Nitrogen Fate and Efficiency of Fertilizer Application under a Rapeseed-Wheat-Rice Rotation System in Southwest China

To evaluate the efficient use of nitrogen (N) for rice in a rapeseed-wheat-rice rotation system, a pot experiment was conducted. The results indicated that in the conventional N-15-labeled (Nc) and reduced N-15-labeled (Nr) urea applications, absorbed N and soil residual N was higher in rapeseed than in wheat. In the rice season, the higher accumulation of N-15 was achieved with an Nr application rate during the rapeseed season and an N fertilizer management model (40% as basal fertilizer, 40% as tillering fertilizer, and 20% as panicle fertilizer) during the rice season (PrNrM3). A high N-15 accumulation was also achieved under the Nc application rate during the wheat season and the N fertilizer management model during the rice season (PwNcM3). The accumulation of N-15 in PrNrM3 and PwNcM3 accounted for 21.35% and 36.72% of the residual N under the Nr application rate in the rapeseed season and the Nc application rate in the wheat season, respectively. Compared with the Nc application rate in the rapeseed season and M3 N management in the rice season (PrNcM3), the N agronomy efficiency (NAE) and the N partial factor efficiency (NPFP) of rice were increased by 23.85% and 1.59%, respectively, in PrNrM3. The annual crop yield was 3.95% lower in PrNrM3, which was not significant. PrNrM3 was a stable yield, N-saving application rate for rapeseed-rice rotation systems in southern China.