Nitrogen fertilisation on lettuce, processing tomato and sweet pepper: yield, nitrogen uptake and the risk of nitrate leaching

Growth, nitrogen uptake and yield response were studied in two cultivars of lettuce (loose leaf type, LL, and butterhead type, BH) and processing tomato (early and late type) and in one cultivar of sweet pepper grown in the field at different levels of nitrogen fertiliser. In all the experiments, increasing nitrogen supply increased crop growth, N uptake and yield but caused a linear decrease of the apparent recovery of the fertiliser nitrogen. In lettuce maximum fresh weight yield was reached at a fertiliser-N rate of 158 kg ha-1 in LL type and 167 kg ha-1 in BH type. At those fertiliser-N rates, the estimated N-uptake was 136 kg ha-1 in LL and 121 kg ha-1 in BH. Significant quantities of mineral nitrogen (i.e. 90 kg ha-1 in LL and 101 kg ha-1 in BH) were left by the crops in the soil at harvest. In processing tomato, both cultivars reached the highest yield at 200 kg N ha-1 rate with a N-uptake of 226 kg ha-1 in early cv and 259 kg ha-1 in late cv. With that N fertiliser rate, "extra" mineral nitrogen left by the crop in the soil at harvest ranged from 73 kg ha-1 (late cv) to 89 kg ha-1 (early cv). Moreover, N in crop residues amounted to about 100 kg ha-1 in both cultivars. In sweet pepper, the maximum marketable yield was achieved with about 310 kg N ha-1 with a N-uptake of 216 kg ha-1. With that N fertiliser rate, "extra" soil mineral nitrogen at harvest was estimated in 223 kg ha-1 and N in crop residues was 130 kg ha-1. The different N uptake rates, apparent N recovery and total N requirements of species and cultivars, together with the amount and fate of crop residues, involve different environmental risks (sweet pepper >> processing tomato @ lettuce).