基于养分平衡的北京市人口-土地-环境问题探讨

人口集聚于城市,大量食物输入的同时,代谢物也不断产生,消纳不充分或不及时就会累积并产生环境污染的隐患。该文以北京市平原区为例,利用遥感技术分析2004—2014年间研究区土地利用结构变化;基于养分平衡原理,估算、比较人口与耕地变化对城市养分排放和吸纳能力的影响;使用景观形状指数度量城市建设用地与外界环境的接触面变化情况。结果表明:1)研究期内因人口增加、食物输入导致人类排泄物及其氮(N)、磷(P)、钾(K)养分增加约0.34×10~5 t,经污水处理后所得污泥中氮磷钾养分增加约282.17 t;2)耕地数量的减少使城市对养分吸纳能力下降约1.17×10~5 t;3)建设用地形状指数由1.48下降至1.37,接触界面的减小使城市代谢物疏散效率下降。研究认为:人口增长-食物消费提高-人类粪尿增加与耕地减少-建设用地界面减小-养分消纳能力降低的"此消彼长"态势无形中造成城市养分富集,增加了对区域水环境污染的风险,而城市结构无法重构,应积极谋求养分的内部消纳。基于北京市农田化肥依赖性强、林地增加的现状,该文建议北京市积极推广粪便处理残渣还田、再生水城市绿化灌溉利用,通过耕地与林地植被消纳新增的养分。

Discussion on population-land-environment problem of Beijing based on nutrient balance

Abstract: More and more people immigrated into metropolises, resulting in the increase of food consumption and waste production. Human waste contains a lot of nutrients, and above all, night soil is the most representative. If the night soil does not be treated well timely, it could bring some serious environmental problems within city, especially for the water environment. Taking the plain area in Beijing as the example, we focused on its land use change between 2004 and 2014 by the method of remote sensing; besides, we also estimated nutrient balance in farmlands due to the changes of population and land use; finally, we also researched the change of interface between urban construction land and outside environment by mean shape index. The results were as follows: 1) Compared with 2004, the increase of population and food consumption caused an increase of about 0.34×105 t nitrogen (N), phosphorus (P), and potassium (K) nutrients from human excrement in 2014, and there was a nutrient increment of 282.17 t in night soil sludge after water treatment; 2) Farmland had a strong capacity for nutrient absorption in Beijing, for instance, the grain crop field of winter wheat and spring maize could absorb 820 kg/hm2 per year, and the vegetable filed of rape could absorb 1148 kg/hm2 per year; the decreased farmland area resulted in an about 1.17×105 t loss of nutrient treatment capacity; 3) The decline of mean shape index from 1.48 to 1.37 demonstrated the shrink of construction land contacting boundary, which made the evacuation efficiency of metabolic flow decline. Urban nutrient surplus and resultant serious water pollution were mainly due to the increase of population, food consumption and human metabolites, as well as the decrease of farmland area, construction land contacting boundary extent, and resultant metabolites absorption capacity. According to statistics, there is still half of night soil sludge stacked or buried in suburb areas without harmless disposal, which increases the risk of regional water environment pollution. City is an open system, in terms of the circulation of matter and energy in city with external environment, we reckon that, compared with mononuclear mega-city, polycentric city with more farmland and forest land interlaced with construction land has a higher nutrient liquidity and ecological stability. Therefore, the multi-center urban structure can avoid the accumulation of nutrients in the city in the process of urban development, and it is worth learning from other cities. However, it is very hard to rebuild the urban construction of Beijing, and thus it is of importance to treat the nutrient within the urban area. In order to better treat the ever-increasing nutrient of human metabolites in the urban area, on the one hand, we suggest to recycle and reuse livestock manure and wasted water into farmlands instead of stacking or burying it, which can help to develop green agriculture; on the other hand, given the increase of forest land due to afforestation of city, the use of reclaimed water in green area irrigation can not only decompose the redundant nutrients, but also save water resource.