Biochar had effects on phosphorus sorption and desorption in three soils with differing acidity

Changes in soil phosphorus (P) availability after biochar application have been reported in a number of glasshouse and field trials. However, the mechanisms underlying these changes remain poorly understood. This study evaluated the effects of four biochar application rates (0, 1%, 5%, and 10%, w/w) on P sorption and desorption in three soil types with different levels of acidity. Results showed that the effects of biochar application on P sorption were highly influenced by soil acidity. As the rate of biochar application increased, P sorption increased in the acidic soil but slightly decreased in the alkaline soil. Desorbed P significantly increased at all levels of biochar application in the studied soils. Inorganic P fractionation revealed that biochar addition sharply increased the Ca-bounded P and slightly enhanced the Al-retained P. However, biochar addition decreased the Fe-bounded P. These changes suggest that the increase in P sorption with biochar addition is attributed to Ca-induced P sorption or precipitation and is less affected by Fe and Al oxides. Biochar application is found to have altered P availability by changing the P sorption and desorption capacities of the soils, and these biochar effects were dependent on soil acidity, which have important implications for improving soil productivity on large scale.