Abstract

With time, different soil-forming processes such as weathering, plant growth, accumulation of organic matter, and cultivation are likely to affect phosphorus (P) speciation. In this study, the depth distribution of P species was investigated for an agricultural clay soil, Lanna, Sweden. Small amounts of apatite-P was demonstrated in the topsoil whereas the speciation of Pat 70-100 cm depth consisted of approximately 86% apatite according to P K-edge XANES (X-ray absorption near-edge structure) spectroscopy. Because there were only minor differences in bulk mineralogy and texture, these variations in P speciation were interpreted as the result of apatite weathering of the topsoil. Speciation modeling on soil extracts supported this idea: hydroxyapatite was not thermodynamically stable in the top 50 cm of the soil. Apatite was enriched in the bulk soil relative to the clay fraction, as expected during apatite dissolution. Combined results from batch experiments, XANES spectroscopy and X-ray diffraction suggested chemical transformations of the topsoil as a result from accumulation of organic matter and airing from tillage followed by enhanced weathering of apatite, amphiboles, clay minerals, and iron oxides. This caused the formation of poorly crystalline secondary iron and aluminum (hydr)oxides in the topsoil, which retained part of the released P from apatite. Other P was incorporated into organic forms. Furthermore, the results also showed that short-term acidification below the current pH value (below 5.5 in the topsoil and 7.2 in the deeper subsoil) caused significant solubilization of P. This is attributed to two different mechanisms: the instability of Al-containing sorbents (e.g. Al hydroxides) at low pH (in the topsoil), and the acid-mediated dissolution of apatite (the subsoil). (C) 2016 Elsevier B.V. All rights reserved.

Keywords

Acidification; Apatite; Clay; Secondary iron and aluminum (hydr)oxides; X-ray adsorption spectroscopy

Published in

Geoderma
2016, Volume: 280, pages: 29-37
Publisher: ELSEVIER SCIENCE BV

SLU Authors

• Eriksson, Ann Kristin

  • Department of Soil and Environment, Swedish University of Agricultural Sciences
    

• Hillier, Stephen

  • Department of Soil and Environment, Swedish University of Agricultural Sciences
    
  • The James Hutton Institute

• Ulen, Barbro

  • Department of Soil and Environment, Swedish University of Agricultural Sciences
    

• Gustafsson, Jon-Petter

  • Department of Soil and Environment, Swedish University of Agricultural Sciences
    
  • Royal Institute of Technology (KTH)

UKÄ Subject classification

Soil Science
Geochemistry


Publication identifier

DOI: https://doi.org/10.1016/j.geoderma.2016.06.004

Permanent link to this page (URI)

https://res.slu.se/id/publ/77307