Approach for predicting P sorption/desorption behaviour of potentially eroded topsoil in watercourses

Abstract

This study focuses on the quantification of the impact of potentially eroded topsoil particles on the available watercourse P concentration. We used 56 topsoil samples for determining the relation existing between the molar ratio of sorption-active Fe- and Al-(hydr)oxides to plant available P, as determined by the commonly used oxalate and Mehlich 3 extractions (P_M3). Our sample set covers the most common combinations of non-calcareous soil types, land and agricultural uses. By using Freundlich isotherms, we found that the concentration of adsorbed/desorbed P (Q) by soil particles in water with specific soluble reactive phosphorus (SRP) concentrations are significantly correlated to P_M3 (Q=a∗P_M3+b) and also to the degree of P saturation as measured by oxalate extraction (DPS_ox; Q=c∗DPS_ox+d). The observed relations varied in slope and intercept parameters for discrete equilibrium P concentrations in water. However, in the environmentally important range of PO₄-P concentrations, i.e., from 20 to 220μgL^-1, a strong non-linear correlation was found between the individual parameters and equilibrium P concentration. Accordingly, we derived equations useful for quantitatively predict P exchange between topsoil particles when potentially eroded into watercourse and the recipient water of either known or targeted P concentrations. This approach might refine erosion models, and thus facilitates the quantification of the impact of soil erosion on the in-stream bioavailable P concentrations.

Keywords: Equilibrium P concentration; Mehlich 3; Soil; Sorption-desorption tests; Watercourse.