[Effects of long-term fertilization on nitrogen fertilizer use efficiency in loessial soil region on the Loess Plateau, Northwest China]

Nitrogen fertilizer use efficiency is a key parameter to determine recommended nitrogen fertilizer amount and evaluate the effects of nitrogen fertilizer application. The effects of long-term fertilization on annual and accumulated nitrogen fertilizer use efficiency and the relationship between them were studied using a 34-year fertilization experiment (1981-2015) in loessial soil region. The results showed that fertilization had significant influence on annual and accumulated nitrogen ferti-lizer use efficiency of wheat, rape and flax in the period of 1983-2015. The highest mean annual nitrogen fertilizer use efficiency of wheat was achieved in N, P and K combination (NPK), thesecond was N combined with P (NP), 77.7% and 62.0% higher than N application alone respectively. The highest mean annual nitrogen fertilizer use efficiency of rape was achieved in NPK, the second was manure (M), N, P and K combination (MNPK), 93.7% and 65.6% higher than N application alone respectively. The annual nitrogenfertilizer use efficiency of M combined with N (MN) increased significantly compared with N application alone. The annual nitrogen fertilizer use efficiency of M, N and P combination (MNP) decreased significantly compared with NP. The annual nitrogen fertilizer use efficiency of MNPK decreased significantly compared with NPK.There was significant positive linear relationship between annual nitrogen fertilizer use efficiency and crop yield. The correlation between accumulated nitrogen fertilizer use efficiency and crop yield was not significant. It indicated that the annual nitrogen fertilizer use efficiency could be more effective in representing fertilizer use efficiency characteristics under given soil fertility levels, crop varieties and environmental conditions compared with accumulated nitrogen fertilizer use efficiency.