The urban carbon cycle is a component of the global carbon cycle [1
]. In China, urban expansion has increased energy consumption and carbon emissions [2
]. Carbon sequestration capacity—the process, activity, and mechanism for removing carbon dioxide from the air—not only refers to the amount of carbon dioxide forests and soils can capture and store, but also to the amount of organic carbon soil can store. The carbon sequestration capacity of densely populated cities is limited, so those cities must transfer the ecological pressure of carbon sequestration to the suburbs [3
]. Urban suburbs have a close ecological relationship with cities [4
]. The creation of urban ecological zones should involve the consideration of ecological security [6
]. The division of the study area into ecological functional units can help to allocate ecosystem resources [7
] and reveal ecological and environmental problems [8
This paper focuses on the spatial distribution of the carbon cycle in cities which can be regard as mono-centric area in a large spatial scales, with relatively low degree of urbanization (mono-centric cities have small human populations, undeveloped transportation systems, and relatively minor industrial production activity). Manoratna et al. described the actual traffic flow along one major intra-urban corridor using a mono-centric city model [9
]. Li et al. found that planned employment decentralization may help resolve the traffic congestion of a city with a mono-centric structure [10
]. Salvati and Carlucci used the mono-centric city model to analyze urban morphology and land use changes [11
]. Those studies represented that the distribution of geographic features of mono-centric city will be radial pattern. In other theories, the von Thünen model is also related to the spatial pattern of urban features. The von Thünen concentric ring model is the most famous model of regional structure, with a single urban market center [12
]. Von Thünen studied how the height and slope of the rent curve for different land uses depend on the distance from the center [13
]; that is, determine the rent through the distance from the urban commercial center, which formed the agricultural land use of the von Thünen ring structure. In von Thünen’s own words: “if this use is chosen with the utmost rationality, what kind of agriculture will develop and how will the distance to the city affect the use of land” [14
]? Angelsen presented a framework for analyzing the tropical deforestation and reforestation based on the von Thünen model, and associated the forest transition (FT) theory, to study changes in forest cover over time [15
]. Kitsikopoulos analyzed the relationship between urban demand and agrarian productivity by the von Thünen model [16
]. Many studies depicted the impact of land cover/use changes on carbon sequestration service without considering the spatial pattern. Human activities have changed the land cover/use type and directly impacted urban carbon sequestration. Urban development, increased demand for construction land, and urban expansion are major factors reducing regional carbon sequestration capacity [17
]. Land cover/use changes, caused by human activity, directly affect soil carbon and the carbon cycle [18
]. Change of land cover/use types affects carbon emissions and carbon sequestration [19
]. Forests and grasslands are typical land cover types possessing strong carbon sequestration capacity [20
]. Green areas in cities capture and store relatively small amounts of the carbon emissions from human activities [21
]. This issue has been discussed; however, no studies depict carbon sequestration service by integrating LUCC and the von Thünen model. This paper therefore surveys the situation with land cover/use and carbon emissions, and processes and evaluates the data on land-use and carbon emissions. We studied the Gang’an area and developed a method for recognizing the distinct characteristics of carbon sequestration and the spatial distribution of those characteristics. We then analyzed the effects of land cover/use on carbon sequestration and its spatial evolution. This study provides a scientific basis for developing low carbon policies and planning for urban or rural land. Mancebo and Salles agree that it is meaningless to talk about urban sustainability if we stop at the city limits and that sustainable urban policies should consider an urban–rural continuum [23
]. Therefore, we take into account a region that has expanded in all directions beyond the urban administrative boundaries.
The main purposes of the simulation of land cover/use changes are to visualize the trends in those changes under different policies and to provide reference information for the modification of government strategy [24
]. It is helpful to investigate the mechanism of land cover/use changes and even to optimize land cover/use allocation for sustainable development [25
]. By simulating land cover/use changes under different total factor productivity (TFP) scenarios, Luo depicted land cover/use competition in socio-economic development and the protection of environment [26
]. Based on simulations of land cover/use using land use planning scenarios, natural development scenarios, ecological-oriented scenarios and farmland protection scenarios, and comparing them with local situations, one study depicted the driving factors of land use [27
]. Thus, the simulation of scenarios is used in this study to elucidate the impact of land cover/use changes on the carbon cycle spatial distribution, and the results can be used to support the formulations of land cover/use.
A generally-accepted definition of urban sustainability is lacking. Taking ecosystem and human well-being into account, urban sustainability can be defined as an adaptive process of facilitating and maintaining virtuous cycle between ecosystem services and human well-being through concerted ecological, economic, and social action response to changes within and beyond the urban landscape [28
]. Urban sustainability is closely associated with ecosystem services and their relationship to society; Nassauer et al. concluded that it is possible to promote urban ecosystem service through urban sustainability planning [31
]. However, a city that derives most of its ecosystem services from other regions, nationally or internationally is subject to myriad environmental and sociopolitical uncertainties, thus is hardly sustainable in the long run [32
]. Thus, timely and effective assessments of the changes of regional ecosystem services are an important issue in the fields of urban ecology and sustainability science [33
]. Therefore, no matter which aspect of urban sustainability is being studied, ecosystem service function, environment and urban landscape pattern are the important parts of urban Sustainable development.
Guang’an is a typical mono-centric spatial structure city, similar to other mono-centric cities in China. The actual city area is small, with sparse internal vegetation. The main park is located on the urban edge, and the ecosystem carbon sequestration capacity of the city is minimal. Increasing population and industrial development have hastened the expansion of housing and commercial construction and the city’s carbon sequestration capacity has decreased significantly. In line with the current urban development trend, local government has implemented policies to reduce emissions and return the farmland to forest, which are helpful for restoring the ecosystem. However, certain ecological and environmental management issues remain.
The urban ecological land is distributed between three administrative regions of Guang’an, thus the carbon pool is scattered across these three administrative areas, which is a problem for unified management.
From the projection of land cover/use, the administrative boundary has broken the integrity of the ecological system. The boundary between the ecological systems and the administrative divisions is inconsistent, which affects government management of the ecological environment. All of the administrative regions need to coordinate efforts to address the combined problems of ecological services and sustainable urban development.
There is a lack of ecological and land cover/use data analysis at the local level. Verification of regional land cover/use planning and policies would be useful.
In this study, we attempted to address the following questions: (1) Does the spatial distribution of carbon cycle modeling follow the von Thünen model? (2) If so, what is the spatial distribution pattern? (3) What are the differences in spatial distribution for different possible land cover/use scenarios?
In this paper, the concentric ring theory and the mono-centric model are used to study the spatial distribution of the carbon cycle in mono-centric cities and suburbs. The results show that the spatial distribution of the carbon cycle in Guang’an follows the concentric ring pattern. Our conclusion is consistent with the concentric ecological model proposed by Li, combined with the concentric ring theory. Our results agree that the ecosystem had a concentric ring distribution with the city at the center [67
]. Through discussing the influence that regulation of the carbon sequestration zone and land cover/use has on the carbon sequestration capacity of the city, we conclude that it is significant for formulating land cover/use policies that maintain the carbon cycle stability of cities.
We analyzed the results and concluded that the government’s afforestation policy is a direct factor in the expansion of the carbon sequestration service zone. Forested land in the study area increased by nearly 12,000 ha over a two-year period, partly due to the government policy of returning farmland to forests. Forests are primary carbon absorbers, and the carbon sequestration function of plantation forests can help mitigate global climate change [68
]. Some scholars have also come to a similar conclusion; that is, an important factor in the change of carbon sequestration eco-services is that the government reduces ecological land area to increase construction land area [70
]. Our results also verify the conclusions of these scholars. Based on the land use policy, we also discuss the impact of energy consumption on the spatial distribution of the carbon cycle. Under the environment of emissions reduction policy, the goals of structural and project emissions reduction have been achieved. These policies helped to reduce the total emissions of the province [73
]. The results of the carbon sequestration zone evaluation indicate that government policy has not changed the structure of the concentric ring, but the original carbon sequestration zone has widened. Afforestation and forest restoration can increase the carbon sequestration function as well as promote water conservation, climate regulation, and resource supply. The absorption rate of urban carbon emissions in Guang’an was 58.84%. The “China Greenhouse Gases Bulletin in 2013” noted that about 55% of
is absorbed by the biosphere and ocean around China, so the absorption rate exceeds this average. The C absorption rate in Guang’an is higher than the national average.
The spatial distribution of the carbon sequestration zone results from the influence of both human activities and natural phenomena. The results of carbon stocks show that forested land is distributed between the city and a large area of cultivated land in the form of a ring because of human activities, urban expansion, and the reclamation of cultivated land. According to the spatial distribution of carbon stocks (Figure 2
), the carbon sequestration in the north of the city is larger than that in the south, and this is closely related to the southerly and northerly wind directions. Southerly winds predominate in Guang’an, accounting for about 85.67% of the region’s winds [74
]. This is subject to further analysis in future research.
China is currently promoting the concept of urban ecosystem services, and urban managers are trying to plan more compact and sustainable cities through urban eco-spatial division and land cover/use change. Based on scenario analysis, the benchmark development scenario was not the best development mode. The different scenarios included different land cover/use structures with the main differences being the proportion of forest land, grassland and cultivated land. The benchmark development scenario trend showed that the proportion of cultivated land in the carbon sequestration zone was the highest, accounting for 47.24% of the area of the area, reducing the coverage of forest land and weakening the carbon sequestration capacity compared to the ecological protection scenarios. With the ecological protection scenario, forest coverage could reach 44%, the carbon sequestration capacity is the strongest, and the carbon stock per square meter was
Mg. Compared with the benchmark development scenario, carbon density increased by 10
, but, in the carbon sequestration zone, the proportion of cultivated land was lower than the minimum required by government policy. In the cultivated land protection scenario, the size of the ring was unchanged but the internal land cover/use was more balanced. The carbon sequestration capacity improved compared to that in the benchmark development scenario, increasing by 7.1
, and the amount of cultivated land was maintained within the range required by government policy, which promised to meet the requirements of food supplying. To maximize the amount of carbon stocks, both ecological protection and human economic activities should be considered [75
]. We cannot blindly change non-ecological lands to ecological land. The sustainable carbon balance is a natural and social combination system. The carbon sequestration capacity of adjacent areas is changed due to these mutual influences, so we should strengthen the spatial link. This allows regions with strong carbon sequestration capacity to spread their ecological effects and improve regions with weak carbon sequestration capacity [76
]. Our scenario simulation results verify the correctness of the view that reasonable land use policy to maintain the stability between human well-being and ecosystem is necessary. Reasonable adjustment of land cover/use structure and spatial distribution is necessary to ensure ecological and economic stability and to achieve the sustained change of urban structure and function. Calthorpe, a famous urban planning scholar, pointedly observed that sustainability is to seek a balance among the social, economic and ecological environment, so that it can exist forever [77
]. Therefore, to achieve the changes that urban scale (population, land, and production) is getting bigger, urban structure is gradually coordinated and urban function is gradually sustainable, urban sustainable development at a certain time and space scale, through long-term sustainable urban growth and its structural evolution, is necessary to not only meet the needs of contemporary reality, but also to meet the requirements of future development. In terms of the sustainable development of Guang’an, the planning of urban eco-service spatial patterns and land cover/use should take the protection of cultivated, protection of ecological areas and the functional requirements of urban development into consideration at the same time.
Many cities have developed from the mono-centric model, not only in China, but also in other world regions [78
]. Many cities have a multi-centric spatial structure developed from a mono-centric structure. For example, in Seoul, its urban spatial structure in the 1880s was a typical mono-centric pattern with only one central business district [80
]. The mono-centric model is still considered an important theory that can describe the spatial structure of cities [81
]. We consider urban structure, development mode, carbon sequestration capacity, and the assessment value of carbon emissions, thus our conclusions have high application value to the sustainable development of Guang’an, and a certain reference value for the spatial distribution of carbon cycle of other cities with similar structure to Guang’an. The results have application value and guiding significance for the planning of mono-centric cities. We can identify and delimit the carbon sequestration functional area in the city according to land cover/use data and urban carbon emission data. Whether, and how, this can be applied to complex urban structure areas are subjects for further study.
Further studies should focus on three areas: First, there should be improved selection and correction of data. The method used for calculating the spatial distribution of carbon sequestration capacity and carbon emissions should also be improved. For example, this paper did not consider the relative carbon sequestration capacity strength of land cover/use patches in a city. An improved InVEST model with this consideration would better reflect the demand and supply values of the city. The accuracy of the estimated spatial distribution of carbon emission concentrations is insufficient and this reduced the computational accuracy of the inner boundary of the carbon sequestration zone. Similarly, the lack of an accurate classification of woodland areas reduced the computational accuracy of carbon stocks. Accurate determination of urban and peripheral concentrations will be the focus of a follow-up study. Second, the changing laws of carbon sequestration zone and the effects on carbon sequestration should consider land cover/use as well as the natural environment (e.g., wind speed and wind direction), urban construction and development needs, and production factors. Integration of a set of programs able to solve multiple questions and balance the ecological service and the reasonable use of land will be the goal of additional studies. Third, a discussion of the carbon sequestration zone of the multi-centric city merits further study because, in addition to Guang’an and other fourth-tier cities, the second- and third- tier cities of China are mostly multi-centric cities.
This paper discussed the spatial distribution of carbon cycle for mono-centric cities, based on the von Thünen concentric ring theory, using an integration of the InVEST model and the revised atmospheric diffusion model, combined with land cover/use, carbon emissions and ecological data, and we discuss the various factors of land cover/use change on urban carbon cycle and its spatial pattern evolution using scenario simulations. This analysis provides a basis for effective planning and using of urban land. The results show that:
Carbon sequestration and carbon emissions in the mono-centric cities follow a concentric ring pattern. The first annular zone (Zone I) represents the carbon emissions, which lie at the concentric ring center; the second annular zone (Zone II) represents the carbon sequestration service; and the third annular zone (Zone III) represents stable carbon stock. From the view of supply and demand, combined with carbon emissions and carbon sequestration capacity data through identification and delimitation of carbon sequestration zone in the mono-centric cities, this paper provides evidence for the government carrying out ecological environment assessment and planning.
In the study area, the carbon sequestration zone is an annular region 6623–14,827 m from the city center in 2014, and an annular region 6351–15,072 m from the city center in 2016. The structure of the concentric ring has not changed, but the spatial distribution of carbon sequestration and carbon density has changed due to fossil energy consumption and land cover/use change. Compared with 2014, the carbon emission zone shrunk while the carbon sequestration service zone expanded in 2016.
The eco-service capacity of the carbon sequestration zone relates to the spatial division and different land cover/use types. In 2016, the carbon density of in the carbon sequestration zone was 417.2 , compared with 392.1 in 2014. It had obviously increased. Compared with 2014, the carbon stocks in 2016 increased by 4023.50382 thousand tons (kt), and this is the main factor of the forest land area increasing. This conclusion coincides with the actual situation of Guang’an. Urban green cover from 2014 to 2016 increased by 12,000 ha, which is closely related to government in Guang’an implementing the policy of returning farmland to forest.
Based on the urban land-use and eco-service time variation simulation, it is found that the current carbon-sequestration eco-service in Guang’an is not the best development condition. Under the ecological protection scenario, the carbon sequestration capacity is the strongest, but the conversion of cultivated land is too great, which is lower than policy demands. Under the cultivated land protection scenario, the government constrains the conversion of cultivated land to other land cover/use types. Although the total carbon stocks are the lowest, the carbon density ranks second to ecological protection scenario because of a higher proportion of forest land than the other scenarios. Under the benchmark development scenario, carbon sequestration capacity was significantly less than the other two scenarios, even though the carbon sequestration zone expanded. Therefore, the function of urban eco-services and its influence factors of change are important components of urban sustainable development. To achieve the sustainability of Guang’an, the planning of urban eco-service spatial patterns and land cover/use should consider the protection of cultivated and ecological areas at the same time. It is possible to provide ideas for coordinating and solving the eco-service, cultivated land protection, urban constructions and other issues through spatial planning and policy guidance in the land-use circle. In Guang’an, there was a positive correlation between the carbon sequestration capacity and the forest coverage of the carbon sequestration zone, with higher forest coverage leading to greater carbon sequestration capacity. There remains a need to rationally allocate the proportion of different types of land cover/use to ensure a balance among ecological services such as carbon sequestration and food supply, and to coordinate sustainable development of urban eco-service, cultivated land protection, and urban construction.
The research method of this paper can be used in other carbon cycle spatial studies of mono-centric cities, and has the flexibility to use different data sources. Data source and processing accuracy will improve in the future. If high-resolution remote sensing data and other environmental data become more readily available, the research of spatial distribution of carbon cycle nad land-use planning in urban ecological zone will be more rational.