Land Application of Biochar and Phosphorus Retention: "an Inconvenient Truth".

The paradox of biochar with high cation exchange capacity and at its ability to adsorb phosphorus (P) at the same time has puzzled scientists. Our research has shown that: i) biochars can constitute a P source when applied to minimally P-impacted sandy soils, and ii) biochar- enhanced P sorption at elevated solution concentrations may be reversible as concentrations revert to background levels. X-ray diffraction analysis did not identify any newly formed crystalline phase in wood-based, poultry litter or biosolids biochars of our studies, though the poultry litter biochar contained a P mineral (Whitlockite, beta-tricalcium phosphate, with or without Mg), a slow P release form. The solution phase in a biochar-soil environment contains Ca, Mg and Fe further indicating that any association of biochar-P with these metals is not stable. Soil, rather than biochar properties dictate environmentally sound P loading rates; the soil P storage capacity (SPSC) could be used as a tool to evaluate “safe” biochar applications at a given site.