Recovery efficiency and loss of 15N-labelled urea in a rice–soil system in the upper reaches of the Yellow River basin

Abstract Chemical N input is essential for high rice yields. However, low recovery efficiency of chemical fertilizer N with flooding irrigation in the anthropogenic-alluvial soil resulted in N lost from the rice field in Ningxia Irrigation Region in the upper reaches of the Yellow River. With the technique of stable isotope 15 N-traced urea, we conducted two years experiment to estimate the recovery efficiency and loss of applied chemical fertilizer in a rice field. The three fertilizer N treatments included 300 kg ha −1 (N300, the conventional application rate), 240 kg ha −1 (the optimized N application amount, N240) and no N fertilizer application treatment (CK). We estimated the recovery of 15 N-labelled urea in grain, straw and root of rice and residues in soil profile. The 15 N not accounted for in the plant and soil was presumably lost. The results showed that more efficient use of N fertilizer could allow current N application rates to reduce by 20%. This would still maintain crop yields while substantially reducing N losses to the environment. The high N fertilizer application increased the N uptake by rice derived from fertilizer, and the amount of N rice taken up from soil reduced correspondingly, which resulted in the higher N surplus in soil. Under the conventional irrigation and fertilizer management level, the recovery rate of 15 N-labelled urea in rice–soil system was about 48–49%. The 15 N-labelled fertilizer recovery in rice plant ( N dff ) was 26–30%. In the paddy soil profiles of 0–90 cm, the residual of 15 N-labelled fertilizer in soil ( N dfs ) were 54–70 kg ha −1 , and N residual rate in soils were 18–23%. The annual N loss from the rice field in the Irrigation Region was 28,865 tons. The distribution of 15 N abundance variability in different soil profile indicated that fertilizer N leached into the deep soil layers along with irrigation water as a result of continuous yearly rice planting. Optimization of nitrogen fertilizer can significantly reduce the amount of N residuals and N loss from the paddy field. Compared with N300, optimized nitrogen fertilizer application could decrease the loss of fertilizer N by 22–34 kg ha −1 , and reduce the amount of N surplus by 26–33 kg ha −1 while the dry matter of rice increased 8–15% and N uptake by rice increased 2–6%. Considering the high food production and the minimum environmental threat, we should fully take into account the optimization application by reducing fertilizer N inputs. However, the interaction between irrigation management and N application rate on N use efficiency in alkaline anthropogenic-alluvial soil needs to be further studied.