Mobilization of phosphate in variable-charge soils amended with biochars derived from crop straws

• Crop straw derived biochars mobilized phosphate in variable-charge soils.
• Biochars increased soil pH and decreased phosphate adsorption.
• Anionic functional groups of biochars competed for adsorption sites with phosphate.
• More negative charge induced by biochars increased soil repulsion to phosphate.

The content of available phosphorus in variable-charge soils is often low due to the strong adsorption of phosphate by the soils. The application of crop straw-derived biochars can change surface chemical properties of variable-charge soils, and thus, should affect phosphate adsorption and phosphorus availability in the soils. However, little information is available on the effect of biochars on the mobility and bioavailability of phosphate in variable-charge soils. The objectives of this study were to investigate the effect of crop straw-derived biochars incorporated on phosphate adsorption by variable-charge soils, and then gain insight into the mechanisms for the effect of biochars on phosphate adsorption. Results showed that crop straw-derived biochars are characterized by large specific surface area, highly negative charged surface, ample of anionic functional groups, and moderate concentrations of divalent Ca2+ and Mg2+. The incorporation of the biochars increased soil CEC (cation-exchange capacity), thus, increased the repulsion of soil surfaces to phosphate, and subsequently decreased the phosphate adsorption by the soils. The ample anionic functional groups on added biochars competed for the adsorption sites on the variable-charge soils with phosphate, and thus, inhibited phosphate adsorption by the soils. The Ca2+ and Mg2+ from the biochars formed precipitates with phosphate in the soils and have the potential to increase apparent phosphate adsorption. Rice straw biochar with the higher CEC and the lowest contents of Ca2+ and Mg2+ showed the greatest inhibition on the phosphate adsorption, and thus, could likely be the best choice as amendment to mobilize phosphate in the variable-charge soils. The phosphate adsorption by both control soil and biochars-amended soils decreased with rising pH. Incorporation of the biochars increased the pH of the amended soils, thereby, further mobilizing phosphate in the soils.