Winter turnip rape as a soil N scavenging catch crop in a cool humid climate

Abstract

Leaching of nitrogen fertilizers such as nitrates from agricultural systems causes watershed eutrophication and is an economic loss for the farmer. This issue may be solved by including a catch crop in a crop rotation. For instance, winter turnip rape is a potential N catch crop for cold climates. Here, we studied winter turnip rape as a catch crop from 2009 to 2011 in Finland. Winter turnip rape was either undersown with barley in May or sown after harvesting barley in late July. In two reference treatments, the barley stubble was either left over the winter or ploughed into the soil in autumn. We collected samples from topsoil, 0-20 cm, and subsoil, 30-50 cm, in early autumn, before snowfall and in the following spring. We measured soil ammonium-N and nitrate-N, and the N content of winter turnip rape plants. Results show that undersown winter turnip rape did not change the yield and quality of barley. Winter turnip rape decreased nitrate-N in the subsoil by 83 % in 2009 and by 61 % in 2010, compared to ploughed barley. By the end of October 2009, winter turnip rape undersown in May took up 74 kg N/ha, whereas the crop sown after barley in July took up 57 kg N/ha. We conclude that winter turnip rape, either undersown with barley or sown after barley, is effective in depleting subsoil nitrates. Even though numerous reports describe the efficiency of different crucifers as catch crops under temperate climate, this is the first article concerning winter turnip rape as a catch crop under cold and humid climate.