Quantitative study on the fate of residual soil nitrate in winter wheat based on a (15)N-labeling method

A considerable amount of surplus nitrogen (N), which primarily takes the form of nitrate, accumulates in the soil profile after harvesting crops from an intensive production system in the North China Plain. The residual soil nitrate (RSN) is a key factor that is included in the N recommendation algorithm. Quantifying the utilization and losses of RSN is a fundamental necessity for optimizing crop N management, improving N use efficiency, and reducing the impact derived from farmland N losses on the environment. In this study, a (15)N-labeling method was introduced to study the fate of the RSN quantitatively during the winter wheat growing season by (15)N tracer technique combined with a soil column study. A soil column with a 2 m height was vertically divided into 10 20-cm layers, and the RSN in each layer was individually labeled with a (15)N tracer before the wheat was sown. The results indicated that approximately 17.68% of the crop N derived from RSN was located in the 0–2 m soil profile prior to wheat sowing. The wheat recovery proportions of RSN at various layers ranged from 0.21% to 33.46%. The percentages that still remained in the soil profile after the wheat harvest ranged from 47.08% to 75.44%, and 19.46–32.64% of the RSN was unaccounted for. Upward and downward movements in the RSN were observed, and the maximum upward and downward distances were 40 cm and 100 cm, respectively. In general, the (15)N-labeling method contributes to a deeper understanding of the fates of the RSN. Considering the low crop recovery of the RSN from deep soil layers, water and N saving practices should be adopted during crop production.