Roles of biochar in improving phosphorus availability in soils: A phosphate adsorbent and a source of available phosphorus

• Role of biochar in improving soil phosphorus availability was studied.
• Direct phosphorus inputs from biochars vary with type of biochars.
• Grass biochar contributes the highest amount of available phosphorus to soils.
• Not all of biochars can be used to increase phosphorus retention in soils.
• With surface calcium carbonate, Mallee biochar has the best P retention capacity.

In this study, the amounts of phosphorus (P) in nine types of biochar (one natural woody biochar and eight manufactured plant derived biochars) extractable by deionized water, 0.5 N NaHCO3 (pH = 8.5) and 0.5 N H2SO4, respectively, and P adsorption on single biochar or soil/biochar mixtures were examined to investigate the potential effect and role of biochar in improving P availability in soils. Results indicated that biochars were able to bring available P into soils, but the amount and form of available P was dependent on biochar types. The results from P adsorption experiments (in phosphate solutions of 100 and 200 mg P L− 1, respectively) revealed that not all the biochars showed favourable P retention abilities and the amount of P retained by each biochar varied with the P concentration in the solution used. Among the nine biochars studied, only biochar derived from Mallee (Eucalyptus polybractea) at 720 °C with a solid residence time of 20 min showed a high P retention ability. Soils amended with Mallee biochar (5% w/w) showed an increase in the P retention ability by 16% after treatment with 200 mg P L− 1 phosphate solution. 55% of the retained P on Mallee biochar was still available for plant uptake. Results from the kinetic study of P adsorption on Mallee biochar indicated that there were two stages in P adsorption on Mallee biochar, a rapid chemisorption stage followed by a surface-diffusion-controlled stage when the surface adsorption sites were saturated. Scanning electron microscopy coupled with energy dispersive X-ray spectroscopy and attenuated total reflectance-Fourier transform infrared spectroscopy revealed that calcium carbonate was mainly responsible for P chemisorption on Mallee biochar.