Fate of fertilizer nitrogen and residual nitrogen in paddy soil in Northeast China

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 (REN) values of the difference subtraction method were 45.23 and 56.98% on average in 2017 and 2018, respectively. The REN 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 (AEN) and physiological efficiency of applied N (PEN) 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.