Effects of cover crops on soil mineral nitrogen and on the yield and nitrogen content of maize

Current agricultural practice favours winter cover crops, which can not only optimize N management in field crop rotation; but also affect subsequent crops. Three field experiments were carried out in Eastern Slovenia to examine the effects of Italian ryegrass (Lolium multiflorum Lam.), winter rape (Brassica napus ssp.oleifera (Metzg.) Sinsk), subclover (Trifolium subterraneum L.), and crimson clover (Trifolium incarnatum L.) as winter cover crops on the mineral N (N_min) content of soil and on the yield and N content of subsequent maize (Zea mays L.), fertilized with 120 kg N ha⁻¹. Italian ryegrass and winter rape decreased soil N_min contents before winter and in spring more than both clovers. In contrast, clovers accumulated significantly higher amounts of N in organic matter and had lower C/N ratios than winter rape and especially Italian ryegrass. In comparison to the control (bare fallow without cover crop), clovers increased the whole above ground maize dry matter yield, maize grain yield and N contents in whole above ground plants and in grain. The yields and N contents of maize following winter rape were on the same level as the control, while yields and N contents of maize following Italian ryegrass were, in two of the experiments, at the same level as the control. The effects of Italian ryegrass on the maize as subsequent crop in the third experiment were markedly negative. Maize in the control treatment exploited N much more efficiently than in treatments with cover crops. Therefore, cover crop N management should be improved, especially with a view to optimizing the timing of net N mineralization in accordance with the N demands of the subsequent crop.