Abstract: The research on nutrient balance is the premise to study agricultural activities and environmental pollution of agroecosystem. This paper, choosing a typical agroforestry ecosystem located in Sichuan hilly area in upper reaches of Three Gorges, discussed the nitrogen balances and appraised their reasonability wit h the way of inspection in farmlands and forestry lands, and of modeling experiments in basins and arable lands and participatory rural appraisal. The results showed that there was a little nitrogen surplus in croplands and in the whole farming system, with balance of 56.1 kg/hm2·a and 42 kg/hm2·a，actual balance rates of 10.4% and 8.2%；contrarily, a little deficit in paddies and in the catchments, with balance of -13.21 kg/hm2·a and -37.5 kg/hm2·a ，actual balance rates of -3.2% and -7.8% respectively. However, the forest showed more deficit with a balance of -165.9 kg/hm\+2·a and with actual balance rate of -39.2%. The allowable rates (23.4% for croplands and -37.2% for paddy) calculated out by mathematical appraisal model showed that, the nitrogen balances of croplands and paddies were relatively reasonable. Comparing nowadays' nitrogen balance with that of 1960s, it is clear that much more attention should be paid to the forest because it plays a great pole in reducing soil and water loss and changing the nitrogen cycling way in ecosystem.

Key words: N balance, nutrient balance appraisal, agroforestry ecosystem