Soil mineral nitrogen and nitrate leaching losses in soil tillage systems combined with a catch crop

Annual nitrogen leaching losses from arable land in south Sweden usually am ount to 15-45 kg ha(-1). The objective of this three-year study was to inve stigate the timing effect of mouldboard ploughing (early autumn, late autum n or spring) on soil mineral nitrogen content and nitrate leaching in a cro pping system with spring-sown small grain crops (barley, oats and wheat). L ate autumn ploughing was studied with and without perennial ryegrass (Loliu m perenne L.) as a catch crop, and with and without preceding stubble culti vation. The effects of removal compared to incorporation of straw were also studied. Soil mineral nitrogen in the 0-90 cm layer was determined on 4-5 occasions each year, Nitrate was determined in soil water sampled with cera mic suction cups at 60 and 90 cm, Total nitrogen in above-ground catch crop s, weeds and volunteer plants was determined at ripeness of the main crop a nd before tillage operations. Time of tillage, as opposed to catch crop or crop residue management, influenced nitrogen leaching. After early tillage, soil mineral nitrogen increased during autumn, In November, the nitrogen c ontent was on an average 68 kg N ha(-1) in 0-90 cm in early ploughed soil a nd 39 kg N ha(-1) when ploughing was delayed to spring. Also nitrate leachi ng was greater in treatments with early than with late tillage. This was pr obably because of increased nitrogen mineralization, However, when tillage was delayed there was also a substantial growth of weeds and volunteer plan ts during autumn, particularly couch-grass (Elymus repens L.), This caused grain yield to decrease by up to 40% when tillage operations were delayed u ntil late autumn or spring. Delaying tillage operations to late autumn or s pring seems to be necessary to reduce the risk of nitrogen leaching. Swedis h regulations for farmers to control nitrogen losses have been changed as a consequence of these results. (C) 1999 Elsevier Science B.V. All rights re served.