EFFECTS OF COVER CROPS ON THE NITROGEN FLUXES IN A SILAGE MAIZE PRODUCTION SYSTEM

Rye and grass cover crops can potentially intercept residual soil mine ral nitrogen (SMN), reduce overwinter leaching, transfer SMN to next g rowing seasons and reduce the fertilizer need of subsequent crops. The se aspects were studied for 6 years in continous silage maize producti on systems with nitrogen (N) input levels ranging from 20 to 304 kg to tal 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 tem peratures and hardly by residual SMN. At low N input levels cover crop s reduced N leaching in accordance with their N uptake. At high N inpu t levels, however, the reduction of leaching losses exceeded the stora ge 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 un der these conditions cover crops did not improve the conversion of SMN into crop N. This was only partly reflected by an increase of residua l SMN on plots where cover crops had been incorporated, as a large par t of the excess N on maize was already lost during the growing season. In N-deficient maize production systems, however, cover crops increas ed the dry matter yield of maize. Their effect was equivalent to the e ffect of fertilizer N rates amounting to 105% and 44% of the abovegrou nd N in rye and grass, respectively. In the first few years cover crop s decomposed incompletely during the growing season following their in corporation. In the course of the years, however, effects on subsequen t maize crops increased. This supports the hypothesis that effects of cover crops can cumulate when grown repeatedly. Averaged over the 6 ye ars, 115% and 73% of the aboveground rye N and grass N, respectively, were recovered in the crop-soil system.