Phosphorus Sorption and Desorption in Soils Amended with Subabul Biochar

Phosphorus sorption mechanism after biochar application is important to understand the bioavailability of P to crops. The knowledge on P adsorption mechanism on biochar is still poorly understood, and therefore the present study was undertaken with varying biochar rates (0, 1, 3, 5%) on two soils, i.e., one taken from control plot (S1) and another from 100% NPK (S2) plots under long-term fertilizer experiment of red soils by fitting the equilibrium solution and sorbed concentrations of P using adsorption isotherms. Results showed that P sorption increased with higher biochar application rates and soils of S2 (701.8–840.1 mg kg−1) sorbed more P than S1 (534.7–742.4 mg kg−1) with 0–5% biochar rate. Phosphate adsorption on biochar was satisfactorily described by the Langmuir equation (r2 = 0.97–0.99, P = 0.01) and Freundlich equation (r2 = 0.89–0.99, P = 0.01). Phosphorus sorption increased with biochar application rates (0–5%). Desorption experiment showed lower P release and suggests a partial or slow irreversibility of the adsorption–desorption phenomena upon biochar addition to soil. In fertilizer-applied soil (S2) loaded with 60 mg P kg−1 (as KH2PO4), desorption increased with increase in biochar application rates. Biochar application thus alters P availability depending upon P sorption and desorption capacities of soils which have implications for improving P use efficiency in soils.