Nitrogen Cycle Of Coupled Human And Natural System—A Case Study Of China

Human activities have intensively altered the global nitrogen (N) cycle of terrestrial ecosystems, increasing system productivity to meet human needs, but also bringing serious environmental and health problems. Therefore, studies on global N cycling under the disturbance of human activities are not only one of the current research focuses of ecology, but also one of the core contents of socioeconomic and environmental sustainability. With the strengthening of human activity disturbances, the structure and function of natural ecosystems have been breaking, forming cropland, urban, plantations, pasture, industry, mine and other functional systems, further self-organizing and upgrading to a coupled human and natural system (CHANS). China is experiencing rapid socioeconomic development and serious environmental N pollution currently, which is on the'hot spot'and'hot moment'of global N cycle in CHNAS. It provides an ideal case for better understanding the N cycling in CHANS, rational use of the positive role of N, and mitigating the negative impact on human and the Earth system.In this paper the horizontal direction boundary definition of CHANS follows China's land borders. The upper boundary in the vertical direction is1km above ground surface, not including atmospheric circulation; the lower boundary is on the surface of bedrock, mineral resources not included. N cycling of CHANS starts from the entry of active N (Nr) that activated from N2into the system or Nr direct input to the system from outside-system, and terminate when Nr is oxidized/reduced to N2or directly output to outside-system. CHANS is divided into four functional groups:processors, consumers, remover and life-supporter. Processor can process the fixed N input into the food chain and biomass products, including cropland, grassland, forest, livestock, aquaculture, industrial, and urban greenland subsystems. Consumer includes humans and pets subsystems. Remover refers to a system that can treat the waste Nr and eliminate its negative impact, including wastewater treatment and garbage disposal subsystems. Life-supporter includes near-surface atmosphere, surface water and groundwater subsystem. All Nr imported from outside-system will access to one or several subsystems, recycling among subsystems within the system or output to the outside-system.There are two core questions of N cycle in CHANS:human N supply consumption and system sustainable development. To address these two questions, I complied the dataset of China's N cycling, including over100thousand Nr fluxes and the related factors of air temperature, precipitation land use and socio-economy, etc. between1980-2008by using of observations, literatures, yearbooks and other sources based on mass balance approach, atmospheric remote sensing and geographic information system (GIS) techniques. On the basis of theories and hypotheses of CHANS and the dataset constructed, I comprehensively conducted the quantitative analysis of China's N cycling in CHANS during the recent30years, and comparison and analysis on food N and industrial N between China and worldwide by using ecological models we built, e.g., NCNA, URCNC. The main results are as follow:1) China's reactive Nr input increased from24.6to59.6Tg N yr-1from1980to2008with an increase fold of1.4, which is2times that of global average increase rate. With7%of world's land area, China consumed30%of global anthropogenic Nr input, indicating China's great contribution to the global N cycle. Compared to the natural Nr input to terrestrial ecosystems, human activities have increased3.6times of Nr input to terrestrial ecosystems in China; however, this value is only1-1.5on global level. During the recent30years, biological N fixation remained unchanged in China, which is different with the increasing trend globally; proportion of Haber-Bosch N fixation to total Nr input increased from50%to69%, higher than the global level. On the basis of consumption, Haber-Bosch Nr mainly distributed in agriculture developed regions, e.g., North China Plain, Northeast China, Yangtze River and Sichuan basin, and industry developed regions, e.g., Yangtze River Delta, Pearl River Delta; although NOX-N emissions from fossil fuel combustion increased3.7times, its N flux was small and the proportion to total Nr input maintained at4%-8%. Therefore, food and industrial N production-consumption are the main drivers of N cycling in China.2) In China, with the increase of Nr input and proportion of Nr from anthropogenic sources, system Nr output was gradually smaller than input. It means that CHANS tends to accumulate Nr along with the enhancement of human activities. This is opposite to previous findings, which suggest that Nr mainly loses through discharged to ocean or atmospheric circulation to nearby regions with the increase of anthropogenic Nr. The neglect of industrial N by previous studies might be the main reason for this opposite conclusion. During the recent30years, Nr accumulated in China increased3.1times to27.0Tg N yr-1in2008, Nr accumulation occurred mainly in the cropland (19.8%), forests (31.3%), grasslands (10.5%), groundwater (17.1%) and human settlement (19.7%). Meanwhile, the Nr concentration of surface water and atmosphere has being on the risen, also increasing the accumulation of Nr in the system.3) Nr input to China's processer functional groups increased by1.3-fold, from59.4to137.8Tg N yr-1during the past30years. The industrial N products providing for human as well as plant and animal proteins increased by8.2,1.0and5.5times, but the per capita consumption is still lower than that of developed countries by a factor of20-25%. High Nr input intensity was significantly related to per capita GDP (PGDP), and annual temperature and precipitation, thus, Eastern and Southern China become the "hot spots" of high Nr input. With technological progress and policy innovation over the past30years, Nr loss rate of processer functional group was slower than its input rate in China which means the N use efficiency (NUE) of processer functional group rose.4) China's per capita food N consumption reached5.2kg N yr-1in2008, lower than the Europe and United States (US)(6.5kg N yr-1), but higher than the global average (4.5kg N yr-1). However, China's per capita consumption proportion of animal protein was33.2%, lower than the global average (38.7%), and the Europe and US (65.2%). Even though, the proportion of grain used as feed in China had been reduced to60%, and the production of animal protein become the main consumer of grain in China. Referencing this proportion of developed countries (>70%), China's grain production would increase been used as feed. Considering the differences of human diets, the proportion of China's animal protein consumption may not reach the level of Europe and US; however, the proportion of animal protein consumption still will increase50%in next20years. It thereby drives China's Nr input to further increase.5) Industrial N is another important N production/consumption type in addition to food N, mainly referring to fiber, houses, furniture, etc. This is the first attempt to classify the industrial Nr into anthropogenic source (NA, such as synthetic fibers, rubber, etc.) and biological sources (NB, such as leather, cotton, etc.), and to estimate the industrial N flux in China and a global scale. In2008, China's per capita consumption of industrial N is only3.4kg N yr-1, lower than the global average (4.3kg N yr-1), far lower than that in the Europe and US (>10kg N yr-1). Per capita NA consumption is significantly related to PGDP and urbanization, resulting in high NA flux occurred mainly in high urbanized and developed regions. NB consumption does not relate to socioeconomic development; therefore NB flux varies little across different regions. However, cultural differences can contribute to the variation of NB flux.6) This paper divides industrial N products into two types:structural and non-structural N. The proportion of structural N is over70%, leading to the accumulation of industrial N in human settlements. In2008, global industrial N accumulation is estimated at～21Tg N, which can explain81% of the global'missing anthropogenic N sink'(-26Tg N yr-1). Structural industrial N can be retained in human settlements for decades to centuries, until burned or decomposed. Delayed Nr release from industrial N would mitigate the environmental N pollution caused by rapid release; however, the delays also lead to difficulty in estimating the amount of waste produced. Policy-makers need to focus on the legacy effect introduced by the delay to avoid 'garbage Besieged City'phenomenon and achieve socioeconomic and environmental sustainability.7) In2008, the total Nr emitted to the atmosphere reached16.9Tg N yr-1, doubled the Nr input over the past30years, and China's NOx emission growth rate was larger than that of NH3and N2O. N2O emission was1.1Tg N yr-1in2008, and its greenhouse effect is equivalent to0.2Pg C yr-1CO2. Atmospheric Nr dynamics estimated by mass balance approach above is consistent with the results retrieved from atmospheric remote sensing inversion (R2=0.80-0.94). Henan, Shandong and Hebei from the North China Plain, and Sichuan and Jiangsu from Yangtze River watershed were the main sources of NH3and N2O, while NOx emissions mainly concentrated in Eastern coastal areas. NH3and N2O emission were mainly driven by agricultural development, while NOx emissions were impacted by socioeconomic development.8) Agricultural nonpoint source pollution contributed～60%, and domestic and industrial point source pollution accounted for～30%of surface water N pollution over the recent30years. Surface water N pollution mainly occurred in North China Plain and the Northeast China and parts of the Northwest China. Although surface water Nr fluxes were large in South China, the sufficient surface water resource made the Nr pollution lightener than North China because of the dilution effect. Human consumption and other related activities are the major causes of the regional differences of surface water Nr pollution, and their contributions rank as follow: population> urbanization> PGDP. China's groundwater was polluted heavily by Nr, especially in the North China Plain, Northeast China and the Yangtze River Delta region, the average groundwater N concentration exceeding20mg N L-1. High population density, PGDP and urbanization significantly affect the groundwater N concentration, which does not related to natural factors, revealing the dominant role of human activities on groundwater N pollution. Through the scenario analysis of future Nr dynamics in China by system modeling, I found that technologies play a key role in atmospheric Nr pollution control; policies mainly contribute to groundwater Nr pollution control, while technology and policy both work on surface water N mitigation within CHANS.