Phosphorus speciation of clay fractions from long-term fertility

19 Phosphorus (P) losses from agricultural soils constitute a main driver for eutrophication of the 20 Baltic Sea. There is limited knowledge about sorption and release processes of P in these soils, 21 especially concerning the effects of fertilization. In this study, P speciation of the clay fractions 22 from six different soils in long-term fertility experiments in Sweden was investigated by P K23 edge XANES spectroscopy. As expected, unfertilized soils had lower concentrations of acid24 digestible P compared with fertilized soils. Based on best-fit standards that emerged from linear 25 combination fitting (LCF) of XANES spectra, phosphate sorbed on iron (Fe) (hydr)oxides was a 26 dominant P species in clay fractions from unfertilized soils containing more than 35 mmol kg of 27 oxalate-extractable Fe. In contrast, P sorbed on aluminum (Al) (hydr)oxides predominated in 28 soils with lower concentrations of oxalate-extractable Fe. A greater proportion of organically 29 bound P was fit for soil samples containing >2 % organic carbon. The soils included one 30 calcareous soil for which a greater proportion of P was fit as apatite. After long-term fertilization, 31 P had accumulated mainly as Al-bound forms (adsorbed species and precipitates) according to 32 the XANES analysis. Our research shows that P speciation in fertilized agricultural soils 33 depended on the level of P buildup and on the soil properties. 34