Potential effects of biochar on the availability of phosphorus — mechanistic insights

•P-contents of eight different biochars were studied using sequential fractionation.•Only a small fraction of total P was present in a labile, readily plant available form.•Both the pyrochar and the hydrochar studied were weak sorbents for phosphate.•Biochar-derived DOM reduced phosphate sorption to iron minerals.

Potential supply shortages of phosphorus (P) rock reserves call for a more efficient use of P fertilizer and exploring new ways of recycling the nutrient within agro-ecosystems. Conditioning arable land with biochar might contribute to achieving both goals. We examined three putative mechanisms governing P availability in biochar amended soils. Potential direct P inputs from eight biochars differing in feedstocks and production techniques were assessed using sequential P fractionation. Phosphorus sorption on one pyrochar and one hydrochar was also studied. Finally, competitive sorption between P and biochar-derived dissolved organic matter (DOM) on goethite was studied at three pH levels. Total P varied from 0.6 to 4.0 mg P g− 1 in biochars derived from phytomass, and was 47 mg P g− 1 in a biochar derived from sewage sludge. In two slightly acidic hydrochars most P was 0.1 M NaOH extractable, Fe-associated, while in the remaining alkaline pyrochars most P was 1 M HCl extractable, Ca-associated. Biochar intrinsic P was mostly of intermediate stability and might slowly replenish labile P in soils. Both the pyrochar and the hydrochar studied were weak sorbents for phosphate. We show that DOM released from biochar, however, can inhibit P sorption on goethite depending on the initial pH, P to DOM ratio and order of sorbate addition. Strong competition between biochar-DOM and P for sorption sites on goethite especially at low pH values suggests that biochar-DOM might enhance P fertilizer use efficiency especially in acidic, highly weathered and strongly P fixing soils.