西双版纳热带季节雨林生态系统氮的生物地球化学循环研究

 用小流域集水区和物质平衡方法，于1999年对西双版纳热带季节雨林生态系统的氮素循环进行了初步研究。西双版纳季节雨林生态系统的氮库总储量为6 481.2 kg·hm-2，其中活体生物量、凋落物层和土壤（0～30 cm）中的氮储量分别为970.9、37.7、5 481.2 kg·hm-2。土壤中的氮占生态系统氮总储量的84.4%，活体生物量占15.0%，凋落物层仅占0.6%。结果表明季节雨林的氮主要分布在土壤中，而在生物量中只占很少部分。大气降水、林内穿透水、树干流及地表径流的氮含量分别为0.565、0.828、0.983和1.042 mg·dm-3，氮通量则分别为8.89、10.97、3.57、5.95 kg·hm-2·a-1。大气降水输入氮8.89 kg·hm-2·a-1，径流输出氮5.95 kg·hm-2·a-1， 收支平衡（输入—输出）为2.94 kg·hm-2·a-1。氮的生物循环：吸收为149.86 kg·hm-2·a-1，存留为69.30 kg·hm-2·a-1，归还为80.56 kg·hm-2·a-1，循环系数为0.54。结果表明未受干扰的季节雨林生态系统处于氮积累的状态，有利于该生态系统的稳定与持续发展。

BIOGEOCHEMICAL CYCLING OF NITROGEN AT A TROPICAL SEASONAL RAIN FOREST IN XISHUANGBANNA, SW CHINA

Xishuangbanna, located in southwest China, bounded by Laos and Myanmar on the upper Mekong River, is on the northernmost edge of tropical Asia. As a result, a mosaic distribution of tropical seasonal rain forest, which mainly appears in the wet valleys, usually with small streams or on the low hills and flats below 1000 m altitude exists in Xishuangbanna. The tropical seasonal rain forest in Xishuangbanna is rich in species and has a complex forest canopy and structure. In 1999, 119 species with DBH>10 cm were recorded in the 1 hm2 study plot. Based on the “small catchment” approach and input-output budgets, we studied on the nitrogen cycling in the seasonal rain forest. The nitrogen stock in the seasonal rain forest ecosystem (6 481.2 kg·hm-2) was higher than that of montane rain forest in Hainan of China (6 345 kg·hm-2), but lower than montane rain forest in New Guinea (20 190 kg·hm-2), lowland rain forest in Brazil (7 537 kg·hm-2), and lowland rain forest in Ghana (7 230 kg·hm-2). The distribution of the nitrogen stock in the seasonal rain forest ecosystem is as follows: 970.9 kg·hm-2 (15.0%) in the living biomass, 37.7 kg· hm-2 (0.6%) in the forest floor, and 5 481.2 kg·hm-2 (84.4%) in the soil (0-30 cm). We found that most of the nitrogen in the seasonal rain forest was stored in the soil, but not in the living biomass. Soil (0-10 cm) net nitrogen mineralization rate was 90.4 kg·hm-2·a-1. Soil played a very important role of stocking and providing N to plants. The nitrogen content in precipitation, throughfall, stemflow and stream flow were 0.565, 0.828, 0.983 and 1.042 mg·dm-3, respectively. The nitrogen input from precipitation, throughfall and stemflow were 8.89, 10.97 and 3.57 kg·hm-2·a-1, respectively. The nitrogen output in stream flow was 5.95 kg·hm-2·a-1. The nitrogen net increase in the seasonal rain forest was 2.94 kg·hm-2·a-1. In the nitrogen biological cycling, 149.86 kg·hm-2·a-1 was taken up by plants，69.30 kg·hm-2·a-1 stored in living biomass，and 80.56 kg·hm-2·a-1 returned to soil. This undisturbed seasonal rain forest had the nitrogen conservation mechanism and remained in the nitrogen accumulation state.