Symbiotic nitrogen fixation enhanced crop production and mitigated nitrous oxide emissions from paddy crops

Feeding a growing population and preventing dangerous climate change are two challenges facing humanity. Biological nitrogen (N) fixation is considered a sustainable production measure that helps maintain crop production and mitigate climate change by supplementing alternative N nutrients. However, the simultaneous impacts of symbiotic N fixation (SNF) by leguminous green manure crops on rice crop production and nitrous oxide (N2O) emissions remain poorly understood. Here, a field experiment of two annual rotation cycles was conducted to monitor rice yield and the N2O emissions under rice (Oryza sativa subsp. japonica Kato.)-wheat (Triticum aestivum L.) (RW) and rice-hairy vetch (Vicia villosa Roth.) (RV) cropping systems at three N input rates (0 (N0), 50 (N1) and 100 (N2) kg ha-1) for the upland crops and 200 kg ha-1 for the rice crop. Growing hairy vetch introduced a significant amount of N as SNF to the agroecosystem, ranging from 123 to 182 kg ha-1. Compared to the treatments without N for the RV system, the N1 and N2 treatments significantly increased the amount of SNF by 44-47% and 19%- 24%, respectively. The cumulative N2O emissions from SNF ranged from 0.28-0.65 kg ha-1, with the N2O emission factor from SNF (EFSNF) ranging from 0.18%- 0.52% for the RV system. The RV system significantly decreased the N2O emission factor from the total N input (EFTN) by an average of 10.0% compared with the RW system. The RV-N1 treatment produced an obviously lower EFTN, averaging 0.37%, than the other treatments receiving N applications. Compared to the RW system, the RV system slightly increased rice yield by an average of 3.5%. Therefore, the most appropriate N application rate was 50 kg ha-1 for the leguminous crop, and SNF of leguminous green manure could serve as complementary to mineral N fertilizers for reducing N2O emissions while improving soil quality and rice yield in paddy field.