| Loess Plateau was a typical degraded eco-system, its productivity was very low. How to increase its agriculture productivity was the key factor to improve eco-environment and warrant ecosystem's sustainable development. Studying mechanism of shaping and transformation relations of crop production potential and actual productivity could find the obstacle factors impeding increasing yield. Through which, we also could provide grounds for agricultural production planning, as to establishment of harmonious development stratagem of population, resources, environment, boosting sustainable development of the eco-system. Based on many years' climate data and experimental results of Changwu demonstration district, Erosion-Productivity Impact Calculator model was used to simulate the cropland ecosystem productivity in Loess Plateau highland. It was showed that EPIC model has a fairly good adaptability in this region, limit factors to increase cropland productivity were analyzed. Through pot experiment, taking summer maize as example, Jensen's Model was applied to study he effect of N> P nutrition on crop's water sensitivity index and physiological and ecological parameters. Main results were as follows:1. The paper computed the temperature and climate potential productivity of changwu district for many years using arid land crop production potential computation equation; compared with practical yield, winter wheat and spring maize being put as example. The result show that: the wheat's average heat potential productivity for many years of the districtwas 6910.5 kg/hm , that of maize 10718.6 kg/hm , variance between years not big. the wheat's average climate potential productivity5904.6 kg/hm2, that of maize is 8148.4 kg/hm2; variance between years very large.2. Taking maize and wheat as example, we tried the use of EPIC model in Loess Plateau, amended some parameters such as soil property and crop physiological ecological characteristics, simulated yield and stress conditions of field crop in Loess Plateau. It was proved that there was a good adaptability for EPIC model to use in loess plateau for cropproductivity simulation research. Results implied that: the average deviation between simulated and measured values for wheat is 7.78%, that of maize 9.60%. It can be concluded that the amendment of parameters was reasonable, EPIC has a good adaptability in this region. Water stress days of wheat is 9.9d, the minimum is 1.7d, the maximum is 23. Id; that of maize 13.4d, Lid, 22.Id; which is in accordance with actual rainfall condition. The model is very accurate in normal years, but in arid years, it is necessary to improve the method of amending the crop and soil parameters.3. Water sensitivity index of summer maize reached the highest value during booting toheading stage and heading to filling stage; it became comparatively low during the stage before booting and after filling. Higher fertilization level increased the water sensibility index greater in key water demand stage than in unimportant water-supply stage.4. Drought stress hindered photosynthesis rate of maize leaf, Higher N P increased photosynthesis rate, plant under higher N was more sensitive than that under low nitrogen, impact of P on Pn under water stress was greater than that under sufficient water condition. Use of N, P boosted maize leaf's transpiration rate, increasing of E affected by higher N was significant than low N when water supply was not sufficient; but it was not significant when water supply was sufficient. Water use efficiency of maize leaf ameliorated as water supply was improved, impact of P on WUE was higher than that of N. The dry root weight under high N level was lower than that under low N level; use of P boosted root growth obviously, and the enhancement under medium drought was greater than that under sufficient water supply. The increasing of above-ground Biomass under different P treatment was higher than that under different N treatment distinctly. The root-stem ratio was increased alo...
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