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

This study analyzed the effects of biochar on the community structure and diversity of nirK-type denitrifying microorganisms in soil under different cropping patterns, and explored the optimized biochar application strategies. Soil samples were collected from the fields under continuous cropping of soybean and rotation of maize and soybean, three years after a single 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. The results demonstrated that the biochar application significantly increased soil pH, and contents of moisture, total organic nitrogen, total phosphorus, alkali-hydrolyzable nitrogen, and available potassium in both cropping systems, while significantly reduced the total potassium content. Under rotation conditions, high-rate biochar (25 t·hm⁻²) application significantly increased the absolute abundance of nirK-type denitrifying microbes. This value was significantly higher than that obtained under continuous cropping. Moreover, abundance was positively influenced by the combined effects of soil nutrients (total nitrogen and total phosphorus) and available nutrients (available nitrogen, phosphorus, and potassium). Moderate biochar application at 15 t·hm⁻² significantly improved community diversity and richness of nirK-type denitrifying microorganisms. The community structures of nirK-type denitrifying microorganisms in the biochar addition treatments differed significantly from those in the control treatment, and pH and ammonium nitrogen were the key driving factors, as revealed by redundancy analysis. Furthermore, the dominant genera of nirK-type denitrifying microorganisms were Bradyrhizobium and Sinorhizobium. Bradyrhizobium positively correlated with soil pH; and Sinorhizobium positively correlated with alkali-hydrolyzable nitrogen, nitrate nitrogen, and ammonium nitrogen. In summary, biochar increased the diversity of nirK-type denitrifying microbial communities, changed community structure and composition, and improved the ecological functions of farmland soil by altering soil nutrient content.