In recent years, Cadmium (Cd) pollution in soybean farmland is severe. Therefore, this study focused on whether biochar influences soil physiochemical properties, the Cd content in soil and soybean grains, and the abundance and community structure of the czcA gene. Four doses of rice husk biochar (0, 5, 15, and 25 t·ha⁻¹) were applied under continuous cropping and crop rotation systems, and soil samples were collected after four years of one-time addition. The results indicated that biochar addition significantly increased soil available nitrogen, phosphorus, and soil organic carbon contents under continuous cropping and rotation. Biochar application significantly reduced the total Cd content of soil samples and soybean grains. Additionally, biochar application reduced czcA gene abundance in soybean soils by 14.26–37.88% and 35.96–48.71%, respectively. Correlation analysis revealed that Cd content and the abundance of the czcA gene significantly correlated with soil nutrients and pH. High-throughput sequencing revealed that the relative abundances of several Cd-resistant microorganisms were decreased by biochar addition. In addition, adding biochar significantly affected the Cd-resistant microbial community structure and diversity by influencing soil properties and Cd content. Therefore, this study has important practical significance for improving the soil environment and ensuring the quality and safety of agricultural products.

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.