关键词: 农业生态系统, 氮生物地球化学循环, 氮平衡, 氮损失, 氮管理

Abstract: It is in general thought that nitrogen (N) fertilizer is overused in Chinese croplands and that the overuse has resulted in severe environmental problems. As the biggest reactive nitrogen producer and N fertilizer consumer in the world, China is facing a great challenge to reduce nitrogen consumption in agriculture. The objectives of this review are to examine the sources and fate of reactive nitrogen in agroecosystems, to find out why N fertilizer consumption reaches such a high level, and provide with suggestions for better N management practices. To understand the current agricultural use of reactive N in China, principles of biogeochemical N cycling are used to discuss N flows in the agroecosystems in the year 2010, with focus on N input/output and balances in crop-soil systems. At the national level, input of reactive N to croplands was excessive in 2010, and the surplus was approximately equal to the quantity of the reactive N recycled back to crop fields by atmospheric N deposition and irrigation with N-polluted water, about 5 Tg N. Generally speaking, the use of N fertilizer in cereal crops is not extraordinarily high since N fertilizer is also distributed for other uses: Forestation, feeding livestock and fishes, and application to the green fields in urban areas. It is common and significant that there are much higher N application rates to fruit tree plantations and vegetable production, especially to the greenhouse vegetable growing system, in comparison with that applied to cereal crops. With the facts of the limited arable lands, low recycling rates of organic wastes, and low input of biological fixed N, crop production has to depend heavily on the use of N fertilizer in China. There is a low acreage of arable land per capita, with 8% of global arable land feeding 20% of the world population. Recycled rate of nutrient N in the organic wastes are lower than 30% and input of biological N fixation to croplands is less than 15%. Therefore, to meet the demands of Chinese population for both food and improving diets under the condition of the predominance of the croplands with medium to low productivities, high N fertilizer input is understandable. However, N fertilizer consumption is much higher than the national average in some highly productive regions, including the Huang-Huai-Hai Plain, the Yangtze Basin, and Zhujiang Delta (Guangdong) regions, and is closely connected with higher crop yields/multiple cropping indices, and smaller proportion of legume crops to the total cropping area. It is clear that the N losses from food production-processing-consumption chain have resulted in resource wasting and environmental risks. On the other hand, part of the environment received reactive N from the losses of croplands and the other pollution sources, returns to the fields via atmospheric deposition and the irrigations with polluted waters, and becomes an important source of N input to croplands. Due to the complexity of N transformation in agroecosystems and biogeochemical N cycling, N losses are unavoidable. Therefore, the best management practices at various spatial levels should be taken as the options to reduce the fertilizer use in croplands to the minimum. Integrated measures, including multi-disciplinary researches and the cooperation of various social sectors, have been suggested to optimize N management practices at each spatial level, in order to reach the fundamental goals of maintaining/improving soil fertility, securing food, reducing nitrogen fertilizer use, and minimizing environmental risks.

Key words: agroecosystem, biogeochemical nitrogen cycling, nitrogen balance, nitrogen losses, nitrogen management