Effects of combined application of fertilizers on nirK- and nirS-type denitrifying bacteria and denitrification potential in rapeseed rhizosphere soil

To investigate the regulatory mechanism of fertilization on denitrifying bacterial communities and activity in the rhizosphere of rape grown in subtropical red soil regions, this study established five treatments under equivalent nutrient principles: sole chemical fertilizer (F0), chemical to organic fertilizer ratio 3:1 (F1), 1:1 (F2), 1:3 (F3), and sole organic fertilizer (F4). This study found soil denitrification potential decreased initially and then increased significantly with higher organic fertilizer proportion, reaching the lowest at F2 treatment 21.75 ng/(g·h) and highest at F4 83.43 ng/(g·h); notably, nirK- and nirS-type denitrifier gene abundance was at its minimum in F1 treatment, with nirK at 2.69×10⁷ copies/g and nirS at 3.75×10⁶ copies/g. The abundances of both nirK- and nirS-type denitrifier genes peaked under the F3 treatment. Soil denitrification potential showed a highly significant positive correlation with total carbon (P< 0.001) but a significant negative correlation with ammonium nitrogen content (P< 0.01).denitrification potential was not significantly associated with the overall diversity of both bacterial types but correlated significantly with 3 key OTUs of nirK-type and 6 key OTUs of nirS-type denitrifiers, with nirS-type bacteria harbouring more critical functional groups. In conclusion, different fertilization regimes affect denitrification potential by reshaping the rhizosphere carbon-nitrogen microenvironment, and the relationship between nirS-type denitrifying bacteria and denitrification potential is closer than that of nirK-type, which provides a microbiological basis for precise nitrogen management in rapeseed fields in red soil regions. Practically, a fertilization regime of 25% organic fertilizer combined with 75% chemical fertilizer is recommended in agricultural practice to balance soil fertility improvement and nitrogen emission reduction.