| With the advancement of the urbanization process,changes of nitrogen (N),phosphorus (P) and carbon (C) flows in food production and consumption system caused serious environmental problems. However, quantitative analyses of the N, P and C flows during urban expansion and information about their use efficiencies and drivers are scarce, especially at the city level. Thus, we report on N, P and C flows of Kunming and their environmental losses between 1989 to 2013, using a combination of statistical data bases,surveys and the substance flow analysis model. Main results are as follows:(1) N input to the crop production of Kunming increased from 48.94Gg/a in 1989 to 113.64Gg/a in 2013 and P input increased from 8.81Gg/a to 25.72Gg/a which were caused by the increased input of chemical fertilizer. Per capital N input increased from 11.6kg/a in 1989 to 17.27kg/a in 2013 and per capital P input increased from 2.09kg/a to 3.91kg/a. The total input increase led to not only per capital N and P increase, but also the increase of N and P environmental losses. Environmental losses increased from 7.48Gg/a in 1989 to 15.97Gg/a in 2013 for N and 0.61Gg/a to 1.04Gg/a for P.Losses to air increased from 5.52Gg/a to 11.78Gg/a and to water from 1.96Gg/a to 4.19Gg/a for N and losses to water increased from 0.61Gg/a to 1.04Gg/a for P.(2) The main input source of the animal production was feed. Influenced by livestock scale expansion, N input increased from 37.91Gg/a in 1989 to 275.78Gg/a in 2013 and P input increased from 7Gg/a to 49.47Gg/a. Per capital input increased from 8.99kg/a to 41.92kg/a for N and 1.66kg/a to 7.52kg/a for P. This showed that the increasing rate of population growth was much slower than feed input growth.Livestock excrement was the main way for N and P losses. N losses increased from 18.2Gg/a in 1989 to 55.48Gg/a in 2013 and for P, from 3.2Gg/a to 7.45Gg/a.(3) In the food consumption system, N and P main input was plant food. But with the development of economy and improvement of living standards, food consumption pattern changed from subsistence to nutrient, thus leading to plant food reduction and animal food growth of N and P. N losses to air decreased apparently while increased continuously to soil and water. P losses to water decreased while increased to soil.(4) The total C sink in crop and animal production increased from 705.44Gg in 1989 to 1195.53Gg in 2013 which was related to the increasing crop yields. The total C emission increased from 496.92Gg to 1012.33Gg which was related to the large input of fertilizer and pesticide. The increasing rate of C emission was faster than C sink,so Kunming still needs a lot of effort to save energy and reduce emission. In the food consumption system,C input decreased from 322.87Gg/a in 1989 to 294.53Gg/a in 2013. Animal food C increased 145.88% while plant food C decreased 21.23%. C losses to air took up the largest proportion, the second was to soil and the smallest was to water. Losses to air decreased and increased to soil and water from 1989 to 2013.(5) In food production and consumption, total N input increased from 64.19Gg/a in 1989 to 331.14Gg/a in 2013 and from 10.83Gg/a to 62.7Gg/a for P which was affected by import feed growth. It was difficult for the local feed production to support enlarging livestock scale. Therefore, a lot of feed was imported from other cities. Among all losses from different processes, animal food production and processing accounted for the largest percent which increased from 18.77Gg/a and 3.44Gg/a to 73.08Gg/a and 10.35 Gg/a, respectively. It indicated that the transformation of food consumption pattern was an important factor of losses growth.(6) From 1989 to 2013, in crop production system, use efficiencies of N and P decreased from 29.85% and 26.73% to 20.21% and 15.85%. In animal production, N and P use efficiencies stabilized at 6%a and 2%. N and P use efficiencies in crop and animal production system decreased from 19.31% and 15.85% to 10.35% and 6.7%and recycling rate of N and P decreased from 35.31% and 45.94% to 17.6% and 19.91%. On the one hand, N and P use efficiencies were too low which indicated that animal food output was small and the majority of N and P input remained in living animals. On the other hand, recycling rate decreased apparently in these years.Therefore, to reduce environmental losses and improve N and P use efficiencies,several key measures were put forward. The first way was to reduce fertilizer input and improve use efficiencies of fertilizer. The second way was to improve feed use efficiencies and realize cyclic utilization of livestock excrement. The third way was to pay more attention to balanced diet, realize resourceful use of human excrement and kitchen waste. Besides, reduce food waste.(7) In crop and animal production system, the intensity of C sink kept stable(2.65t/hm2). The intensity of C emission increased from 1.86t/hm2 in 1989 to 2.24t/hm2 in 2013. The net C sink decreased from 208.52Gg to 183.2Gg and C sinklevel also decreased from 141.96% to 118.1%. As a result, Kunming city needs to advocate low carbon agriculture model and reduce greenhouse gas emission so as to improve C sink capacity of all the agriculture production. |
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