| The precipitation is not stable in dryland farming area, it is also difficult to support water according to crop demand in irrigation area, water and fertilizer can not cooperation well according to people expect. In order to develop the effects of water and nutrient fully, it must to be done firstly to decide the key stage and compensatory effect of water and nutrient cooperation, so that it fits to unstable precipitation. Therefore, it is very important to study the condition and mechanism of water and nutrient cooperation, in order to develop the water and nutrient interaction, to improve the use efficiency. of water and fertilizer. The pot and field experiments were conducted to study the effects and mechanism of nitrogen application at different growth stage on the growth and development of winter wheat and summer maize under different status of soil moisture by the complete and quadratic general spinning design in this paper. The main results are as follows: 1. Under normal irrigated conditions, Nitrogen stress at tillering and jointing stage could reduce the tillering number and dry matter weight of shoot and root significantly, had significant effect on the growth and development of winter wheat root. It could reduce not only the root length,number and volume remarkably, but also the total absorbing area, active absorbing area, active absorbing area percentage and TTC reducing capacity. Therefore, it could reduce the field of winter wheat. The results indicated that the sensitive stage of winter wheat responses to nitrogen stress is tillering and jointing stage, the compensative stage is jointing stage.2. The effects of N fertilizer were inhibited seriously by water stress, only with the condition of water and Fertilizer coordination, it was developed abundantly. Thereby, N fertilizer could not only increase winter wheat yield, but also improve the qualities of wheat grains. The jointing stage is the key and sensitive stage of winter wheat responses to water and nitrogen coordination.The N Fertilizer applied at this stage could not only increase the winter wheat yield ,but also increase the content of free amino acid and protein of grain. The mechanism is that it could promote the growth and development of shoot, accelerate the absorption and usage of N,P,K nutrients, while, postpone the root to be old, contain the root activities at late stage.3. The diversity of different levels of three factors was striking, jointing stage was the best one among three growth stage, N3 and H3 were the best one among different nitrogen and water levels respectively. The interactive effect of water and nitrogen was positive in seedling stage and jointing stage. Under the condition of this experiment, the nitrogen effect is more great than water effect. So far as the growth stage corporation, jointing stage and trumpet mouth stage corporation was the best one among different corporations for water supply; seedling stage and jointing stage corporation was the best one for nitrogen supply and the cooperation of water and nitrogen. After of all, jointing stage is the key stage of water and nitrogen supply for summer maize. 4. The pot experiment was conducted to study the effects of nitrogen application rate and stage on summer maize water use efficiency under different status of soil moisture by using the quadratic general spinning design. The results showed that the effect of nitrogen fertilizer on WUE is more great than that of soil moisture, but the N application stage for WUE is different between WUE base to grain yield and biological yield. Effects of N application in seedling stage and milking stage on WUE base to grain yield are more great than in other stage ,while that of N application in seedling stage and jointing stage on WUE base to biological yield are more great than in other stage ï¼›From the single factor effect,we found that the best N application rate and soil moisture are 0.2g N/kg soil and 21% respectively ,not the more N application rate and soil moisture ,the more WUE ;The key stage fo...
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