Fractionation of dissolved organic matter and associated P during sorption to agricultural soil

Kurzfassung

The mobility of dissolved organic matter (DOM) is a major factor affecting the export of nutrients as N and P from soils to surface waters. It also provides a medium for microbiological activity. In order to study the mobility and sorption of dissolved organic C (DOC) and dissolved organic P (DOP) in soil, the pH-dependent sorption of DOM to an agricultural top- and subsoil was investigated and a model concept tested. Sorption experiments of 1-72 hours duration were conducted using soil samples from the Ap-, EB-, and Bt- horizons of a Humic Hapludult, applying two pH levels (pH 5.0 and 7.0), and six initial concentration levels of DOC (0-4.7 mM).
The model concept, an empirical, kinetic expression describing DOM sorption/desorption, was capable to simulated the DOC and DOP sorption/desorption at all concentrations applied and both pH values. In general, DOM sorption/desorption is relatively fast, achieving equilibrium within 24 hours. DOC and DOP sorption strongly decreased with increase in pH. Actually, desorption was dominated at pH 7, whereas at pH 5 almost no desorption occurred. Desorption was most pronounced for DOC and to a lesser extend for DOP. The results support the findings that a fractionation occurs after DOM sorption/desorption making the soluble fraction of DOM enriched with P. The fractionation depends on DOM concentration as fractionation only occurred at relatively low concentrations (< 2 mM) of DOC. However, agricultural soils often contain DOC below this concentration and the fractionation is therefore important when quantifying the leaching of DOC and DOP.