Uptake and utilization of nitrogen applied to the foliage of winter wheat
Foliar urea was applied as a source of late-N between flag leaf emergence and anthesis to winter wheat crops that had received varying rates of basal-N fertilizer in the spring in order to produce crops with differently sized GAI). The crops grown were: no N fertilizer applied (NO), conventionally fertilized (Net), a Canopy Managed crop grown to a GAI 5 (GAI 5), at IACR-Rothamsted in 1995, with the addition of a GAI 3 crop (GAI 3) at IACR-Rothamsted in 1994 and at Sutton Bonington in 1995. Each of the applications of late-N as foliar urea resulted in the prolongation of GAI of Canopy Managed crops, irrespective of the timing, amount of N applied, or whether adjuvants were used. The date of complete death of canopy green area was similar for all foliar urea treatments due to the sunny, warm, dry weather at the end of July in both 1994 and 1995, at both sites. The duration of canopy green area was associated with its N content at anthesis, as well as with water availability and the prevailing weather conditions, such that Ncf crops, containing significantly more N than GAI 5 crops at anthesis, retained green area for a longer period than the GAI 5 crops. The application of foliar urea did not always result in an increase in grain yield or quality and the partitioning of biomass and N to the grain was also seemingly unaffected by the application of foliar urea. However, yields from GAI 5 crops receiving late-N as foliar urea, irrespective of the method of application, were not significantly different to those obtained from Ncf crops. The amount of N deposited and the pattern of deposition were affected by canopy size. Applications made prior to ear emergence penetrated more deeply into the canopy. The top half of the canopy, flag leaf to flag-1 and the ear when present, was the most important site for both N interception and uptake. A maximum of 60 % of the applied N was intercepted by the GAI 5 crops and 10 % remained on the surface of the crop 96 hours later. 35 % of the N 'lost' from the crop surface was taken up over 96 hours. Of the remaining 40 % of the applied N, an estimated 10 % was lost by volatilization, 5 % by drift and 25 % penetrated to the soil surface. N uptake from the leaf surface probably followed an exponential pattern through time. The time for half of the N present initially to be lost (t0.5), was unaffected by the side of the leaf to which N was applied, the age of the leaf growth stage of the plant or the amount of N applied. t0.5 was improved only by the addition of a spreader or a penetrant. By harvest up to 64 % of the fertilizer N applied was recovered by the GAI 5 crop of which 87 % was present in the grain. Studies using N15 labelled urea suggested that N was transported away from the flag leaf immediately after application, but it was not clear whether N was transported directly to the ear or used in leaf metabolism elsewhere.