Nutrient losses by surface run-off from soils with winter cover crops and spring-ploughed soils in the south of Sweden

Winter cover crops are used as a method of reducing nitrogen (N) losses from arable land in several countries, but their effect on phosphorus (P) losses is poorly documented. Run-off and losses of nutrients and soil were measured from a clay loam with autumn-ploughed and spring-ploughed plots and from plots with winter wheat during three winter seasons (1993–1996) in Holland County in south western Sweden. The run-off water was collected in troughs dug into the soil at the end of collecting slopes placed in the experimental plots. As a result of the weather, there was only one winter in which surface run-off occurred to any great extent. On average, 75% of P was in particulate form (P_part). Neither winter wheat (Triticum aestivum L.) nor catch crops of English ryegrass (Lolium perenne L.) reduced losses of P_part when compared with losses from autumn-ploughed soil; and losses from spring-ploughed soil containing stubble and weeds were no lower than those from autumn-ploughed soil. Losses of P_part from all treatments were moderate considering its low bio-availability. Concentrations of phosphate phosphorus (PO₄-P) were low, with a mean 0.04 mg 1⁻¹. Despite a significant increase in losses of PO₄-P from spring-ploughed soil covered with stubble and catch crops or weeds compared with that in autumn-ploughed soil, the extra input from this P source was at most 2 g ha⁻¹ yr⁻¹. This mass loss was equal to 0.5 g kg⁻¹ of the total mass of P in the vegetation. Thus, only very small extra P surface losses were found with winter cover crops compared with those with bare soils. N losses in run-off were low in all treatments.