The relationship between the fate of nitrogen (N) fertilizer and the N application rate in paddy fields in Northeast China is unclear, as is the fate of residual N.  To clarify these issues, paddy field and ¹⁵N microplot experiments were carried out in 2017 and 2018, with N applications at five levels: 0, 75, 105, 135 and 165 kg N ha^–1 (N0, N75, N105, N135 and N165, respectively).  ¹⁵N-labeled urea was applied to the microplots in 2017, and the same amount of unlabeled urea was applied in 2018.  Ammonia (NH₃) volatilization, leaching, surface runoff, rice yield, the N contents and ¹⁵N abundances of both plants and soil were analyzed.  The results indicated a linear platform model for rice yield and the application rate of N fertilizer, and the optimal rate was 135 kg N ha^–1.  N uptake increased with an increasing N rate, and the recovery efficiency of applied N (RE_N) values of the difference subtraction method were 45.23 and 56.98% on average in 2017 and 2018, respectively.  The RE_N was the highest at the N rate of 135 kg ha^–1 in 2017 and it was insignificantly affected by the N application rate in 2018, while the agronomic efficiency of applied N (AE_N) and physiological efficiency of applied N (PE_N) decreased significantly when excessive N was applied.  N loss through NH₃ volatilization, leaching and surface runoff was low in the paddy fields in Northeast China.  NH₃ volatilization accounted for 0.81 and 2.99% of the total N application in 2017 and 2018, respectively.  On average, the leaching and surface runoff rates were 4.45% and less than 1.05%, respectively, but the apparent denitrification loss was approximately 42.63%.  The residual N fertilizer in the soil layer (0–40 cm) was 18.37–31.81 kg N ha^–1 in 2017, and the residual rate was 19.28–24.50%.  Residual ¹⁵N from fertilizer in the soil increased significantly with increasing N fertilizer, which was mainly concentrated in the 0–10 cm soil layer, accounting for 58.45–83.54% of the total residual N, and decreased with increasing depth.  While the ratio of residual N in the 0–10 cm soil layer to that in the 0–40 cm soil layer was decreased with increasing N application.  Furthermore, of the residual N, approximately 5.4% was taken up on average in the following season and 50.2% was lost, but 44.4% remained in the soil.  Hence, the amount of applied N fertilizer should be reduced appropriately due to the high residual N in paddy fields in Northeast China.  The appropriate N fertilizer rate in the northern fields in China was determined to be 105–135 kg N ha^–1 in order to achieve a balance between rice yield and high N fertilizer uptake.

A plot experiment including four treatments, CK (N 105 kg ha-1 as urea, including a basal N application of 35 kg ha-1 and a topdressing N 70 kg ha-1 at turned green stage) and optimized N management (OPT1, OPT2 and OPT3, applied two-thirds, one-third and two-fifths N at jointing stage, respectively, total N 60 kg ha-1), was conducted to evaluate the effects of nitrogen management on growth and N uptake of winter wheat (Triticum aestivum), Dongnong 1, which is the first highly cold tolerant winter wheat in China. Index of population quality, N uptake and yield were determined. The ear-bearing tiller rate was increased by above 12%, and the leaf area index, biomass and N uptake were significantly decreased (P<0.05) at jointing stage. OPT treatments increased the grain to leaf area ratio at heading stage, the dry matter weight and N uptake after heading by 14.3-27.9%, 11.6-28.7% and 118.1-161.8 %, respectively. The yield of the OPT treatments was increased by 14.2-37.5% compared with CK, and there was a significant difference (P<0.05) between CK and OPT1 treatments. Harvest index and N partial factor productivity (PFP, kg grain yield per kg N applied) was clearly enhanced from 0.4 and 35.6 kg, respectively for CK to an average of 0.48 (P<0.05) and 77.6 kg (P<0.05) in the OPT treatments. These results indicated that the optimized N management increased the harvest index, yield and N use efficiency by decreasing the N application rate and postponing N application time, improved wheat population quality, controlled excessive growth in the vegetative stages and increased dry matter and N accumulation rates after heading.