基于养分平衡和环境保护的北京市城市结构探讨

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。

The Multi-cluster City Structure is Good for Nutrient Recycle and Environmental Protection

City sprawling causes cultivated land shrinking.Since city is the biggest market where all kinds of agriculture products are imported to,and the suburb farmers also increase the inputs(include fertilizer especially) on their cultivated lands tend to get a higher output to meet the growing citizen demand.Thus city is also the consumption center for energy and material.If these energy and nutrients were not efficiently used or disposed properly,the fertilizer resources could be wasted,and at the same time they could make the environmental pollution more serious.Thus,on the base of the nutrient cycling principles and with greater Beijing municipality as example,we calculated the nutrients supply from human and livestock excreta and crops,as well as nutrients holding capacity of cultivated land under the 15000 kg grain per ha yield condition.Our results showed that the nutrients flowed into Beijing municipals were higher than outflow to cultivated land.Therefore,we recommend organic farming development to fully utilize nutrients from human and livestock excreta.In order to facilitate the organic farming and reduce pressure on the environment,it is a good way to adapt multi-cluster city structure in favor of increasing the interface between urban and rural farming,and ultimately increasing the energy and nutrient exchange efficiency in between.