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

Abstract

Annual nitrogen leaching losses from arable land in south Sweden usually amount to 15–45 kg ha −1. The objective of this three-year study was to investigate the timing effect of mouldboard ploughing (early autumn, late autumn or spring) on soil mineral nitrogen content and nitrate leaching in a cropping system with spring-sown small grain crops (barley, oats and wheat). Late autumn ploughing was studied with and without perennial ryegrass ( Lolium perenne L.) as a catch crop, and with and without preceding stubble cultivation. 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 ceramic suction cups at 60 and 90 cm. Total nitrogen in above-ground catch crops, weeds and volunteer plants was determined at ripeness of the main crop and 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 content was on an average 68 kg N ha −1 in 0–90 cm in early ploughed soil and 39 kg N ha −1 when ploughing was delayed to spring. Also nitrate leaching was greater in treatments with early than with late tillage. This was probably because of increased nitrogen mineralization. However, when tillage was delayed there was also a substantial growth of weeds and volunteer plants during autumn, particularly couch-grass ( Elymus repens L.). This caused grain yield to decrease by up to 40% when tillage operations were delayed until late autumn or spring. Delaying tillage operations to late autumn or spring seems to be necessary to reduce the risk of nitrogen leaching. Swedish regulations for farmers to control nitrogen losses have been changed as a consequence of these results.