Ingrid Kaag Thomsen, Department of Agroecology, Aarhus University, DK-8830 Tjele, DENMARK and Elly M. Hansen, Department of Agroecology, Aarhus University, DK-8830 Tjele, Denmark
In many temperate areas with excess rainfall, preventive measures have been introduced to reduce nitrate leaching in fall and winter. With more than 60% of the land farmed, Denmark has implemented a series of policy action plans for protection of its vulnerable water environments. Cultivation of catch crops (CCs) is one of the most important initiatives and CCs are required by law on a certain proportion of farmland. To allow for any residual effect, the CCs are followed by a mandatory reduction in the amounts of nitrogen (N) that can be applied to the succeeding crop. As the use of CCs is widespread and is even expected to increase in the future, an accurate determination of potential leaching reduction and residual value is essential. We have started two field experiments with spring barley (Hordeum vulgare) followed by different CCs, and we compare nitrate leaching and residual effect of the CCs with spring barley followed by bare soil or by volunteers. Also included is winter rye (Secale cereale), which like CCs provides soil cover in fall but has a higher yield potential than spring barley. Aboveground fall N uptake in catch crops containing fodder radish (Raphanus sativus) or ryegrass (Lolium perenne) ranged from 25-59 kg N/ha on the two sites. Winter rye only held 2-5 kg N/ha. The reduction in N leaching generally reflected fall N uptake and indicated an additional 30-50% N retention in the roots. Spring barley volunteers had a relatively high N uptake but did not lead to a similar reduction in leaching during winter. The first-year results showed that the residual value of the CCs for the succeeding spring barley grain yield was low or absent while higher N concentrations in the grains resulted in increased total N uptake after the most productive CCs.