Effects of crop stubbles in different cropping systems on the number and community structure of denitrifying bacteria in black soil farmland

Objectives Denitrification is an important process in soil nitrogen loss and greenhouse gas emissions. We studied the effects of crop stubbles in different cropping systems on soil denitrifying bacterial community structures, aiming to reveal the related mechanisms of crop stubbles affecting N₂O emission.

Methods The experiment was conducted in Hailun City, Heilongjiang Province (47°23′N, 126°51′E). Continuous cropping of corn, soybean, and corn–soybean rotation was set up in the experiment. At sampling, the corn was three years, and the soybean was two years old. Topsoil (0–15 cm) samples were collected after harvesting maize and soybean. The abundance and community composition of the soil nirS and nirK denitrification bacteria were analyzed using real-time quantitative PCR (qPCR) and high-throughput sequencing techniques.

Results The continuous corn (CC) plot soil had the highest denitrification rate. Rotation corn (CSC) and rotation soybean (SSC) had higher soil denitrification rate (P<0.05) than continuous soybean (SS). The abundance of nirS and nirK denitrifying bacterial genes in CSC and SSC was higher than that in SS, and there was no significant difference between CSC and CC (P>0.05). PCoA results showed variation in nirS denitrifying bacterial communities in SSC and CSC, while CC and SS had different nirK bacterial communities. RDA analysis showed that NO₃^–-N content and C/N were the main factors regulating the nirS and nirK denitrifying bacterial communities, respectively. SEM revealed that nirS denitrifying bacterial communities were positively (R²=0.92) correlated with the denitrification rate (P<0.05) , but the gene abundance of nirS and nirK did not correlate with the denitrification rate.

Conclusions The abundance and community composition of denitrifying bacteria vary among the farmlands under different crops. Denitrifying bacterial community composition, rather than their abundance, plays essential roles in determining the denitrification rate, while nirS denitrifying bacteria contributes more to soil denitrification.