A field micro-plot experiment for winter wheat was conducted in an irrigated winter wheat ( Triticum aestivum)-summer maize ( Zea mays L.) rotation system in Mazhuang, Xinji of Hebei province in the North China Plain, using the 15N isotope method to determine the effects of N application (rates and timing), and irrigation frequency on urea- 15N fate, residual-N and N recovery efficiency (NRE) of wheat. The experiment was conducted under two irrigation treatments (I2 and I3, representing for two and three irrigations, respectively), at three N rates (150, 210, and 270, kg ha -1), divided between two 15N-labeled applications of basal- 15N (90 kg ha -1) and topdress- 15N (60, 120, and 180, kg N ha -1, respectively). The total N uptake by wheat (ranging from 186 to 238 kg ha -1) and the fertilizer-derived N (Ndff, about 34-55%) were measured. The Ndff from labeled basal- 15N and from labeled topdress- 15N were about 15-22% and 16-40%, respectively. The NRE (measured either as recovery in grain or as the total N recovery in the plant) was higher with I3 (39-41 or 47-49%) than with I2 (35-40 or 42-47%), showing maximum NRE in grain of about 40% both at N210 with I2 and at N150 with I3 treatment. The NRE by the first wheat crop (in grain or the total N recovery in plant) was higher with labeled topdress- 15N (39-48 or 45-56%) as compared to that with labeled basal- 15N (30-37 or 36-45%), while the unaccounted N losses were lower with labeled basal- 15N (14-22%) relative to labeled topdress- 15N (14-35%). Higher residual N in soils was found with labeled basal- 15N (41-51%), as compared to labeled topdress- 15N (18-35%). Residual N in the 0- to 150-cm soil depth ranged from 26 to 44% while the unaccounted N losses ranged from 14 to 30%. Recovery of residual N by the 2nd and 3rd crops in the rotation was 5-10% in the maize crop and a further 1.7-3.5% in the subsequent wheat crop. The accumulated N recovery and the unaccounted N losses in continuous wheat-maize-wheat rotations derived from labeled topdress- 15N were 54-64% and 16-37%, respectively while they were 47-53% and 16-28%, respectively from labeled basal- 15N. This study also suggested that an N rate of 210 kg ha -1 (with a ratio of basal-N to topdress-N of 1:1.3) with two irrigation applications could optimize wheat grain yields and NRE, under the water limited conditions in North China Plain.
