Real nitrogen use efficiency of crop as affected by long-term fertilization in typical soils of China

【Objectives】 Nitrogen use efficiency (NUE) or apparent NUE (ANUE) is the percentage of applied N uptaken by one season crop, which does not consider the N residue in soil. Long-term ferlilization experiment is able to reflect the variation of soil N storage. In this article, we analyzed the ANUE and real NUE (RNUE) in different fertilization trentments using long-term experiments to calculate real contribution and real loss of N fertilizer in agricultural production. 【Methods】 Four long-term experiments (31 years) were chosen in this study, aqni-cinnamon soil in Beijing and fluvo-aquic soil in Kaifeng with cropping system of wheat-corn rotation, and purplish soil in Sichuan and yellow-brown soil in Hubei with cropping system of rice-wheat rotation. Crop and soil samples were collected from the treatments of no fertilizer(CK), chemical N(N), chemical N, P and K fertilizers(NPK), and NPK comlined with manure(NPKM). The aurnual N uptake by crop, soil total N content were analyzed, and the ANUE and RNUE were calculated. 【Results】 The average RNUEs are 47.6%, 56.6%, 57.0% and 56.3% in aqui-cinnamon soil, fluvo-aquic soil, purplish soil and yellow-brown soil under the three fertilization, respectively, which are significantly higher than ANUE (33.6%, 42.1%, 37.8% and 25.8%). RNUEs in purplish soil and yellow-brown soil are 18.6%-32.9% higher, and in aqui-cinnamon soil and fluvo-aquic soil 10.9%-17.5% higher than ANUEs, indicating that the traditional NUE is underestimated at a higher extent in upland-paddy soil than in upland soil. The ANUE and RNUE are increased annually at rates of 1.76%-2.49% and 1.50%-2.29%, respectively, in upland soils, under the NPK applied treatments (NPK and NPKM), attributed to the N 17.5-37.3 mg/(kga) increase of soil total N content, significant increases are observed at RNUE but not at ANUE during the fertilization period in yellow-brown soil, indicating that of the NPK and NPKM treatments could improve the N accumulation in soil with a higher extent in upland-paddy soils than in upland soils.【Conclusions】 NUE is generally underestimated about 20% in China, especially in the paddy-upland rotation with a high change of soil total N.