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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 15N-labeling method was introduced to study the fate of the RSN quantitatively during the winter wheat growing season by 15N 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 15N 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 15N-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.

Cropping systems is potentially useful practice for mitigating nitrate leaching and increasing nitrogen use efficiency (NUE) because deeper rooted crops could capture the leached N from previous shallower rooted crops. The recovery of fertilizer nitrogen (FN) applied to maize (Zea mays L.) by the succeeding wheat (Triticum aestivum L.) in a winter wheat―summer maize rotation in North China Plain (NCP) was studied in 2003―2004 and 2005―2006 by creating 15 N-labeled soil columns. We found that the in-season maize plant use efficiency ofFN was about 46 to 48% and the total residual fertilizer nitrogen (RFN) in a 2-m soil profile at maize harvest was 34% in 2003―2004 and 41% in 2005―2006, the ratios ofFN unaccounted for were about 19 and 12%, respectively. The RFN declined significantly down the soil profile. About 13 and 10% of total RFN, or 4.4 and 4.0% of total applied FN were observed below the 1.2 m soil depth. Wheat was able to recover the RFN up to the 2.0-m soil depth, the recovery ratio from upper soil layers was higher than that from lower layers and ranged from 4.4 to 23% in 2003―2004 and 4.6 to 34% in 2005― 2 006. Corresponding total recovery ratios were 13.7 and 21.4% of total RFN, or 4.9 and 8.7% of total applied FN, respectively. The annual recovery ratios were 51.6% in 2003―2004 and 56.2% in 2005―2006. The deep-rooting characteristics of winter wheat for deep N uptake should be taken as an important index in N fertilizer management in the rotation system.

In winter wheat ( Triticum aestivum L.)–summer maize ( Zea mays L.) rotation system in the North China Plain, maize roots do not extend beyond 1.2 m in the vertical soil profile, but wheat roots can reach up to 2.0 m. Increases in soil nitrate content at maize harvest and significant reductions after winter wheat harvest were observed in the 1.4–2.0 m depth under field conditions. The recovery of 15 N isotope (calcium nitrate) from various (1.0, 1.2, 1.4, 1.6, 1.8 and 2.0 m) soil depths showed that deep-rooting winter wheat could use soil nitrate up to the 2.0 m depth. This accounted partially, for the reduced nitrate in the 1.4–2.0 m depth of the soil after harvest of wheat in the rotation system.