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Recovery of fertilizer nitrogen (N) applied to winter wheat crops at tillering in spring is lower than that of N applied at later growth stages because of higher losses and immobilization of N. Two strategies to reduce early N losses and N immobilization and to increase N availability for winter wheat, which should result in an improved N use efficiency (= higher N uptake and/or increased yield per unit fertilizer N), were evaluated. First, 16 winter wheat trials (eight sites in each of 1996 and 1997) were conducted to investigate the effects of reduced and increased N application rates at tillering and stem elongation, respectively, on yield and N uptake of grain. In  treatment 90‐70‐60 (90 kg N ha−1 at tillering, 70 kg N ha−1 at stem elongation and 60 kg N ha−1 at ear emergence), the average values for grain yield and grain N removal were up to 3.1 and 5.0 % higher than in treatment 120‐40‐60, reflecting conventional fertilizer practice. Higher grain N removal for the treatment with reduced N rates at tillering, 90‐70‐60, was attributed to lower N immobilization (and N losses), which increased fertilizer N availability. Secondly, as microorganisms prefer NH4+ to NO3− for N immobilization, higher net N immobilization would be expected after application of the ammonium‐N form. In a pot experiment, net N immobilization was higher and dry matter yields and crop N contents at harvest were lower with ammonium (ammonium sulphate  +  nitrification inhibitor Dicyandiamide) than with nitrate (calcium nitrate) nutrition. Five field trials were then conducted to compare calcium nitrate (CN) and calcium ammonium nitrate (CAN) nutrition at tillering, followed by two CAN applications for both treatments. At harvest, crop N and grain yield were higher in the CN than in the CAN treatment at each N supply level. In conclusion, fertilizer N use efficiency in winter wheat can be improved if N availability to the crops is increased as a result of reduced N immobilization (and N losses) early in the growth period. N application systems could be modified towards strategies with lower N applications at tillering compensated by higher N dressing applications later. An additional advantage is expected to result from use of nitrate‐N fertilizers at tillering.

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Previous experiments have shown that, at harvest of winter wheat, recovery of fertilizer N applied in early spring [tillering, Zadok's growth stage (GS) 25] is lower than that of N applied later in the growth period. This can be explained by losses and immobilization of N, which might be higher between GS 25 and stem elongation (GS 31). It was hypothesized that a higher crop density (i.e. more plants per unit area) results in an increased uptake of fertilizer N applied at GS 25, so that less fertilizer N is subject to losses and immobilization. Dierent crop densities of winter wheat at GS 25 were established by sowing densities of 100 seeds m ±2 (S low ), 375 seeds m ±2 (S cfp common farming practice) and 650 seeds m ±2 (S high ) in autumn. The eect of sowing density on crop N uptake and apparent fertilizer N recovery (aFNrec N in fertilized treatments ) N in unfertilized treatments) in crops and soil mineral N (N min ), as well as on lost and immobilized N (i.e. non-recovered N N rate ) aFNrec), was investigated for two periods after N application at GS 25 [i.e. from GS 25 to 15 days later (GS 25 + 15d), and from GS 25 + 15d to GS 31] and in a third period between GS 31 and harvest (i.e. after second and third N applications). Fertilizer N rates varied at GS 25 (0, 43 and 103 kg N ha ±1 ), GS 31 (0 and 30 kg N ha ±1 ) and ear emergence (0, 30 and 60 kg ha ±1 ). At GS 25 + 15d, non-recovered N was highest (up to 33 kg N ha ±1 and up to 74 kg N ha ±1 at N rates of 43 and 103 kg N ha ±1 , respectively) due to low crop N uptake after the ®rst N dressing. Non-recovered N was not aected by sowing density. Re-mineralization during later growth stages indicated that non-recovered N had been immobilized. N uptake rates from the second and third N applications were lowest for S low , so non-recovered N at harvest was highest for S low . Although non-recovered N was similar for S cfp and S high , the highest grain yields were found at S cfp and N dressings of 43 + 30 + 60 kg N ha ±1 . This combination of sowing density and N rates was the closest to common farming practice. Grain yields were lower for S high than for S cfp , presumably due to high competition between plants for nutrients and water. In conclusion, reducing or increasing sowing density compared to S cfp did not reduce immobilization (and losses) of fertilizer N and did not result in increased fertilizer N use eciency or grain yields.