Non-overwintering cover crops: a significant source of nitrogen

In field experiments in 1982-89 at 2 sites in the Netherlands, potatoes cv. Bintje and sugarbeet cv. Monohil or Ovatio in a wheat/potatoes/wheat/sugarbeet rotation were preceded during winter by fallow or a green manure crop of Lolium multiflorum cv. Tetila with 0 (G0), 100 (G100) or 200 kg N/ha (G200) or Trifolium pratense cv. Rotra with no N (RC). Green manure crops were undersown to winter wheat cv. Durin and ploughed in in the first half of November. On average, G0, G100, G200 and RC had then accumulated 22, 93, 125 and 57 kg N/ha, respectively, in the above-ground plant parts. G0 crops tended to immobilize soil mineral N in spring and generally had non-significant effects on the yields of potatoes and sugarbeet, whereas G100, G200 and RC increased the N yields and marketable yields significantly. Effects were mainly attributable to the release of N by the green manures. The fertilizer value of the green manures was evaluated by their effect on economic optimum N rates, the marketable yield and the N yield when no mineral fertilizer N was supplied. The last two methods appeared to be most appropriate for the present experiments. Fertilizer values ranged from -21 to 108 kg N/ha, depending on the type of green manure crop and the calculation method. About half of the N accumulated in above-ground parts of the green manure crop was available to potatoes and sugarbeet from G100 when evaluated by its effect on N yields. From G200 this was even greater, although this may partly have resulted from the transfer of fertilizer N that had not been taken up by the green manure crop. Due to a high content of N in roots and stubble, RC provided, on average, almost double the amount of N accumulated in the above-ground plant parts. Green manure crops also had a minor positive effect on grain yields of winter wheat following potatoes and sugarbeet. At least 35% of the above-ground N in L. multiflorum green manure crops was not utilized within the first 18 months after their incorporation