Abstract. Acceptability of novel bone char fertilizers depends on their P release but reactions at bone char surfaces and impacts to soil P speciation are insufficiently known. By 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, consequently, has affected the P speciation of amended soils. Therefore, two different kinds of bone char particles (BC, produced by pyrolysis of degreased animal bone chips at 800 ºC and BCplus, 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 as compared to the control. The most P increase between all treatments significantly occurred in the NaOH-P and resin-P fractions in response to BCplus application in both, incubation-leaching and ryegrass cultivation experiments. This increase of the readily available P fraction in BCplus treated soils was confirmed by linear combination fitting (LCF) analysis on P K-edge XANES spectra of BCs particles and amended soils. The proportion of Ca-hydroxyapatite decreased, whereas the proportion of CaHPO4 increased in BCplus 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 BCplus application. These results indicated that soil amendment with BCplus particles leads to elevated P concentration and maintains more soluble P species than BC particles even after 230-days of ryegrass cultivation.
Bone char effects on soil: sequential fractionations and XANES spectroscopy