Intrinsic soil properties and microbial controls on denitrification rates across contrasting agricultural soils

Denitrification is a key microbial process in nitrogen cycling and a major source of N₂O emissions from agricultural soils. In this study, we investigated denitrification rates and their intrinsic controls across four contrasting agricultural soils from China (black, yellow loessal, purple, and red soils) using anaerobic microcosm incubations with the acetylene inhibition method. Denitrification rate constants were derived from a pseudo-first-order kinetic model. Microbial community composition and the abundance of denitrification functional genes (narG, napA, nirK, nirS, norB, and nosZ) were characterized using 16S rRNA gene sequencing and quantitative polymerase chain reaction (qPCR), respectively. Denitrification rate constants differed significantly among soils, decreasing from 12.0 ± 1.3 to 0.1 ± 0.1 d^-1 in the order black > yellow loessal > purple > red. Soil type explained 92.1% of the variation in bacterial community composition (PERMANOVA, p < 0.001), demonstrating pronounced soil-specific microbial assemblages. Soil physicochemical properties, especially pH, soil organic carbon (SOC), and cation exchange capacity (CEC), were strongly associated with microbial community structure and denitrification potential. Microbial diversity correlated positively with denitrification rates, and norB abundance closely tracked differences in denitrification activity across soils. However, functional gene abundance did not always correspond to the microbial structure or denitrification rates, as illustrated by the high abundance but non-significant relationship of narG with denitrification rate constants. Together, these results highlight the integrated roles of intrinsic soil properties, microbial community characteristics, and functional potential in explaining denitrification variability among soils. By providing a quantitative baseline, this study offers a reference framework for future investigations assessing how external environmental disturbances may alter soil nitrogen cycling.