Model predictions and long-term trends in phosphorus transport from arable lands in Sweden

As a part of nation-wide monitoring of the impact of agriculture on the aquatic system, phosphorus (P) transport was measured from subsurface drained fields in Sweden. The measured transports was a mixture of P from surface, subsurface and groundwater contribution. Fourteen of these fields together with a similar experimental field have been studied for at least 10 years and most of them for much longer. Transport of total phosphorus (TP) from four of the fields together accounted for 74% of the total transport. Five other fields together accounted for another 19%, while transport from the other five fields was more or less negligible. Based on factor combinations, a simple regression model for calculating TP concentration was used for prediction of the losses. The parameters included were livestock density (LD), HCl extractable P (P–HCl) in the topsoil, duration of high water flow (DHF) and soil specific areas (SSAs). The model was found to satisfactorily predict the TP transport in the streams of 32 small agriculture-dominated catchments in different parts of the country, with a P loss from arable land that was calculated as varying from 0.03 to 0.50 kg ha−1 yr−1. However, for a very few catchments, the model worked only for transport by drainage water and not for P transport measured in the stream water.