Effects of Lathyrus sativus return to the field and nitrogen rate reduction on paddy soil labile organic carbon and soil enzyme activities

In this study, we aimed at exploring how returning Lathyrus sativus green manure to the field with nitrogen fertilizer affects soil improvement as well as determining the appropriate proportion of green manure with nitrogen fertilizer, 2 × 4. We performed two-factor experiments, comprising two nitrogen application 60% (N₁) and 80% (N₂) of conventional and four green manure turnover 15 000 (M₁), 22 500 (M₂), 30 000 (M₃), and 37 500 (M₄) kg·ha⁻¹ rates, respectively. We studied how the ratio of green manure and nitrogen fertilizer affects soil active organic carbon pool components, carbon pool management index, and enzyme activity. Our results demonstrated that compared with conventional fertilization (CF), green manure and a certain amount of nitrogen fertilizer could effectively improve the total organic, active organic, soluble organic, and microbial biomass carbon contents as well as the carbon pool management index of rice fields. Moreover, this improvement effect differed fertilization proportion-dependently, with M₄N₁ and M₄N₂ treatments yielding the best improvement effects. At the same nitrogen application level, the organic carbon component content, carbon pool management index, and overall enzyme activity displayed an increasing trend with a turning over amount increase. Compared with the CF treatment, turning green manure and applying nitrogen fertilizer did not significantly affect soil catalase in the paddy fields, but significantly affected the cellulase and invertase β-Glucomannase activities. The overall enzyme activity showed upon M₄N₂ > M₄N₁ > M₃N₁ > M₃N₂ > M₂N₁ > M₂N₂ > M₁N₁ > M₁N₂ > CF > CK treatment. We observed a significant or extremely significant positive correlation between organic carbon components, except for catalase, sucrase β-Glucomannase, and cellulase were positively correlated with active organic carbon in the soil. We comprehensively analyzed the grey correlation degree of the soil active organic carbon content and soil enzyme activity-influencing factors and determined the best comprehensive evaluation effect when applying the 60% nitrogen fertilizer + 37 500 kg·ha⁻¹ green fertilizer model.