Winter cover crops favor cereal crop in N competition against creeping thistle Cirsium arvense (L.) Scop

Weed control is important to improve crop yield in organic farming. In Northern Europe, the use of mechanical weed control restricts the ability to use green manure-cover crops that could be a key source of N supply. In a two-year experiment repeated twice, the objective of this study was to compare the effect of cover crop- and autumn tillage-based systems on N uptake of the succeeding barley crop (Hordeum vulgare L.) and creeping thistle (Cirsium arvense (L.) Scop.). Each autumn, a 15NO3- tracer was applied to the topsoil, and in spring its recovery was measured in the soil, barley and thistle. The effect of cover crops on soil N distribution was significant: 66–85% of the total recovered 15NO3- was in the upper 0.5 m after the cover crop treatments, while after the autumn tillage treatment only 4–8% of the recovered 15NO3- was in this soil layer. The 15NO3- retained by cover crops in the topsoil postponed its availability to the deep-rooted thistle, while making it available to the shallow-rooted spring barley, thus strengthening the barley’s ability to compete against thistle. In contrast, following autumn tillage, 15NO3- clearly leached downward; much of it leached below the barley root zone, but remained at depths where it could be taken up by thistle. Cover cropping was found to have a post-harvest control effect on perennial weeds.