Biochar Chemistry in a Weathered Tropical Soil: Kinetics and Equilibrium of Phosphorus Sorption and Desorption.

The phosphorus chemistry of biochar amended soils is unknown. This statement is based on the lack of published research attempting a comprehensive characterization of biochar’s influence on P sorption and desorption. Therefore, we addressed the kinetic limitations these processes as well as their equilibrium characterization. This was accomplished using a fast pyrolysis biochar made from waste materials and a highly weathered tropical Ultisol from São Paulo, Brazil. Standard methods for kinetic and equilibrium studies were used, including an adaptation in some analyses using dialysis tubing. The kinetic studies indicated that while soil required the normal accepted 12 to 24 h to come to quasi-equilibrium, biochar alone or soil+biochar required as much as 2 to 3 days (24 to 72 h). Equilibrium studies showed that P sorption was decreased when biochar was added to soil, due to biochar’s high water soluble P content (909 mg P kg^-1 biochar; 61% of total biochar P) making it a net desorber of P.  Easily desorbed biochar P was presumably sorbed to the mineral soil sorption sites; reducing the mineral soil’s P sorption potential when additional P was added. The soil-char complex did not become a net P sorber until greater than 80 mg P kg^-1soil were added. Regardless of the initial point of desorption, desorption isotherms converged to a uniform isotherm with a partition coefficient of approximately 1540 and a buffered solution concentration of 0.1 to 0.2 mg P L^-1.  When biochar is in the soil system, the significantly greater time required to come to equilibrium suggests that P chemistry modeling with an assumption of equilibrium is difficult to justify.