Phosphorus Sorption and Availability from Biochars and Soil/Biochar Mixtures

In an energy-limited world, biomass may be converted to energy products through pyrolysis. A byproduct of this process is biochar. A better understanding is needed of the sorption characteristics of biochars, which can influence the availability of plant essential nutrients and potential water contaminants such as phosphorus (P) in soil. Knowledge of P retention and release mechanisms when applying carbon-rich amendments such as biochar to soil is needed. The objectives of this study were to quantify the P sorption and availability from biochars produced from the fast pyrolysis of corn stover (Zea mays L.), Ponderosa pine (Pinus ponderosa Lawson and C. Lawson) wood residue, and switchgrass (Panicum virgatum L.). We determined the impact of biochar application to soils with different chemical characteristics on P sorption and availability. Sorption of P by biochars and soil-biochar mixtures was studied by fitting the equilibrium solution and sorbed concentrations of P using Freundlich and Langmuir isotherms. Biochar produced from Ponderosa pine wood residue had very different chemical characteristics than corn stover and switchgrass. Corn stover biochar had the highest P sorption (in average 79% of the initial solution P concentration) followed by switchgrass biochar (in average 76%) and Ponderosa pine wood residue biochar (in average 31%). Ponderosa pine wood residue biochar had higher bicarbonate extractable (available) P (in average 43%) followed by switchgrass biochar (33% of sorbed P) and corn stover biochar (25% of sorbed P). The incorporation of biochars to acidic soil at 40g/kg (4%) increased the equilibrium solution P concentration (reduced the sorption) and increased available sorbed P. In calcareous soil, application of alkaline biochars (corn stover and switchgrass biochars) significantly increased the sorption of P and decreased the availability of sorbed P. Biochar effects on soil P was aligned with their chemical composition and surface characteristics.