Biochar-induced soil phosphate sorption and availability depend on soil properties: a microcosm study

The effects of soil properties on biochar-induced soil phosphate sorption and availability are not well investigated. An alkaline biochar-induced soil phosphate sorption, phosphorus (P) availability, and related mechanisms (including the effects of calcium/magnesium (Ca/Mg) minerals in biochar and the liming effect) in different soils were studied in the laboratory. Two soils (sandy soil and clay soil) with contrasting P-retention capacities were amended with four rates of alkaline peanut biochar. Soil phosphate sorption–desorption indices were characterized by a multiple-point isotherm batch experiment. Soil P availability was determined by sequential fractionation method. The alkaline biochar addition increased the phosphate sorption of sandy soil but decreased the phosphate sorption of the clay soil. The interaction between the biochar and external phosphate on the biochar surface was mainly controlled by adsorption process, rather than precipitation. The liming effect of the biochar decreased the Langmuir phosphate sorption maxima (Smax) of clay soil. However, the biochar increased Smax of clay soil when the liming effect was eliminated, because Ca/Mg minerals in biochar can provide phosphate sorption sites. The dominant P fraction of the biochar was Ca-bounded P, and therefore the biochar addition increased HCl-extractable P of both sandy and clay soils. Generally, the biochar addition increased P availability of clay soil because it increased NaHCO3-extractable P content and decreased phosphate sorption. P mobility of sandy soil was also increased by biochar addition, because water-extractable P and EPC0 were increased. Our results suggest that both the biochar (particularly the contents of Ca/Mg minerals within) and soil properties (particularly the initial phosphate sorption capacity) determine phosphate sorption and availability following the biochar application. The alkaline biochar application to sandy soils with low P buffering capacity should consider the amount of P input from biochar and the corresponding environmental risk, although biochar addition can enhance phosphate sorption in these soils in the short term.