Response of soil nirK denitrifying microorganisms to biochar under different cropping patterns

To analyze the effects of biochar on the community structure and diversity of nirK-type denitrifying microorganisms in soil under different cropping patterns, and then to explore the optimized biochar application strategies, this study collected soil samples from the fields under continuous cropping of soybean and rotation of maize and soybean, three years after one-time application of biochar at 0, 5, 15, and 25 t·hm⁻². The abundance and community composition of soil nirK-type denitrifying microorganisms were analyzed using quantitative real-time PCR and Illumina MiSeq high-throughput sequencing technologies. The results demonstrated that biochar rate significantly increased soil pH, moisture content, total organic nitrogen, total phosphorus, available nitrogen, and available potassium in both cropping systems, while significantly reduced total potassium content. Under rotation condition, high-rate biochar (25 t·hm⁻²) significantly increased the absolute abundance of nirK denitrifying. which was significantly higher than that under continuous cropping. Moreover, the abundance was positively influenced by the combined effects of soil nutrients (total nitrogen, total phosphorus) and available nutrients (available nitrogen, phosphorus and potassium). Moderate biochar application with 15 t t·hm⁻² significantly improved community diversity and richness. The community structures of nirK denitrifying microorganism of biochar addition treatments were significantly different from that of the control treatment, and pH and ammonium nitrogen were the key driving factors revealed by redundancy analysis. Furthermore, the dominant genera of nirK denitrifying microorganisms were Bradyrhizobium which was positively correlated with soil pH, and Sinorhizobium positively correlated with available nitrogen, nitrate nitrogen, and ammonium nitrogen. In summary, biochar increased the diversity of nirK denitrifying microbial communities, changed community structure and composition, and finally would improve the ecological functions of farmland soil by altering soil nutrient content.