Simulation of dissolved phosphorus from cropped and grassed clayey soils in southern Finland

Agricultural phosphorus (P) loading is a major contributor to eutrophication of surface waters in Finland. Of the various forms of P in runoff from cultivated fields, dissolved reactive phosphorus (DRP) is immediately available for algal growth and can directly accelerate eutrophication. The applicability of an empirical model developed in southeastern USA was evaluated by simulating DRP in surface runoff from two cropped and grassed clayey soils (Vertic Cambisols) in southwestern Finland. The model relates DRP in a runoff event, e.g. to desorbable soil P (P-D), runoff volume (V) and the concentration of total suspended solids (TSS) in runoff. The model overestimated the mean concentration of DRP by a factor of 110-1645. In addition, the observed and simulated mean concentrations of DRP in plots with different winter covers, crops and P status did not correlate with each other. The predictions improved when P-D was estimated by water extractions instead of Bray extractions, and the model was simplified by excluding the dependence of DRP concentration on V. The correct level of results could, however, only be achieved by calibration. In order to improve the model lit, the dependence of DRP concentration on V runoff duration and TSS should be assessed under Finnish conditions and the model should be modified accordingly. (C) 1999 Elsevier Science B.V. All rights reserved.