Nitrogen balance of natural forest ecosystem in China
中国森林生态系统N平衡现状

The view of forest ecosystem as a nitrogen sink system was challenged with the discovery of nitrogen saturation forest ecosystem. The information of nitrogen cycling from different areas over China during recent decade was collected. According to the climatic zone and the forest characteristics, the natural forest ecosystem in China was divided into the following 6 areas: (1) Northeast China, which includes Heilongjiang Province, Jilin Province and Liaoning Province; (2) North China, which includes Beijing, Tianjin, Shandong Province, Hebei Province and Henan Province; (3) Central China, which includes Anhui Province, Hunan Province, Hubei Province and Shanghai city; (4) Northwest China, which includes Xinjiang, Ning xia, Qinghai, Gansu, Shanxi, Shaanxi and Inner Mongolia; (5)Southeast coastal area, which includes Fujian Province, Zhejiang Province, Guangdong Province, Hainan Province and Taiwan Province; (6) Southwest China, which includes Guangxi, Guizhou Province, Yunnan Province, Sichuan Province, Tibet and Jiangxi Province. All the parameters of N input and N output were sorted out based on the divided forest area and analyzed by the method of estimating the nutrient balance of farmland system. The result shows that the N input of natural forest ecosystem in China mainly is biological N fixation and N deposition. The biological N fixation flux of natural forest varied from 33.1 Nkg hm-2 a-1 to 47 Nkg hm-2 a-1 , and the biological N fixation flux of the forest in central China and south China is higher than those of northwest China, northeast China and north China. The N deposition flux of natural forest is very different in different areas. The N deposition flux is only 6.14 Nkg hm-2 a-1 in northwest China and it increased to 18.54 Nkg hm-2 a-1 in northeast coastal area. The N output mainly consists of N runoff, erosion and N volatilization. However, the amount of N output of natural forest is much lower than that of N input.The total flux of N output varied from 1.05 Nkg hm-2 a-1 in northwest China to 3.79 Nkg hm-2 a-1 in northeast China. The balance of N input and N output in the natural forest ecosystem is 38.19 Nkg hm-2 a-1 in northwest China, 48.2 Nkg hm-2 a-1 in north China, 48.57 Nkg hm-2 a-1 in northeast China, 56.51 Nkg hm-2 a-1 in central China, 58.79 Nkg hm-2 a-1 in southwest China, and 62.55 Nkg hm-2 a-1 in southeast coastal area. About 7.36 million tons of nitrogen, in it about 1.76 million tons are from nitrogen deposition and 5.99 million tons from biological fixation, was stored in natural forest ecosystem of China per year. Among the 7.36 million tons of nitrogen entered the natural forest, about 0.16 million tons was taken up and stored in wood. The rest was stored in soil, consequently, the soil nitrogen content of natural forest ecosystem increased by 0.002 percent per year. However, the surplus nitrogen has not induced forest ecosystem to be a nitrogen saturation system because the wood volume of the natural forest in Ch