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Characteristics of energy input and output in fruit tree-grain crop lntercropping ecosystem--A case study from dry valley region in the upper reaches of the MinJinng River Chemical fertilizers input and soil nutrient resources in winter wheat-corn rotation system in Huantai County Nitrogen balance of agro-ecosystem in a typical watershed in the hilly area of central Sichuan Basin Changes of soil nutrient contents and input of nutrients in arable of China