Sorption and Fractionation of Dissolved Organic Matter and Associated Phosphorus in Agricultural Soil

Abstract

Mobility of dissolved organic matter (DOM) strongly affects the export of nitrogen (N) and phosphorus (P) from soils to surface waters. To study the sorption and mobility of dissolved organic C and P (DOC, DOP) in soil, the pH-dependent sorption of DOM to samples from Ap, EB, and Bt horizons from a Danish agricultural Humic Hapludult was investigated and a kinetic model applicable in fieldscale models tested. Sorption experiments of 1 to 72 h duration were conducted at two pH levels (pH 5.0 and 7.0) and six initial DOC concentrations (0-4.7 mmol L 21 ). Most sorption/desorption occurred during the first few hours. Dissolved organic carbon and DOP sorption decreased strongly with increased pH and desorption dominated at pH 7, especially for DOC. Due to fractionation during DOM sorption/ desorption at DOC concentrations up to 2 mmol L 21 , the solution fraction of DOM was enriched in P indicating preferred leaching of DOP. The kinetics of sorption was expressed as a function of how far the solution DOC or DOP concentrations deviate from "equilibrium."The model was able to simulate the kinetics of DOC and DOP sorption/ desorption at all concentrations investigated and at both pH levels making it useful for incorporation in field-scale models for quantifying DOC and DOP dynamics.

Challenges in modelling dissolved organic matter dynamics in agricultural soil using DAISY

“…The distribution of DOM between the dissolved and solid phase is described based on the kinetics of the exchange process (Gjettermann et al, 2007). Hence, it is assumed that sorption/desorption depends on the deviation between the actual solute concentration and the equilibrium concentration of DOC, C eq .…”

“…The DAISY model was calibrated on experimental data from a field experiment of a sandy loamy Humic Hapludult (Soil Survey Staff, 1999) (Table 1) at Research Centre Foulum located in the central part of Jutland, Denmark (Vinther et al, 2006;Gjettermann et al, 2007). The field experiment was established to study the first-year effects on crop uptake and nutrient fluxes after ploughing grassclover swards.…”

“…DON was calculated as the difference between total N and inorganic N. Details about the field experiment and measurements are given in Vinther et al (2006). Experiments on mobilization of DOM were carried out on soil samples collected from the same field (Gjettermann et al, 2007).…”

“…To parameterize the equilibrium concentration of DOC, C eq , a pedotransfer function producing parameters for the initial mass (IM) isotherm has been applied (Gjettermann et al, 2007). The IM isotherm developed by Nodvin et al (1986) is able to quantify the amount of DOC and DON removed or released to the solution by the soil (Moore et al, 1992;Kaiser and Zech, 2000).…”

“…Iron and aluminium oxides are important sorbents of DOM with sorption usually described as surface complexation of DOM carboxylic acid groups (Kaiser et al, 1997). Additionally, it is often found that soil is able to release DOM when exposed to aqueous solution containing no or very low concentrations of DOM (Nodvin et al, 1986;Kaiser, 2001;Gjettermann et al, 2007). Thus, the soil solid phase may not only sorb DOM but may also release it.…”