Globally, urea is the cheapest nitrogenous fertilizer and covers around 50% of the other inorganic N fertilizers used for agricultural production. Efforts are being continuing to increase N utilization efficiency and to minimize its loss from agroecosystems through either developing improved management practices or limiting its release and transformations by chemical means. Research findings revealed slow release and urease/nitrification inhibition characteristics of large urea granules. Thus, we compared the influence of prilled urea (PU) mixed into soil with urea super granules (USG, ~0.70 g), which was point-placed either at 7.5 cm (experiment 1) or at 2.5, 5.0 and 7.5 cm soil depths (experiment 2) on the distribution of fertilizer N, yield response and N recovery by spring wheat in a loess silt loam using ¹⁵N isotope technique. Both experiments were carried out under greenhouse conditions using polyvinyl boxes (110 cm x 70 cm x 40 cm). Each box was filled with 5 cm sand on the bottom followed by 20 cm subsurface and 15 cm surface soil, which was packed to reach a bulk density of 1.33 g cm^-3. ¹⁵N labelled urea was applied at a rate of 88.2 (2.87 atom %) and 91.7 (5.11 atom %) kg N ha^-1 for experiment 1 and 2, respectively. Both PU and USG were applied as a single dose before sowing of spring wheat (Triticum aestivum L., cv. Thassos). Composite soil samples were collected next to (1.5 cm) and 5.0 cm away from the USG placement zone for ¹⁵N analyses. The soil sampling depths were 0-15, 15-30 and 30-40 cm for experiment 1, and 0-10, 10-20 and 20-40 cm for experiment 2. Plant samples were collected 3-4 times during the growth periods to analyse ¹⁵N enrichment. The crop was harvested at maturity (116 days after sowing, DAS) for experiment 1 and at 70 DAS for experiment 2. High concentration of total ¹⁵N at the zone of USG placement decreased with time and its movement occurred up to 5 cm distance. The share of total ¹⁵N in the surface layer was >50% compared to the lower depths. Urea sizes had significant influences on total dry matter (TDM) yields at harvest, unlike during early growth periods (until 67 DAS). TDM at 70 DAS responded identically to various depths of the USG placement, which differed significantly only with the unfertilized control. The USG delayed but did not limit fertilizer N uptake by the crop. The translocation of ¹⁵N from vegetative parts to grain portions during grain-filling stage (from 67 to 116 DAS) was 34.9% with the USG and 28.7% with the PU, resulting in higher grain yield (711 kg ha^-1) with the former than the latter. The uptake of fertilizer N by the crop took place earlier in experiment 2, attributing to the relatively higher (ranges, 51-68% water-filled pore space, WFPS) but constant soil water content than experiment 1 (43-67% WFPS). Irrespective of urea sizes, fertilizer N recovery in crop (FNR_c) increased linearly until day 67 or 70 for both experiments, being highest with the USG point-placed at 5.0-7.5 cm (75-78%). At maturity for experiment 1, the FNR_c was 70.5% for the USG point-placed at 7.5 cm depth and 56.6% for the PU mixed into the soil.  As such, the enrichment of soil ¹⁵N pool was greater with the PU (28.2%) than the USG applied in either experiment (14.6-16.4%). Priming effect was evidenced in both experiments, ranging from 10.8 to 44.8 kg N ha^-1. The increased soil N uptake by the crop over time implied the influence of rhizosphere priming effect during rapid growth periods. Considering residual ¹⁵N in crown and roots, the unaccounted ¹⁵N for PU mixed into the soil (6.0% of the added N) and USG point-placed at 2.5 cm (15.7%) was higher than the USG point-placed at 5.0-7.5 cm (2.6-4.8%). Results suggest that the USG technique could increase yields and fertilizer N use efficiency of spring wheat and decrease gaseous N losses over the PU.

Keywords: ¹⁵N-urea, urea granule sizes, N use efficiency, spring wheat, priming effect