Strategies to improve the use efficiency of mineral fertilizer nitrogen applied to winter wheat

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 microorganisins 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.