Effect of continuous reduction of nitrogen application to a rice-wheat rotation system in the middle-lower Yangtze River region (2013-2015)

Rice (Oryza sativa L.) and wheat (Triticum aestivum L.) are two stable food crops that play a vital role in national food security. Summer rice-winter wheat double-cropping rotation is the dominant crop rotation practice used in the middle-lower Yangtze River region of China. However, excessive application of nitrogen (N) fertilizer in this region has led to low nitrogen use efficiency (NUE). To our knowledge, no studies have investigated the effects of reducing N fertilizer applications during both the rice and wheat seasons on crop yield, NUE, and soil fertility. In this study, we conducted a two-year field experiment using N fertilizer rates of 180, 240, and 300 kg N ha−1 during the rice growing season and 135, 180, and 225 kg N ha−1 during the wheat growing season. No differences in yield were observed among the treatments during the rice growing season; however, reduced N fertilizer application significantly affected yield during the wheat growing season. Reducing the amount of N fertilizer applied during the previous season and current season and the interactions between these seasons (R × W) had no effect on rice yield but did affect wheat yield. In addition, the N application rates significantly influenced N uptake and NUE. During the rice growing season, the N agronomic efficiency (NAE), N physiological efficiency (NPE), N partial factor productivity (NPF), and N recovery efficiency (NRE) increased by 20.6-42.5%, 11.1-15.8%, 23.9-40.2%, and 4.8-28.2%, respectively. During the wheat growing season, the NAE, NPE, NPF, and NRE increased by 20.4-54.9%, 8.2-16.5%, 21.8-58.3%, and 11.6-24.4%, respectively. We applied 15N-labelled urea in the remainder of the soil N fertilization treatments, which indicated that the addition of 15N fertilizer resulted in no difference in the 0-60 cm soil profile after rice harvest. Collectively, reducing N fertilizer application rates can effectively improve NUE and decrease N losses, and short-term reductions in N fertilizer application do not affect soil fertility.