Abstract: To explore the effect of Chinese milk vetch (Astragalus sinicus L.) as green manure on the abundance of denitrifying functional genes in paddy soils and their driving factors, we employed real-time quantitative PCR to assess the abundances of nirS, nirK, nosZ Ⅰ, and nosZ Ⅱ genes, along with soil physicochemical properties and potential N₂O emissions. The field trial had eight treatments followed a randomized block design, incorporating three factors: planting green manure, straw return, and chemical nitrogen reduction. Results showed that planting green manure significantly increased soil ammonium content while decreasing soil pH. Neither straw return nor chemical nitrogen reduction had significant effects on the abundance of any denitrifying functional genes. Planting green manure did not affect the abundance of nirS, nirK and nosZ Ⅰ genes, but reduced the abundance of nosZ Ⅱ genes (P<0.01) by 26.6%. The abundance of nirS gene was significantly higher than that of nirK and the abundance of nosZ Ⅰ gene was significantly higher than that of nosZ Ⅱ, indicating that nirS and nosZ Ⅰ were the dominant nitrite-reducing and nitrous oxide-reducing bacteria in paddy soils, respectively. Random forest and correlation analyses revealed that nosZ Ⅱ gene abundance was significantly negatively correlated with soil ammonium content (P<0.001) and positively correlated with soil pH (P<0.01). Moreover, planting green manure significantly increased potential N₂O emissions, while straw return and chemical nitrogen reduction did not affect emissions. Overall, planting green manure increased soil ammonium nitrogen supply, potentially allowing for reduced chemical nitrogen fertilizer application. However, the reduction in nosZ Ⅱ abundance and the increase in potential N₂O emissions due to planting green manure warrant special attention.

Key words: paddy soil, green manure, chemical nitrogen reduction, denitrifying microorganisms, nosZ Ⅱ