Phosphorus speciation and bioavailability in diverse biochars
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Background and aims
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.
KeywordsBioavailability Biochar Phosphorus XANES
X-ray absorption spectroscopy was funded by a beamtime award to LL, TJR and ALR by the National Synchrotron Radiation Research Center, Taiwan (proposal 2013-3-037-1). We thank the beamline scientist, Dr. LY (Peter) Jang, for his assistance with the XANES measurements.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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