Utlakningsbart kväve i åkermark vid höst- respektive vårbearbetning i system med och utan fånggröda samt vid långvarig halmnedbrukning och fånggrödeodling

This field experiment was established in 1993 on a sandy loam at Mellby, Halland, south-western Sweden. The objective was to investigate the effect of different tillage strategies on soil mineral nitrogen (SMN) content and nitrate leaching in a cropping system with spring-sown small grain crops. This report presents results from 2006-2011. For more detailed measure­ments and results for the period 1993-2005, see Stenberg et al. (1999a) and Myrbeck et al. (2006). Tillage strategies during 2006-2011 included mouldboard ploughing (20-25 cm depth) in early autumn, late autumn or spring, together with different options for straw management and catch crops (perennial ryegrass). SMN in the 0-30, 30-60 and 60-90 cm layers was measured on several occasions during autumn and early spring to estimate the risk of nitrate leaching during autumn and winter. Due to an increase in couchgrass in spring-ploughed plots, weed topping using a rotary mower was introduced in autumn 2001 in half these plots. Results from the period 2006-2011 confirmed the conclusions from 1993-2005 that timing of tillage has a major influence on nitrogen (N) accumulation in soil during autumn and winter, and thereby also on the risk of N leaching. The soil profile contained considerably more mineral N during autumn after early mouldboard ploughing than after delayed tillage. Use of a catch crop, incorporated in late autumn or in spring, reduced SMN during autumn. Compared with a system with early ploughing and no catch crop, incorporation in spring reduced the amount of SMN (measured in December) by 25 kg ha-1, of which 60% was attributable to postponement of tillage and 40% to the catch crop. Catch crop incorporation in late autumn gave a 20 kg ha-1 reduction in SMN, of which 70% was attributable to postponement of tillage and 30% to the catch crop. The lowest accumulation of SMN during winter (December-March) was observed for spring ploughing without a catch crop (17 kg ha-1) and the highest for catch crop incorporation in late autumn (37 kg ha-1). An intermediate increase was observed for spring ploughing with a catch crop (27 kg ha-1). The effect of the catch crop remained unchanged during 1993-2005 and comprised an approximately 10 kg N ha-1 reduction in late autumn. However, during 2006-2011 the corresponding reduction was only 2-6 kg ha-1, with even higher SMN content in the catch crop treatment in some years. This suggests that N uptake by the catch crop might no longer have exceeded the potential increase in N release from incorporated green material. The effect of incorporation or removal of crop residues was comparatively small. There was a trend for higher SMN content in treatments with straw incorporation, indicating increased amounts of mineralisable organic N in the soil after continuous incorporation. Crop yields during 2006-2011 were not influenced by the timing of tillage. However, spring ploughing with a catch crop gave higher yield than other treatments, suggesting that possible N immobilisation in recently incorporated ryegrass material did not negatively affect yield in this experiment.