Influence Of Urbanization On The Nitrogen Flow In Food Consumption System And Nitrogen Cycling In Agro-Ecosystem Of China

With the growing of income and urbanization,the food consumption structure in urban and rural area of China changes the nitrogen flow model significantly,which has led to tremendous pressure of ecological environment arid serious pollution problems in cities and towns.At the same time,the alternative food consumption structure drives the adjustment of agricultural structure,which Will inevitably affect the nutrient cycling in agro-ecosystem.Therefore,this study preliminarily probed into some impacts of urbanization through survey data,literatures,Statistical data and material flow analysis. The impacts of urbanization on cnaracteristics of nitrogen flow and nitrogen reutilization, nitrogen excretion quantity and its mount emitted to water body in China food consumption system were studies and the relatlonships between urbanization and regional nitrogen balance in agro-ecosystem were analyzed based on the survey of different years in all the country,and companson of various counties with different urbanization rate in Beijing city and Hebei province.The results were shown as follows.The nitrogen consumption oi urban residents per capita in 2002 was 4.770 kg which was 10%higher than that of rural residents(4.314 kg).With the raising of urbanization rate from1982 to 2002,food nitrogen consumption of urban residents increased by 145.3%, while a decrease of 8.1%in rural areas.Nitrogen discharged from urban into the environment increased by 18.4 times,while the utilization rate by nitrogen recycle was only 13.0 percent,lowered by 40 percentage points.Scenario analysis Showed that,if the food structure met state standards of China Food and Nutrition Programs,Japan,Europe and USA following the population growth,China's total nitrogen consumption in 2010 would increase by 196.1-419.3 million tons more than that in 2002,and nitrogen in waste would increase by 179.7-368.3 million tons.Therefore,rapid development of urbanization not only produced food nitrogen demands and environmental nitrogen emissions,but also brought tremendbus pressure to animal and plant production.Compared with rural residents,both the consumption and excretion of nitrogen from urban residents were higher than that from rural residents.The per capita nitrogen discharged into the water from urban and rural residents were roughly the Same in 1982 while the amount from urban residents was 2.8 times higher than that from rural residents in 1992 and 2002.In 1982,the ratio of nitrogen discharged into water for rural and urban residents were comparative with the value of 7.5%.However,in 2002,this ratio for urban residents went up to 56.7%which was much higher than that of rural residents with 34%. Scenario analysis showed that,with the population growth,if the food structure was improved to increase protein intake at the same time,nitrogen in waste in 2020 will increase by 121-418 million tons and the nitrogen discharged into the water will also increase by 41-182 million tons.Improving the waste recycling rate,the ratio of sewage treatment and removal rate of nitrogen in waste treatment can reduce nitrogen discharged into the water.The size of city also affects food consumption of nitrogen.The per capita consumption nitrogen of the major cities was higher than that of medium-sized and small cities with 7.5%and 18.5%respectively.With the increased size of the cities,the nitrogen content of atmosphere and water are increasing continually,and particularly,the nitrogen entering into the water environment in the big cities was 1.7 times as much as small cities. However,the nitrogen returned to the field was with a reduction of 3.5 times than that of the small cities.Compared with the medium and small cities and small towns,the nitrogen cycle utilization of major cities was only 5.0%,lowered by 5.1 and 12.5 percentage points respectively.In 2002,the urban population of China has come to 502.12 million.If such a population is concentrated in large cities,the nitrogen would be excessive of 179,000 tons discharging into water,pure nitrogen 534,000 tons more than that in medium-sized cities and small towns.Instead,if the population is concentrated in small and medium-sized cities or small towns,there would be 179,000 tons or 534,000 tons pure nitrogen returning to farmland.The larger size of the city decreases the lower the utilization rate of nitrogen cycle.With the development of town and the change of the people's life style,the nitrogen cycle utilization will decline significantly.As for farmland nutrient recycle,according to nationwide indicators,input amount of fertilizer nitrogen per hector was becoming more and more.For example,it increase from 95 kg / ha in 1980 to 258 kg / ha in 2005.The amplitude of organic fertilizer per hector was lower than fertilizer nitrogen,for instance,it increased from 61kg/ha in 1980 to 127 kg / ha in 2005.The input and residues of total fertilizer nitrogen increased greatly.For instance,it increased from 156kg / ha in 1980 to 385kg / ha in 2005,about 2.5 times.The more the input amount of total nitrogen,the higher soil nutrient surplus.Scenario analysis showed that,in 2005 when other conditions remained unchanged,if only urbanization increased by 10 percentage points,the nitrogen demand inputted into farmland would come to 1674000 tons.If the food structure is also improved and the population is still growing,fertilizer nitrogen,organic nitrogen and total nitrogen input per unit area will increase more.Input amount and surplus of nitrogen per hector in areas of Beijing in 2005 has a substantial increase than that in 1995 with the increase of urbanization rate.The input amount of fertilizer in vegetable,aquaculture and fruit areas which located in suburbs are large,So the rank of average fertilizer nitrogen input amount per unit area is outskirts＞exurb＞urban area.According to the correlation analysis of urbanization rate and farmland nutrient cycling,the relationship in areas of urban in 1995 is significant.However,in suburbs and exurbs,it was not significant.The input amount of nutrient is increasing with the raise of urbanization rate in areas of urban.But in the suburbs and exurb,the significant correlation between the urbanization rate and farmland nutrient cycling indicators was not observed.Total nitrogen input and surplus per hector in area of Beijing is 1.4-2.0 times as much as the average amount of whole country.Hebei province,which belongs to rural agriculture,lies in the middle-east of China.It doesn't have distinct zoning like urban agriculture.The nutrient cycling of rural agro-ecosystem is mainly affected by agricultural structure.Hebei as the important province of stock raising and planting,the farmland nitrogen flow has been increasing in the whole areas.With the time going on,the input amount of fertilize nitrogen per hector and soil nutrient surplus capacity is still increasing.The input amounts of nutrient in vegetable and fruit areas are large.The relation of urbanization rate and indicators of soil nutrient cycling is not significant.The urbanization rate of Hebei is low,about 2.5%-30%.The changes of local urbanization rate does not affect farmland nutrient cycling directively.