Fertilizer nitrogen use efficiency and its fate in the spring wheat-soil system under varying N-fertilizer rates: A two-year field study using 15N tracer

Wheat ( Triticum aestivum L.) is a globally significant staple crop and a primary source of food. Excessive nitrogen (N) fertilization in wheat farming systems has resulted in lower N use efficiency (NUE) in arid regions of China. We hypothesize that prolonged N fertilization alters the fate of fertilizer-derived N, thus affecting NUE. To test this hypothesis, we conducted two on-farm 15 N tracer experiments during the 2021–2022 growing seasons, within a long-term N fertilization trial on a monoculture spring wheat field. The trial included five N-fertilizer rate treatments: N1 (0 kg N ha −1 ), N2 (52.5 kg N ha −1 ), N3 (105 kg N ha −1 ), N4 (157.5 kg N ha −1 ), and N5 (210 kg N ha −1 ). The results showed that the majority of N absorbed by spring wheat was sourced from soil N, accounting for 65.90–81.45 % at maturity, while fertilizer-derived N contributed 18.55–34.10 %. The fertilizer N recovery rate (N fr ) ranged from 21.47 % to 43.98 %, the residual N in 0–100 cm soil varied between 14.32 % and 46.59 %, and the loss rate ranged from 9.43 % to 62.41 %. As N fertilizer rates increased, N fr declined, while residual and loss rates significantly increased. The optimal N fertilization rate was approximately 105 kg N ha −1 , at which we observed a high N fertilizer contribution rate (30.59 %), N fertilizer physiological efficiency of 10.28 kg kg −1 , and a soil N dependency rate of 53.97 %. These findings suggest that the balance between external N accumulation and soil N utilization is a key determinant for maintaining sustainable soil N fertility. Future research should prioritize the investigation of long-term dynamics of fertilizer NUE, with a particular emphasis on the role of nitrogen fertilizers in soil nitrogen depletion and accrual. • Soil N was the primary N source, and the proportion of N uptake from fertilizers was only 18.55–34.10 %. • Higher N-fertilizer rates enhance crop fertilizer N uptake, but inhibit soil N uptake. • 105 kg N ha −1 was the best rate for improving the N fertilizer contribution rate.