MINERAL AND SLURRY NITROGEN EFFECTS ON YIELD, N-UPTAKE, AND APPARENT N-USE EFFICIENCY OF OILSEED RAPE (BRASSICA-NAPUS)

In NW Europe, autumn-grown oilseed rape normally receives nitrogen (N) in autumn as seedbed N and in the spring as a split application at th e beginning of growth and at stem elongation. In the growing seasons 1 990/91 to 1992/93, the effects of slurry and mineral N fertilization o n yield, N uptake by the seed and apparent N-use efficiency (NUE) by o ilseed rape (Brassica napus) were investigated in a factorial field ex periment at Hohenschulen Experimental Station near Kiel, NW Germany. T he crop rotation was oilseed rape-winter wheat-winter barley, and soil tillage (conservation tillage without ploughing, conventional tillage ), application of pig slurry (none, autumn, spring, autumn + spring) a nd mineral N fertilization (0 to 200 kg N ha(-1)) were all varied. Eac h year, the treatments were applied to all three crops of the rotation and were located on the same plots. Between the years, average seed y ield ranged from 3.04 to 3.78 t ha(-1), while the corresponding N upta ke by the seed varied from 107 to 131 kg N ha(-1). Slurry application in spring increased the seed yield and N uptake by the seed in all yea rs, whereas the effect of autumn slurry alone or in combination with s pring slurry was negligible. Mineral N fertilizer increased seed yield and N uptake by the seeds except in 1991/92, when N amounts exceeded 160 kg N ha(-1). No significant slurry x mineral N interaction occurre d. Apparent NUE of mineral N was larger than that of slurry N, but dec reased with increasing mineral fertilizer N rates. Only 5% of the autu mn slurry N was apparently utilized by the seeds, compared with 24% of the spring slurry N. Despite its ability to take up substantial quant ities of N before the winter, oilseed rape utilized very little autumn slurry N for seed production. To minimize environmental impacts, slur ry should be applied in the spring, when plants are more able to use N for yield formation, even if NUE of slurry N is lower than that of mi neral N. However, since NUE changes with the amount of applied N, it i s difficult to find the best combination of slurry and mineral N ferti lization to avoid negative environmental effects.