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

Abstract

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 continous silage maize production systems with nitrogen (N) input levels ranging from 20 to 304 kg total N ha-1, on a sandy soil in The Netherlands. Rye and grass cover crops were able to absorb on average 40 kg N ha-1 in the aboveground plant parts. The actual N uptake was largely determined by winter temperatures and hardly 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 can have stimulated the loss of N via denitrification or immobilisation. 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 on 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 aboveground N in rye and grass, 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 effects of cover crops can cumulate when grown repeatedly. Averaged over the 6 years, 115% and 73% of the aboveground rye N and grass N, respectively, were recovered in the crop-soil system.