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城市化进程中,随着人口不断向城市积聚,城市成为各种农产品的输入地,农民也不断增加对城市周围耕地的投入以获取更多的农产品。城市及其郊区成为各种物流、能流的聚集地。如果聚集的能量、养分得不到有效利用,不仅造成资源的浪费,还会给环境带来一定的压力。文章基于养分循环原理,以北京市为例,对分别计算了北京市人畜排泄物和作物产出可以提供的养分,与北京市耕地在吨粮田情况下所能够吸纳的养分,结果显示养分是富余的。与单中心大城市相比,多中心中小城市的城市建成区与周围农田之间的物流能流交换界面更大,有利于充分利用人畜排泄物作为作物养分,发展有机农业;同时,也有利于减少城市环境压力和改善城市生态环境。因此,要坚持分散集团式的城市空间布局,避免“城市大饼”无限扩大,促进养分循环和土地可持续利用,建设生态环境良好的宜居城市。
In the process of urbanization, as the population keeps accumulating in the cities, the cities become the input fields for various kinds of agricultural products, and peasants also increase their input to the cultivated land around the cities in order to obtain more agricultural products. The city and its suburbs have become a gathering place for all kinds of logistics and energy flow. If the accumulated energy and nutrients are not effectively utilized, it will not only result in the waste of resources but also bring some pressure to the environment. Based on the principle of nutrient cycling and taking Beijing as an example, the nutrients that can be provided by the livestock and poultry excrement and crop output in Beijing are calculated separately from the nutrients that can be absorbed by the cultivated land in Beijing under the condition of the grain crop. The results show that the nutrients are Surplus. Compared with a single-center metropolitan area, the interface of logistics and energy flow between urban built-up area and surrounding farmland in multi-center small and medium-sized cities is larger, which is conducive to making full use of human and livestock excretions as crop nutrients and developing organic agriculture. At the same time, Reduce the pressure on the urban environment and improve the urban ecological environment. Therefore, we must adhere to the decentralization of group-style urban space layout, to avoid unlimited expansion of city cake, promote nutrient cycling and sustainable land use, and build a livable city with good ecological environment.