planting treatments, strong positive relationships between N2O fluxes and soil-dissolved organic carbon (DOC), ammonium (NH4+), nitrate (NO3-), and functional genes (nirS, nirK, and nosZ genes) were observed, implying that soil available C and N, and related functional genes are key regulatory factors controlling N2O emissions. Compared with the RW-CP treatment, the higher abundance of the nirK gene and lower nosZ/(nirS + nirK) ratio from the RR-CP treatment may facilitate greater N2O production, while reducing conversion of N2O to N2, resulting in increased N2O emissions. Furthermore, structural equation model (SEM) showed that field flood depth (FD), soil available C and N, and the abundance of nirS gene, together, displayed more than 70 % impact effect on N2O emission for RR-CP treatment, while, FD, soil available N, and nirS and nirK genes, together, displayed more than 60 % impact effect on N2O emission for RW-CP treatment. Our results suggest that suitable upland crop rice cultivation could be beneficial for mitigating soil N2O emissions.