Estimation of nitrogen supply for winter wheat production through a long-term field trial in China

Excessive synthetic nitrogen (N) applications, high mineral N accumulation and low N use efficiency (NUE) are current issues in intensively cultivated winter wheat production system impeding the sustainable development of agriculture in China. To solve these problems, soil accumulated N in the top 1 m of the soil profile before sowing (N_soil), returned straw-N from the previous maize crop (N_straw) and fertilizer N application (N_fertilizer) should be comprehensively considered N supply sources in N management. As such, the objective of this research was to determine the optimal total N supply (TN_supply) level needed to meet crop requirements while minimizing environmental impacts. A 9-year on-farm experiment was conducted in accordance with a split-plot design involving two different fertilizer management systems (main treatments) and three N application strategies (sub treatments). Extensive TN_supply levels (ranging from 61 kg ha^-1 to 813 kg ha^-1) were detected, and relative yield (RY), N input and N output in response to the TN_supply were measured. The relationships between TN_supply and RY, N input, and N output strongly fit linear-plateau, linear, and linear-plateau models, respectively. The minimum TN_supply levels needed to achieve the maximum RY and N output were 325 and 392 kg ha^-1, respectively. On the basis of N supply capacity, the TN_supply was removed from the growing system by 61% (N input). As the N input increased past 209 kg ha^-1, the NUE declined, at which point the TN_supply reached 433 kg ha^-1. Therefore, the suitable TN_supply should range from 325 kg ha^-1 (ensuring a total N supply for high yield and N uptake) to 433 kg ha^-1 (obtaining a relatively higher NUE and less N loss to the environment). The TN_supply was highlighted to be an indicator for use in N management recommendations. Considering the average high N accumulation in winter wheat production systems, N management should essentially take into account the consumption of N_soil, the levels of N_straw and the minimum application of N_fertilizer to obtain high yields while minimizing environmental impacts under suitable TN_supply levels.