Phosphorus speciation in Swedish agricultural clay soils : influence of fertilisation and mineralogy

Sammanfattning: Phosphorus (P) is an important element for crop production, but build-up of excess soil P can promote P leaching and eutrophication of surface waters. To better understand the dynamics of P release from soil to waters, more knowledge is needed about sorption patterns and P speciation in agricultural soils. Two new indices were developed to assess the importance of P sorption to hydroxy-interlayered clay minerals, and to evaluate the amount of hydroxy-interlayering and hydroxy-interlayer stability. A strong relationship was found between oxalate-extractable aluminium (Al) and the amount of hydroxy-interlayering in soil, suggesting a common source. This makes it difficult to analytically distinguish between phosphate (PO4) adsorbed to hydroxy-interlayers from PO4 adsorbed to Al hydroxide-type precipitates. Application of X-ray absorption near-edge structure (XANES) spectroscopy to evaluate P speciation in soil profiles and the effects of P depletion and fertilisation revealed a distinct change in P speciation with increasing soil depth in an agricultural clay soil profile. The results indicated that the subsoil P predominantly occurred as apatite, whereas PO4 adsorbed with Al-hydroxides or hydroxy-interlayers dominated in the topsoil. Organic P and PO4 adsorbed to iron (Fe)-(hydr)oxides were observed only in the topsoil. This can be explained by long-term weathering of apatite and silicates, particularly ferromagnesian forms such as amphibole, in the upper soil horizons, causing an association of the released PO4 with secondary metal (hydr)oxides. Collectively, the XANES results showed that the most important phases governing P retention and release in agricultural clay soils are adsorption of PO4 to Al and to Fe in (hydr)oxide minerals or interlayers. After fertilisation, P was adsorbed to Al hydroxide phases in six different soil types studied, but in two of the soils there was also an increase in calcium phosphate. Moreover, P solubility was shown to be lowest at pH values ranging from 4.5 to 7.5 and increased with decreasing pH, probably as a result of the dissolution of apatite and PO4-bearing hydroxy-Al precipitates at low pH.

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