Bone char effects on soil: sequential fractionations and XANES spectroscopy

Abstract. The acceptability of novel bone char fertilizers depends on their P release, but reactions at bone char surfaces and impacts on soil P speciation are insufficiently known. By using sequential fractionation and synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy we investigated whether and how the chemical composition of bone char particles has been altered in soil and has consequently affected the P speciation of amended soils. Therefore, two different kinds of bone char particles (BC produced by the pyrolysis of degreased animal bone chips at 800 °C and BC^plus, a BC enriched with reduced sulfur compounds) were manually separated from the soil at the end of two different experiments: incubation leaching and ryegrass cultivation. Sequential P fractionation of amended soils showed P enrichment in all fractions compared to the control. The most P increase between all treatments significantly occurred in the NaOH–P and resin-P fractions in response to BC^plus application in both incubation-leaching and ryegrass cultivation experiments. This increase in the readily available P fraction in BC^plus-treated soils was confirmed by linear combination fitting (LCF) analysis on P K-edge XANES spectra of BC particles and amended soils. The proportion of Ca hydroxyapatite decreased, whereas the proportion of CaHPO₄ increased in BC^plus particles after amended soils had been incubated and leached and cropped by ryegrass. Based on P XANES speciation as determined by LCF analysis, the proportion of inorganic Ca(H₂PO₄)₂ increased in amended soils after BC^plus application. These results indicate that soil amendment with BC^plus particles leads to elevated P concentration and maintains more soluble P species than BC particles even after 230 days of ryegrass cultivation.