Effects of cover crops on the nitrogen fluxes in a silage maize production system

Rye and grass cover crops can potentially intercept residual soil mineral nitrogen (SMN), reduce overwinter leaching, transfer SMN to next growing seasons and reduce the fertilizer need of subsequent crops. These aspects were studied for 6 years in continuous silage maize cv. LG 2080 production systems receiving 20-304 kg N/ha, on a sandy soil in the Netherlands. Rye cv. Admiraal and grass (Lolium multiflorum cv. Combita) cover crops were able to absorb on average 40 kg N/ha into the shoots. The actual N uptake was largely determined by winter temperatures and hardly affected by residual SMN. At low N input levels cover crops reduced N leaching in accordance with their N uptake. At high N input levels, however, the reduction of leaching losses exceeded the storage capacity of the cover crop, suggesting that cover cropping stimulated the loss of N via denitrification or immobilization. Cover crops had no positive effect on maize yields at larger N rates and under these conditions cover crops did not improve the conversion of SMN into crop N. This was only partly reflected by an increase of residual SMN on plots where cover crops had been incorporated, as a large part of the excess N applied to maize was already lost during the growing season. In N-deficient maize production systems, however, cover crops increased the dry matter yield of maize. Their effect was equivalent to the effect of fertilizer N rates amounting to 105 and 44% of the shoot N in rye and L. multiflorum, respectively. In the first few years cover crops decomposed incompletely during the growing season following their incorporation. In the course of the years, however, effects on subsequent maize crops increased. This supports the hypothesis that the effects of cover crops are cumulative when grown repeatedly. Averaged over the 6 years, 115% and 73% of the shoot rye N and L. multiflorum N, respectively, were recovered in the crop-soil system