Losses of nitrogen and phosphorus via the drainage system from organic crop rotations with and without livestock on a clay soil in southwest Sweden

Abstract

Leaching losses of N and P were examined in separately tile-drained field plots on a clay soil with two 6-year organic crop rotations (1998–2006). Two different farming systems (with dairy cows (+L) and without stock (−L)) were evaluated to identify parts of the crop rotations with the greatest risks of N and P leaching losses and to examine the scope for improvement. Although N and P leaching losses tended to be higher without livestock, the mean annual leaching loads from both systems were low and did not differ significantly (6.8 and 9.1 kg N ha⁻¹ year⁻¹ and 0.39 and 0.55 kg P ha⁻¹ year⁻¹ for +L and −L, respectively). For both systems, there were increased amounts of N and P in drainage water in the period following sowing of winter wheat after incorporation of clover–grass ley (CG). This could be attributed to the early date of CG incorporation, as late incorporation followed by bare fallow gave lower nutrient leaching. Drainage from bare fallow after a broad bean crop was identified as a critical part of the crop rotation for P leaching, with P losses possibly enhanced by macropore formation by the taproot of broad bean. The lowest leaching losses were observed during CG growth, demonstrating that CG had a buffering effect on leaching during heavy precipitation events. It was concluded that in organic farming on clay soils, countermeasures such as undersown CG and late incorporation of this CG can be effective in reducing N and P leaching losses.