Changes And Efficiency Of Active Nitrogen Flux In Food Production And Consumption In Nanjing

Nitrogen(N)is one of the main essential elements for all living organisms.It flows as different forms in the crop-livestock-food-environment system.The understanding of N flow characteristics and use efficiencies in each subsystem will help put forward effective measures to reduce N environmental load.Based on the method of Material Flow Analysis,we established N flow model in the food production and consumption system and analyzed its flow characteristics,use efficiencies and environmental load changes in recent ten years of Nanjing.Main results are as follows:(1)Main N input in the farmland system was from N fertilizer.During the period of 1995 to 2012,the total input decreased from 91.97 GgN/a to 62.55 GgN/a.This may be related to the urbanization development,reduction of sowing area and planting structure changes.Due to rapid increase of population and planting structure changes in Nanjing,per capita N input decreased by more than half.The N environmental load decreased from 23.06 GgN/a to 15.05 GgN/a,probably due to the total N input reduction.(2)Main N input in the livestock system was from feed.Due to livestock scale reduction and local population increase,the total N input decreased from 52.44 GgN/a to 35.84 GgN/a.Per capita N input showed downward trend,and decreased from 10.05 kg/a to 4.39 kg/a,The animal excrement is the main source of N environmental load in the livestock system.Improvement of recycling rate of livestock excrement is an efficient way to reduce environmental load.(3)Consumption of animal and plant-derived foods is the main source of reactive Nin the food consumption system.During the period of 1995 to 2012,N consumption of plant-derived food decreased from 15.85 GgN/a to 13.23 GgN/a while animal-derived food increased from 5.30 GgN/a to 11.33 GgN/a.This showed that,with food consumption transforming from the stage of necessity to quantity,food consumption structure turned from plant-derived food oriented to animal and plant-derived food balanced.Per capital food consumption of rural and urban residents decreased from 5.09 kg/(per·a)to 4.11 kg/(per·a)and 3.04 kg/(per·a)to 2.65 kg/(per·a),respectively.Human excrement,kitchen waste,processing loss and household loss are main sources for N entering the environment.N losses to water and soil increased by 1.03 GgN/a and 0.30 GgN/a from 1995 to 2012,respectively,while losses to air decreased by 1.33 GgN/a.These may be related to the improvement of economic level and changes of life style.Along with the rising economic development,the urban population has continued to increase as well as city sewage pipe coverage rate,which changed the forms of reactive N discharged to the environment.(4)Total input of N in the food production and consumption system decreased from 162.48 GgN/a in 1995 to 116.42 GgN/a in 2012,due to the increased levels of urbanization and transformed food supply from self-sufficiency to partially relied on imports.However,with the improvement of urbanization level,imported food increased,leading to the increased N environmental load.In addition,changes of the food consumption pattern(from quantity to quality)may increase the per capita N consumption like Beijing and the total input of N too.Thus,a certain degree of urbanization will benefit the development of urbanization areas.(5)Mean N use efficiency in food production increased from 18.71%in 1995 to 24.34%in 2012.Although use efficiencies in each subsystem increased,it was still lower than the mean national level(27%).Increased use efficiency can further reduce environmental load.N cost of food consumption of Nanjing decreased from 7.68 kg/kg in 1995 to 4.74 kg/kg in 2012,lower than the mean national level(11 kg/kg)and higher than that of Beijing(2.5-4.5 kg/kg).The rapid increase of population from 1995 onwards has greatly contributed to the increased food import and low food N cost.