| Based on the principle of environmental economics and value science, the dissertation aims at orientating and recognizing Beijing cropland ecosystem from a new integrative perspective of negative and positive effects, which not only provide the service function value for city but also bring out the environmental influences because of excessive use of agriculture resources. The results are helpful for not only preventing environmental destroy and excessive resources exploration but also avoiding the wrong trend, i.e. some people are negating and making light of agriculture nearby city in their cognition. In the meantime, the integrative valuation targets are to make a calculation tool for cost-benefit analysis of sustainable agricultural development, as well as to provide the methodology for green GDP accounting of regional agriculture.The research are systematically designed to assess the service value from cropland and the environmental costs on the basis of trans-disciplinary study, including agronomy, ecology, environmental economics, resource economics, mathematics, value science and so on, combination of microcosmic and macroscopical study and of qualitative and quantitative study. Based on a large amount of the first-hand investigation and statistical data, theoretical analysis, surveys together with model simulations to fulfill the comprehensive value assessment of cropland in Beijing rural areas, some important results are concluded as the followings:(1) The positive and negative effects of Beijing cropland were analyzed by above theories and the service function of cropland is taken as the positive effects of cropland, but the environmental influences are taken as the negative effects which are divided into two parts i.e. resource exhausting and environmental pollution. The assessmental process with five stepes including counting extent and environmental factors, valuation of resource status, selection of service function types and environmental factors, physical quantifying, monetary quantifying, summing up monetary value of all these items, are adopted. Many related methods of environmental economics are tried to assess the positive and negative value brought by Beijing cropland. The cost-benefit analysis of agricultural production in Beijing cropland is conducted.(2) The various service function values from different type of crops in Beijing rural areas are estimated by many methods. The total service value provided by cropland is decreased in the region from 45.13 billion yuan RMB in 1996 to 34.27 billion yuan RMB in 2002, in which the value of cereals decreased rapidly, and the cash-crops increased to some extent. The total average value of farmland services during the period of 6 years was 8 times of the production value, with indirect values of 34.62 billion yuan RMB, which equals to 6 times of the direct values. The value of farming products make up only 12.41% of the total service value in cropland in present cultivation system, Whereas the values, including Carbon fixation, oxygen production and environment purification, are very great and make up 77%, of which Carbon fixation and oxygen production makes up 39.48% and environment purification 37.51% respectively, the value of OM accumulation in soil 4.4%, that of maintaining nutrients circle 1.27%, that of water-holding and agricultural tourism 2.21% and 3.8% respectively and that of soil conservation only 0.01%.(3) The annual total environmental cost caused by agricultural operation is 928 million yuan RMB on average, accounting for 18.8% of the agricultural net value in 2002. Among those negative external factors and on average per year, the water resources loss exhausted by cropland is the utmost one, about 670 million yuan RMB; the loss caused by soil erosion is about 95.62 million yuan RMB; the loss led to environmental pollution by sewage irrigation, 71.08 million yuan RMB; the loss of ground water contamination, brought by excessive application of nitrogen fertilizer, are about 66.72 million yuan RMB; the economic loss of GHG emission in cropland about 11.29 million yuan RMB. The soil pollution lost by film remnants is 8.11 million yuan RMB. The loss caused by every hectare cropland is 3725 yuan RMB. In the meantime, it is concluded that the environmental cost of 0.17 yuan RMB is paid by society in 2002 while the agricultural net production value of 1 yuan RMB is gained in terms of boundary opportunity cost analysis.(4) The negative and positive value of corpland are integrated. The total positive value of its effects is about 34.27billion yuan RMB, and the negative value caused by excessive exploration 996 million yuan RMB and their ratio is 34.4:1. To sum up, the positive effects of cropland are far more than the negatives. Taking external benefits and cost brought by cropland into account, the ratio of the total benefits and cost of cropland in Beijing is about 6.86:1, and the external benefits of cropland are 27 times of its external cost. As the agricultural production value has decreased continuously, the environmental function of agriculture will be more and more prominent, and cannot be substituted by cropland in other regions.(5)The study is aimed at to recognize the service function of agriculture in Beijing rural areas from a whole new perspective. This is helpful to improving the positive benefits of cropland and alleviating the negative effects, further maintaining the important ecological action of cropland to Beijing region. Several proposals are provided to construct urban ecological agriculture on the basis of the results of the study.
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