土壤碳储量减少:中国农业之隐患──中美农业生态系统碳循环对比研究

LOSS OF SOIL CARBON THREATENS CHINESE AGRICULTURE: A COMPARISON ON AGROECOSYSTEM CARBON POOL IN CHINA AND THE U.S.

我们使用一个生物地球化学模型ＤＮＤＣ对中国和美国农业生态系统中土壤有机碳（ＳＯＣ）的动态进行了全国范围的预测。模型模拟结果表明，美国农业土地每年净增７２．４Ｔｇ碳，而中国农业土地每年丢失７３．８Ｔｇ碳。每一年，美国土壤以二氧化碳形式释放入大气的碳为８１２Ｔｇ但从农作物残留物（根和秸秆）获得８８４Ｔｇ的碳。中国土壤释放３６６Ｔｇ的碳于大气中，仅从农作物残留物收回２９３Ｔｇ的碳。中美对农作物残留物的不同管理是造成两国农业土壤动态背道而驰的重要原因之一。中国目前日益恶化的土地沙化及沙尘暴问题即是大区域ＳＯＣ减少的一个必然结果。

A biogeochemical model, DNC, was employed for predicting soil organic carbon (SOC) dynamics in agricultural ecosystems in both China and the U. S. at national scale. Data of 1990 climate, soil properties, crop types and acreage, and cropping management at county scale were collected from various sources and integrated into a Geographic Information System (GIS) dhabase to support the model runs at the regional scale. The predicted results revealed that the agricultural lands in the U. S. were gaining SOC at a rate of 72.4Tg C / year, and the agricultural lands in China were loosing SOC at a rate of 73.8Tg C/ year. The U, S. soils released 812Tg C as carbon dioxide (CO2) into the atmosphere, and obtained 884Tg C as crop residue annually. China soils released 366Tg C as CO2, and obtained 293Tg C as crop residue annually. The difference in crop residue management was one of the major reasons, which caused SOC increase in the U. S. and decrease in China. In comparison to the U S., China soils provide less available nitrogen (N) due to less SOC turnover every year. That is partially why the Chinese farmers have to use much more fertilizer (abollt 16 million tons N per year) for their 95.9 million ha cropland, and the U. S. farmers only use about 8 million tons of N fertilizer in their 143.6 million ha cropland. Overuse of fertilizers will not only degrade the soil quality but also cause regional and global environmental problems. Since SOC is an essential element determining soil physical and chemical properties, loss of SOC will cause soil degradation, which does not only undermine sustainable yield but also affect environmental safety. Desertification and dust storm currently prevailing in China are one of the inherent consequences of SOC loss at regional scale. The DNDC model tested several alternative management, such as land cover changes, conservative tillage and increased percentage of crop residue returned back to the field after harvest, and found they were able to effectively increase SOC in the cropland in China. China needs a long-term policy based on scientific analysis to protect her soil resources if she wants to maintain the soil fertility, sustainable yield, and environmental safety in the county.