Does biochar affect the availability and chemical fractionation of phosphate in soils?

Biochar as a soil amendment has been reported to affect the content and availability of soil nutrients. In this study, we aimed to test whether the biochar addition to soils would change the availability and chemical fractionation of phosphate in soils. Two soils (Ultisol and Alfisol) were amended with five kinds of biochars at application rate of 0, 1, and 2% (w/w). After 3-month incubation, availability and chemical forms of P were measured to investigate the potential effect and role of biochar in improving P availability in soils. The biochars used here had a lager variation of P content, depending on their feedstocks. Compared to the untreated soils, application of biochars derived from deciduous tree leaves (DLB), reed (RB), and rice straw (RSB) significantly increased the pH of two soils. The total P content of biochar-amended soils was increased with the addition of biochars. However, only RSB exhibited a significant increase (p < 0.05) of total P content. Application of biochars significantly increased the NH₄Cl-extractable P content of two soils, indicating that biochars were able to increase the availability of phosphate in soils, but the amount of available P was dependent on biochar types. Ultisol and Alfisol amended with RSB (2% w/w) showed an increase in the P availability (0.5 M NaHCO₃-extractable P) by 46 and 39%, respectively. For strongly acidic Ultisol, addition of biochar significantly increased Al-P and Ca-P content, as well as decreased Fe-P content. The P desorption test indicated the release of P from soils increased with the addition of biochar. Results suggested that biochar would change the P sorption affinity of the soil and help to increase the availability of fixed P. The increase of P availability with biochar application was due to the pH change and direct P contribution from biochar. Our results concluded that biochar affected the availability, chemical forms, and sorption capability of phosphate in soil. The extent of biochar effects on soil P varied greatly with the type of feedstock of biochar and soil type.