Human-dominated Carbon Cycle And Ecosystem Services In Urban And Rural Coupling Systems

Human activities have intensively modified and domianted the carbon cycle of terrestrial ecosystems. While satisfying basic needs for food and housing, human activities have brought serious environmental and health problems. Since industrial revolution, the human disturbances have broken the structure and differentiate the function of pristine ecosystems, forming functional units (e.g. croplands, city or wastewater treatment) with single goal to meet human needs. These units are in turn coupling, self-organizing and upgrading into new systems existing in real world with city as the core, i.e. Urban and Rural Coupling Systems (URCS). Although the structure and function of URCS is approaching perfect, its carbon cycle and ecosystem services remain largely unknown. In order to understand the carbon cycle and ecosystem services of different functional sub-systems, this study tried to construct the framework of human dominated carbon cycle in URCS. Based on the framework, we investigated the change of carbon cycle and ecosystem services during typical human dominated processes (e.g. agricultural intensification), and explored their natural, social and economic effect factors.This study devided URCS into four functional subsystems:production, decomposition, human settlement and life-supporter. The specific processes of human dominated carbon cycle for URCS were explained using typical artificial ecosystems. Then, the framework of carbon cycle among production-decomposition-human settlement and life-supporter sub-systems were constructed with mass balance method. Finally, this paper used greenhouse agriculture, constructed wetland and urban ecosystem as case studies to investigate the human dominated carbon balance and ecosystem services during typical processes of URCS. Furthermore, the natural and socialeconomic factors affected the carbon cycle are also considered. This paper use meta-analysis to compile dataset with multiple data sources from experiments and literatures. Next, the carbon balance and ecosystem services during corresponding processes are quantified using mass balance and life cycle analysis. The effect factors are explained with statistial methods. The main results are as follows: 1) Human domination has increased the pathways of carbon fluxes and diversities of carbon pools among different functional sub-systems in URCS. As a result, carbon accumulation also increased in human dominated systems. In2009, the photosynthetic carbon inputs of croplands is623.1Tg C yr-1. About88%of the photosynthetic carbon inputs are appropriated by humans and exported into industry, human settlement and livestock sub-systems. The pathways of carbon outputs for croplands are more than that of decomposition and human settlement. The carbon output intensity of deompositon sub-systems into life-supporter sub-systems is lower than that of croplands and human settlements, In2009, the decomposition sub-system can treat～79%waste carbon outputs from human settlements. Human settlements can accumulate1.9-and3.2-fold carbon that of croplands and decomposition sub-systems in2009. The inorganic carbon sequestration in human settlement is similar to that of croplands. These resutls indicates human activities tend to accumulate carbon in human settlements. The life cycle carbon balances of croplands and human settlement in2009act as carbon sources while that of decompositon sub-systems is carbon sinks. However, decompostion sub-systems are also a net greenhouse gas souces (12.8-69.1Tg CO2-eq), contributing to greenhouse effect.2) As intensification of food production in URCS, the conversion from conventional farming to plastic greenhouse agriculture can reduce carbon emissions under five climatic zones (cold-temperate, mid-temperate, Tibet, northern and southern subtropical zones) in China. The average carbon emission and greenhouse gas reduction is1.51t C hm-2yr-1and5.42t CO2-eq hm-2yr-1under life cycle analysis, respectively. This is because the increase of net primary production and soil carbon sequestration following the conversion can more than offset the increment of the external organic fertilizer carbon inputs and fossil fuel carbon emissions. This intensification can strengthen the basic function of production sub-systems, i.e. food supply while simutaneously increase mutiple ecosystem services and dis-services. The economic value of provision services is about84%that of total services from plastic greenhouse. Conversion from conventional farming to plastic greenhouses increase both ecosystem services and dis-services under five climatic zones in China. However, all the net econimic values are positive.3) As the typical case for diversification of artificial decomposition sub-system, constructed wetlands can increase the capacity of wastewater treatment in URCS. Constructed wetlands is one of important substitute approaches for wastewater treatment, treating wastewater nitrogen while producing biofuel. The life cycle carbo emissions and greehouse gas emissions of constructed wetlands are14t C t-1N removal and575t CO2-eq t-1N removal less than that of wastewater treatment plants, respectively. This is mainly because the low construct and operation cost lead to low material and energy consumption. The economic value of provision service (clean water supply) from constructed wetlands can reach97%of total economic values, either as wastewater treatment or biofuel production system. As wastewater treatment, the provision and regulation services from constructed wetland is9.5-and10.4-fold that of wastewater treatment plants, while its dis-service is only0.09%. As biofuel production systems, the total economic value of construted wetlands is slightly higher than microalgea systems, and is153-602fold that of other biofuel production systems (e.g. switchgrass and corn).4) As the regulation center of URCS, urban ecosystem include urban greenspace and human settlement. Urbanization increased carbon density release large amount of carbon. The average carbon density of human settlement (177.8t C hm-2) is higher than that of urban greenspace (120.1t C hm"2) and adjacent natural ecosystems (109.8t C hm-2) across built-up area of16cities in China. Considering the urban ecosystem as an integrative functional unit in real world, the average total carbon density of built-up area (297.9t C hm-2) can be higher than that of forest ecosystem (208.5t C hm-2) and tropical rain forests (40-250t C hm"2). This may be due to increase of greenspace soil carbon density and diversity of carbon pools in human settlements. As the life-support sub-systems, urban greenspace can provide ecosystem services such as aesthetic value, climate regulation, air purification and improve urban environment. The economic value of cultural services from urban greenspace is on average～84%that of total ecosystem services. Moreover, urban greenspace, though managed by human, can reduce carbon emissions under life cycle analysis and provide low dis-services. 5) The carbon balance of production, decomposition and urban ecosystem in URCS is directly or indirectly affected by natural factors, namely temperature. For example, the conversion from conventional farming to plastic greenhouses have increased the accumulated temperature and thus prolonged the growth seasons, which in turn increase net primary production and soil carbon sequestration; Temperature can affect the microorganism activities and change CH4and N2O emissions; Carbon density of urban greenspace significantily decrease with increasing annual mean temperature, suggesting high temperature would increase soil respiration; Carbon density of human settlements in built-up area peaks in the cities with medium annual mean temperature. This may be because people tend to live in mild climate. Socioeconomic factors affecting carbon balance is more diverse. For instance, farmers are willing to apply more organic fertilizers in cold regions than warm regions due to higher economic return, increasing net primary production and soil carbon sequestration; Net primary production of constructed wetlands could be improved by reasonable nitrogen level, vertical constructed wetland and species with high productivity; Carbon density of urban greenspace is lowest in cities with medium energy intensity, while that of human settlement increase with household density and energy intensity and decreas with per capita GDP.Above all, this study reveals the carbon cycle of subsytems with different functions in URCS, supplementing the parts of human dominated carbon cycle in the context of global carbon cycle. Furthermore, this study use intensification of food production, replacement of conventional artifical decomposition approaches and urbanization to understand human dominated carbon balance and ecosystem services, and their natural and socioeconomic factors. This study will provide scientific foundations for sustainable development in the future.