Phosphorus speciation and bioavailability in diverse biochars

Abstract

Erosion of phosphorus (P)-rich soil into waterways is a major contributor to eutrophication. To minimize the build-up of P in agricultural soils, greater knowledge of the bioavailability and fate of P from soil amendments is required. We used X-ray Absorption Near Edge Structure (XANES) spectroscopy to resolve the major P species in nine diverse biochars. We then examined the relationship between biochar P extracted using a range of typical soil (water, Bray2 and Colwell) and plant (2% citric acid, and 2% formic acid) assays. We compared these with ryegrass P uptake via bioassay. Linear combination fitting indicated Al-phosphate (variscite) was the dominant P species in biochars derived from cattle feedlot manure, sugarcane trash and sugarcane bagasse, reflecting the likely Al content of the feedstock. Non-apatite Ca-phosphates (monocalcium phosphate or CaHPO4) were the major P species in poultry litter, green waste, papermill sludge, wheat chaff, sugarcane mill mud and rice husk biochars. Biochar P was poorly water soluble but largely soluble in weak acids (formic and citric acids). Despite this, biochar P extracted by citric and formic acid was a poor predictor of P bioavailability to ryegrass, with the percentage of total P extracted by water or by the Bray2 reagent providing the best prediction of ryegrass P uptake. The P in biochar was identified by XANES spectroscopy as predominantly Ca and/or Al-P. Water and Bray2 extraction provided the best predictors of plant available P from biochars in a plant bioassay.