Fate of labeled urea-15N as basal and topdressing applications in an irrigated wheat-maize rotation system in North China plain: II summer maize

A field micro-plot experiment for summer maize 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) on urea-15N fate, residual N effects and N recovery efficiency (NRE) by maize. The experiment included three N rates (90, 180, and 270 kg ha−1), divided by two 15N-labeled groups of basal-15N (30, 60, and 90 kg ha−1, respectively) and topdress-15N (60, 120, and 180 kg N ha−1, respectively). All of the treatments were irrigated two times, once at seeding time and once at topdressing time. The absorbed N in the maize plant derived from basal-N (6.8–13%) and topdress-N (17–30%) was identified. The residual N in the 0–150-cm soil depth ranged from 45 to 60% at the first maize harvest, mainly retained in the top 20-cm layers. Both NRE in grain and total N recovery in plant in the first maize crop were higher from topdress-15N (26–31 or 41–51%, respectively) than from basal-15N (18–23 or 34–43%, respectively). The residual N in the 0–150-cm soil layer was lower from topdress-15N (45–47%) than from basal-15N (55–60%) after the first maize harvest. Residual N recovery was 6–11% in the second and 1.5–3.5% in the third crop. Cumulative N recovery in the maize-wheat-maize rotations was higher from the topdress-15N (49–59%) than from basal-15N and (45–55%). The unaccounted N loss was 14–24% from the basal-15N and 20–33% from the topdress-15N, with a double dose of basal-15N application. An N rate of approximately 180 kg ha−1 appears to be an effective application rate to optimum maize yield and NRE on North China Plain, depending on the residual N and the crop yield potential.