南京城市化食物生产消费系统氮素流动变化

城市化的发展对食物生产消费过程中氮素的流动产生了一定的影响。以1995—2012年南京市食物生产消费变化为基础,分析了食物生产消费过程中氮素的流动变化及其引起的环境负荷。结果表明,农村和城镇人均食物氮消费量分别由1995年的5.09 kg人~(-1)a~(-1)和3.04 kg人~(-1)a~(-1)下降至2012年的4.11 kg人~(-1)a~(-1)和2.65 kg人~(-1)a~(-1);与1995年相比,南京市食物消费代价降低了39.29%;农田系统和畜禽养殖系统氮素综合利用率由1995年的18.71%增加至2012年的24.34%,整体低于全国水平,大量的氮素进入环境;1995年食物链引起氮素的环境负荷为100.49 Gg N/a,到2012年下降至69.90 Gg N/a,下降了30.44%。南京城市化的发展增加了食物进口,会使食物生产地的氮环境负荷大幅度增加。

Nitrogen flow associated with food production and consumption in Nanjing City

With rapid urbanization, the food production-consumption system is associated with increased nitrogen (N) flow resulting in environmental problems. However, quantitative analyses of N flow during urban expansion and information about their drivers and trajectories are limited, especially at the city level. In the present study, we analyzed the changes in N flow and environmental load in the food production and consumption system of Nanjing, which experienced remarkable population and economic growth between 1995 and 2012, using a combination of statistical databases, literature surveys, and the mass balance model. The results showed that population density increased at the expense of agricultural land and livestock breeding. Food production transformed from self-sufficiency to deficiency, resulting in increased food imports. The rapid increase of the migrant population coincided with a rapid increase in gross domestic production, with changes in food demand and consumption patterns. The structure of food consumption in Nanjing transformed from grain-oriented to grain-vegetable-meat-oriented, and the per capita N consumption of food decreased from 1995 to 2012. In addition, due to different living standards and food consumption patterns between rural and urban residents, the percentage of the per capita decline for N consumption of food was different. The rural and urban per capita N consumption decreased from 5.09 kg person^-1 a^-1 and 3.04 kg person^-1 a^-1 in 1995 to 4.11 kg person^-1 a^-1 and 2.65 kg person^-1 a^-1 in 2012, respectively. The N cost of food consumption of Nanjing decreased from 8.40 kg/kg in 1995 to 5.10 kg/kg in 2012, and declined by 39.29%. The N cost of the food consumption was relatively low when compared to the mean in China, and decreased over time. Mean N use efficiency in food production increased from 18.71% in 1995 to 24.34% in 2012; however, it was still lower than the mean national level. A large amount of N entered the environment and caused severe air, water, and soil pollution. The decrease of total N input, combined with increased N use efficiency in crop and animal production systems, resulted in a decreased environmental N load from food production and consumption of 30.44%, from 100.49 GgN/a in1995 to 69.90 GgN/a in 2012. Regarding food production in Nanjing, environmental N load could be efficiently reduced by optimizing N fertilizer management and using slow-release fertilizer. Regarding food consumption, environmental N load is produced primarily from food waste and human sewage discharge. The effective collection and treatment of these wastes and the utilization of these in rural areas is urgently needed. The rapid increase in the urban population from 1995 onwards has greatly contributed to the increasing import of food and apparent low food N cost. As food imports increased with rapid growth of urban areas in Nanjing, the N cost associated with the production and processing of imported food and feed was transferred to areas where it was produced. Other regions with increased food production will be affected by large increases in N load.