Response of Soil Ammonia−Oxidizing Microorganisms to Biochar Amendment Under Continuous Cropping and Crop Rotation Systems

Ammonia−oxidizing archaea (AOA), ammonia−oxidizing bacteria (AOB), and Comammox are key drivers of soil nitrification and play crucial roles in soil nitrogen cycling, with distinct functional contributions. AOA and AOB primarily drive the first step of nitrification by oxidizing ammonia to nitrite, while comammox bacteria can directly oxidize ammonia to nitrate. In this study, quantitative real−time polymerase chain reaction (qPCR) and Illumina MiSeq high−throughput sequencing were employed to determine the absolute abundance, diversity, and community structure of AOA, AOB, and Comammox in alkaline soybean farmland soil under single biochar application. Combined with soil chemical properties, the driving factors shaping ammonia−oxidizing microbial community structure were analyzed to clarify the impact of biochar on ammonia−oxidizing microorganisms from the perspective of soil microbial ecology. The results showed that biochar application effectively modified soil physicochemical properties, increased the absolute abundance of AOA, AOB, and Comammox, influenced their α–diversity, and reshaped community structure. Redundancy analysis revealed that after biochar application, AOA, AOB, and Comammox exhibited positive correlations with soil total phosphorus (TP), total nitrogen (TN), available nitrogen (AN), available phosphorus (AP), total organic carbon (TOC), and available potassium (AK). In conclusion, biochar application regulates soil nutrient cycling, particularly by modulating the ammonia oxidation and nitrite oxidation processes, thereby regulating soil nitrogen cycling and ammonia−oxidizing microbial communities, with potential benefits for sustaining soil quality in agricultural systems.