Abstract
The soil chemistry of phosphorus (P) is important for understanding the processes governing plant availability as well as the risk of environmental losses of P. The objective of this research was to investigate both the speciation and the pH-dependent solubility patterns of P in clayey agricultural soils in relation to soil mineralogy and fertilization history. The study focused on soil samples from six fields that were subjected to different P fertilization regimes for periods of 45 to 57years. Soil P speciation was analyzed by P K-edge XANES spectroscopy and chemical fractionation, sorption isotherms were constructed, and dissolved P was measured as a function of pH. The XANES fitting results showed that organic P and P adsorbed to Fe and Al (hydr)oxides were common P constituents in all soils. Calcium phosphates were identified in five of six soil samples. The XANES results also indicated an increase in P adsorbed to Al and to a lesser extent Fe (hydr)oxides as a result of fertilization. Moreover, the fluorescence intensity from the P K-edge XANES analysis was most strongly correlated with HCl-digestible P (r=0.81***). Consistent with the XANES analysis, laboratory sorption isotherm models showed that the Freundlich sorption coefficient (KF) was most closely related to oxalate-extractable Al. Greater proportions of Ca phosphate in two of the heavily fertilized soils in combination with enhanced PO4 solubilization upon sample acidification indicated neoformation of Ca-phosphate precipitates. The results for the unfertilized soil samples generally showed a minimum in dissolved PO4 between pH6.5 and 7.5, with increases particularly at lower pH. This behavior can be explained either by the dissolution of Al-hydroxide-type sorbents or Ca phosphates at lower pH. In fertilized soils, there was no consistent trend in pH-dependent solubilization of P, with a complex relationship to solid-phase speciation. To conclude, inorganic P species changed most dynamically in agricultural clay soils over a period of several decades, and the role of pH in the solubilization of P depended mainly on P fertilization history and the content of reactive Ca phosphates.
Highlights
Phosphorus sorption and dissolution in soils affect plant availability and transport of P to surface and subsurface waters
Chemical speciation of soil P has historically been determined by sequential extractions (e.g. Chang and Jackson, 1957; Williams et al, 1967; Hedley et al, 1982), but X-ray adsorption near edge structure (XANES) spectroscopy has gained use in the past two decades (e.g. Hesterberg et al, 1999; Beauchemin et al, 2003; Eveborn et al, 2014)
According to XANES fitting results, the P species that increased most after P fertilization, was P adsorbed to Al and Feoxides, with Al hydroxides being more important in a larger number of soils
Summary
Phosphorus sorption and dissolution in soils affect plant availability and transport of P to surface and subsurface waters. Chemical speciation of soil P has historically been determined by sequential extractions XANES spectroscopy measures soil P speciation without any chemical pretreatment and results have been compared with those from sequential extractions (Beauchemin et al, 2003; Kar et al, 2011). Sato et al, 2005; Lombi et al, 2006; Khatiwada et al, 2012; Khatiwada et al, 2014) These results show that the most likely P species accumulating after fertilization are P sorbed to Al- and Fe (hydr)oxides, and Ca-, Fe- and Al-phosphate compounds. To improve our understanding of P reactions in fertilized soils, P speciation must be complemented with soil chemical properties such as P sorption and pH-dependent solubility
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