中国森林生态系统N平衡现状

由于N饱和生态系统的出现,森林生态系统作为环境污染储蓄库的认识受到挑战。收集了近十余年来全国各地森林N素循环的研究资料,通过对目前大气N沉降、森林生物固N、森林生态系统N的流失、淋失、挥发等各项收支参数的分析,借助农田养分收支平衡的估算思路和方法对全国森林生态系统N平衡进行了估算。结果表明,我国森林生态系统N的输入大于输出,全国森林生态系统年容纳大气N约为736万t,其中约176万t来自于大气N沉降,约599万t来自于生物固N。而进入到森林生态系统中的N约16万t固定在木材中用以维持森林蓄积的增加,其余绝大部分则保存于森林土壤,使得森林土壤全N含量大约以每年0.002%的速率增长。但不断增加的N素输入并未导致森林生态系统N饱和,全国的森林蓄积仍保持增长的趋势,森林生态系统在N的生物地球化学循环过程中起着重要的调节作用,仍是环境N的储蓄库,对于调节气候,防治污染具有重大作用。

Nitrogen balance of natural forest ecosystem in China

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 China increases by 1.21% per year. It indicated that forest ecosystem is still a nitrogen sink system and plays an important role in regulating climate and controlling pollution.