| Statistical analysis on Nitrogen Use Efficiency (NUE) and relevant data of rice, wheat and maize growing on Black soil, Fluvo-aquic soil, Aquic soil, Loess soil, Purple soil , Red soil, Yellow brown soil and Paddy soil under 15～24 year long term fertilization at 13 representative points over the country was conducted in order to understand the change in the nitrogen temporal evolution and spatial variability under a long-term fertilization, which could provide information for guiding the rational fertilization in future in the country. The main results were as follows:1. The nitrogen use efficiency under different long term fertilizations varied over time significantly. The nitrogen use efficiency by maize and wheat with only nitrogen fertilizer applied decreased at 1.0%~3.2% per year over time, which was significant from that of other treatments. If nitrogen and phosphorus (NP) were applied together, the nitrogen use efficiency by maize and rice did not vary significantly over time, The nitrogen use efficiency by all three kinds of crops were among 30.9～73.8% per year. If nitrogen and potassium (NK) were applied together, the nitrogen use efficiency by wheat, maize and rice showed different change trends over time. The nitrogen use efficiency by wheat decreased over time on Fluvo-aquic soil, Aquic soil, Purple soil and Red soil at 1.5% , 1.9% , 2.1% and 3.7% annually. The nitrogen use efficiency by maize and rice appeared stable mainly, which explained when phosphate fertilizer was not applied, the wheat's ability using nitrogen was smaller than that of maize and rice. When nitrogen, phosphorus and potassium (NPK) were applied together with organic fertilizer (NPKM), crop nitrogen use efficiency did not change significantly or increased slightly over time. The nitrogen use efficiency between NPK and NPKM was not significantly different over time.2. The nitrogen use efficiency of wheat and maize in different soils appeared obviously different with spatial difference. When nitrogen, phosphorus and potassium were applied together the nitrogen use efficiency by wheat on Loess soil, Fluvo-aquic soil, Aquic soil in north warm temperate zone (68.9%) was higher than that on Purple soil in subtropics (41.7%) and Black soil in north middle temperate zone (33.8%). The nitrogen use efficiency by maize on Purple soil in southern subtropics and on Black soil in north middle temperate region was higher than that on Fluvo-aquic soil, Aquic soil and Loess soil in north warm temperate zone. The nitrogen use efficiency by rice did not show obvious difference over climatic zones. This was maybe because paddy soils were mostly located in southern China and artificial management effects was more important than in non-rice growing practice.3. Crop nitrogen use efficiency space-time difference was mainly affected by soil characters and crop rotations. When nitrogen, phosphorus and potassium were applied together, three kinds of crops'nitrogen use efficiency was significantly correlated with initial organic matter content and initial pH values. The nitrogen use efficiency by wheat was negatively correlated with total soil nitrogen content. The nitrogen use efficiency by maize was positively correlated with available soil nitrogen content. The nitrogen use efficiency by rice was positively correlated with soil organic matter content.The nitrogen use efficiency by wheat at wheat- maize rotation (68.9%) was greater than that at two crops a year rice field-upland field rotation (41.7%) and one crop a year (33.8%); the nitrogen use efficiency by maize at two crops a year with continuous maize cropping (94.9%) and one crop a year only maize (90.3%) was greater than that at two crops a year wheat- maize rotation (40.8%). The nitrogen use efficiency by rice at two crops a year rice- rice rotation (55.5%) was slightly greater than at rice field-upland field rotation (47.7%).
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