Enhancing phosphorus availability in two variable charge soils by the amendments of crop straw biochars

More recent works implied that biochar enhanced P availability in variable charge soils. However, the involved mechanisms have not been clearly defined. In the present study, we evaluated the effects of peanut and rice straw biochars on improving P availability (Olsen-P) in an Oxisol and an Ultisol. Incubation experiments, with different amendments including peanut straw biochar (PC), rice straw biochar (RC), acid-washed peanut straw biochar (APC), acid-washed rice straw biochar (ARC), and Ca(OH)(2), were performed. After 56-day incubation without and with 50 mg P kg(-1) loading, Olsen-P, pH, electrical conductivity (EC), and exchangeable acidity in the amended soils were analyzed. Results showed that PC greatly increased soil pH, EC, cation exchange capacity (CEC), and Olsen-P and decreased exchangeable acidity in biochar-amended soils followed by RC when compared with the control and other treatments. However, APC, ARC, and Ca(OH)(2) application did not increase P availability greatly. The results manifested that an adequate amount of functional groups together with high alkalinity in PC and RC was responsible for the increase in Olsen-P in both soils. Thus, application of biochar with higher functional groups and alkali mutually inhibited P adsorption and increased its availability in variable charge soils. The increase in soil pH enhanced the dissociation of acidic functional groups on the biochars and thus increased the ability of these anionic functional groups to compete for adsorption sites on the soils with phosphate and decreased phosphate adsorption by the soils, which was the main mechanism for the increase in Olsen-P content in the soils induced by the biochars.