Mechanisms of rice straw biochar effects on phosphorus sorption characteristics of acid upland red soils.

An important pathway for biochar to alter the availability of soil phosphorus (P) is to change P sorption characteristics of the soil. The aim of this study was to understand the mechanisms of biochar effects on P sorption in acid upland red soils in the presence of different concentrations of exogenous P. Rice straw biochar (RSB) was prepared and applied at rates of 0, 1%, 3%, and 5% (w/w) to three red soils (MZ, MZ, and QY) differing in initial pH (pH = 4.31, 4.82, and 5.68, respectively). The P sorption characteristics of these red soils were described using the Langmuir and Temkin equations and their relationships with soil basic physicochemical properties were analyzed. Furthermore, a representative red soil (MZ) was selected to analyze the zeta potential of soil colloids and the chemical properties of sorption equilibrium solution, in order to understand their relationships with P sorption characteristics. Results showed that within a certain range of P concentration in the equilibrium solution, the amount of P sorbed by the three red soils decreased and the corresponding amount of P desorbed increased with increasing amendment rate of RSB. RSB showed the greatest effect on P desorption characteristics of MZ soil in the presence of higher exogenous P concentration. With increasing RSB amendment rate, the maximum P sorption of MZ soil decreased, while those of MZ and QY soils increased after an initial decrease. Phosphate sorption equilibrium constant and maximum P buffer capacity of each soil first increased and then decreased. However, a single physicochemical property could not interpret complex changes in multi-factors that jointly determine the P sorption characteristics of red soils. In the case of MZ soil, RSB amendment shifted the zeta potential of soil colloids to the negative direction; this decreased the positive charge and increased the negative charge on the soil surface, thus reducing P sorption in the MZ soil. In the presence of the same concentration of exogenous P, RSB amendment altered the pH, dissolved organic C (DOC), humification index (HIX), and maximum fluorescence intensity (F) in the sorption equilibrium solution. In most cases, the amount of P sorbed by the MZ soil was negatively correlated with the pH value, DOC concentration, HIX value, and F value of humic-like dissolved organic matter (DOM), and positively correlated with the F value of protein-like DOM (P < 0.05 or P < 0.01). The relative fractional distribution of the contents for humic-like and protein-like DOM might determine the difference in the P sorption characteristics of MZ soil. In conclusion, different amendment rates of RSB affected the release of phosphate from soil surfaces into the solution by altering basic physicochemical and electrochemical properties of red soils and chemical properties of sorption equilibrium solution.