Four-year biochar study: Positive response of acidic soil microenvironment and citrus growth to biochar under potassium deficiency conditions

Biochar has direct or indirect effects on soil microorganisms, but the changes in soil metabolism are rarely monitored and analyzed. In addition, the potassium (K) effect of biochar has not attracted much attention. This study set up a four-year experiment with acid soil and citrus as the test soil and plants, respectively. The long-term effects of biochar on the acid soil microenvironment and citrus growth were explored from soil properties (nutrient contents, microbial communities, and metabolites) and citrus growth (nutrient contents, reactive oxygen species (ROS), and root endophytes). The results showed that the four-year amendment of biochar in acid soil was very significant, in which the soil pH was increased by 1 unit, organic matter and cation exchange capacity (CEC) increased by 120.77% and 16.21%, respectively. Biochar improved the K availability of soil by increasing the number and metabolic activity of Azotobacter and Pseudomonas, and finally effectively alleviated the K deficiency of citrus. From the perspective of available K content, 2% biochar reduced the 20% conventional K application rate. The pH, organic matter, and cation exchange capacity (CEC) were the most important factors affecting the bacterial community structure, while the fungal community was more sensitive to the change in the nutrient environment. Biochar mainly stimulated the progress of soil metabolism by affecting the metabolic activity of bacterial communities. Biochar application increased some of the beneficial bacteria in the soil, i.e., the relative abundance of Pseudarthrobacter increased by 700 times. However, biochar and exogenous K did not significantly affect arbuscular mycorrhizal fungi (AMF) and endophytic bacteria in citrus roots. In general, biochar has a long-term and positive response to the acidic soil microenvironment and citrus growth, as well as promotion value in the agricultural field.