Effect of N supply on apparent recovery of fertilizer N as crop N and N-min in soil during and after cultivation of winter cereals

Field trials were conducted over two years to investigate the effect of inc reasing N supply on apparent fertilizer N recovery by winter cereal crops ( 4 X wheat and 2 X barley) and on nonrecovered N. Apparent fertilizer N reco very was calculated by comparing N in fertilized and unfertilized crops. No n-recovered N is defined as N which was neither found in crops nor soil min eral N (N-min = NH4-N + NO3-N), At N supply levels according to common farm ing practice (N-cfp = 190 to 220 kg N/ha), 60- 93% of the fertilizer N was recovered in crops at harvest, while at high N supply levels of 265 to 273 kg N/ha 58-76% of fertilizer N was recovered. There were small differences in soil N-min, in 0-200 cm between N-cfp and unfertilized plots, but substa ntial increases in N-min, occurred at the highest N supply. Amounts of non- recovered N differed substantially between sites (maximum value of 84 kg N/ ha). Non-recovered N increased with increasing N rate on only 3 out of the 6 sites, indicating that N immobilization was not necessarily dependent on N rate. The fate of non-recovered N was studied for a further year by growi ng catch crops on the sites after cereal harvest. N re-mineralization deduc ed from changes in catch crop N and in N-min, indicated that non-recovered N had been immobilized in the soil. At three sites, crop N uptake was found between milk-ripe stage and harvest (19 to 60 kg N/ha) suggesting substant ial uptake of N mineralized from soil. However, grain yields were lower wit h N rates below N-cfp, indicating that late net soil N mineralization could not compensate for reductions in N fertilizer rate in these trials.