农林复合生态系统氮素平衡及其评价

农业生态系统中养分的平衡状况及其合理性评价，是探讨农业生产和环境变化的前提和基础。选择三峡库区上游川中丘陵区一典型农林复合生态系统，依据系统养分循环概念模型，进行了农田和林地定位监测、耕地和盆钵模拟试验以及参与式农户调查等，研究系统的氮素各收入支出参数，计算系统的氮素平衡并对平衡合理性进行评价。结果表明，本复合生态系统旱地和整个农田子系统氮素略有盈余，盈亏量分别为56.1 kg/hm2·a和42 kg/hm\2·a，实际盈亏率为10.4%和8.2%；而水稻田和复合系统有少量亏缺，盈亏量分别为-13.21 kg/hm2·a和-37.5 kg/hm2·a，实际盈亏率分别为为-3.2%和-7.8%；林地亏缺较大，盈亏量为-165.9 kg/hm2·a，实际盈亏率为-39.2%。模型计算的允许盈亏率（旱地23.4%，水稻田-37.2%）评价表明，旱地和水稻田目前的氮素平衡基本上是合理的。与1960s系统氮平衡比较，可以看到林地在减少水土流失、促进养分的合理循环方面发挥了重要作用，应受到充分的重视。

NITROGEN BALANCES AND APPRAISALS IN AGRO-FORESTRY ECOSYSTEM--A CASE STUDY IN YANTING EXPERIMENT STATION

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.