Simulation And Prediction Of The Effects Of Urbanization On Carbon Cycle In Nanjing City

The process of urbanization and its impact on ecosystems is an important part of the study of the Earth's biochemical processes.With the rapid development of global urbanization,more and more attentions have been paid on the research of urban ecosystem.The process of urbanization has caused artificial soil sealing in urban area,which leads to the degradation of soil function and the decrease of vegetation coverage.It also affects the carbon cycle of urban ecosystem.With the development of the urban scale and population,urban soil sealing areas will continue to increase and change the urban carbon cycle profoundly.The relationship between the change of the urban carbon cycle and the process of the urbanization has become a global concern.At present,the simulation of carbon cycle in urban areas was mainly focused on the simulation of urban green space and urban forest carbon fluxes.There were few studies on the decomposition process of organic carbon under impervious surfaces.Therefore,the study on the spatial simulation of the process of urban carbon cycle has great significance to the coordination of urban economy and ecological construction.In this study,the SOC storage and qualities of the soil under the impervious surfaces were investigated on the basis of the field sampling.By combining soil sealing carbon cycle model and open soil carbon cycle model,we have simulated the carbon cycle process in urban areas of Nanjing from 1980 to 2015,which had rapid urbanization these years.The CA model was used to simulate the urban expansion trends in Nanjing under four different scenarios.The urban carbon cycle model was applied in different urban expansion scenarios to analyze the different changes of urban carbon cycle and put forward suggestions to the development of Nanjing city in the future.The main results in this paper are as follows:(1)Based on the study of the process of organic carbon decomposition under urban impervious surfaces,the model of urban soil sealing carbon cycle was established with the consideration of temperature,water content and inert organic carbon content limitation.The urban carbon cycle model was established by combining the urban soil sealing carbon cycle model and urban open soil carbon cycle model to simulate the carbon cycle of urban areas.The results show that the expansion of impervious surfaces can cause the decrease of annual NPP,soil organic carbon density and soil carbon pool and affect the carbon cycle of adjacent open soils.The simulation results are in good agreement with the measured results,which shows that the model can simulate the carbon cycle of urban area accurately.(2)We found that the process of urban carbon cycle in Nanjing is seriously affected by human activities and shows strong temporal and spatial heterogeneity by simulating the carbon cycle in Nanjing city from 1980 to 2015.The annual NPP,soil organic carbon density and soil organic carbon pool content in Nanjing showed a decreasing trend with time and decreased by 27.1%,6.3%and 5.8%in 30 years,respectively.On the other hand,the annual NPP,soil organic carbon density and soil organic carbon pool content in the center of the Nanjing city are lower,which are higher in areas that are away from the main city.The results showed an increased trend from the city center to the surrounding areas.The change of annual NPP,soil organic carbon density and soil organic carbon pool content in Nanjing showed a certain correlation with the expansion of urban impervious surfaces.The annual NPP,soil organic carbon density and soil organic carbon pool content is lower in areas which have more impervious surfaces.The urban carbon cycle was affected by the expansion of impervious surface obviously.(3)We calculated the urban expansion and adaptability index of the study area in different scenarios with the consideration of the current urban development rate and the limiting factors of urban development in the future.The expansion of the city from 2015 to 2025 was predicted by the CA model,which used the urban expansion and adaptability index.In the next 10 years,the expansion of impervious surfaces will happen in the center of the city in the densification scenarios.In the sprawl scenarios,the areas of impervious surfaces will increase in Jiangning District,Luhe District and Pukou District as well as the center of the city,which lead to a situation of multi-core urban expansion.Under the conditions of giving priority to the protection of cultivated land resources,the urban expansion will be mainly occupied in non-cultivated areas and the rate of impervious surfaces expansion will be limited.Under the condition of preferentially protecting the ecological land,the urban expansion will be happened in cultivated areas mostly and the impervious surface areas will be increased obviously(4)In the four urban expansion scenarios,the annual NPP,soil organic carbon density and soil organic carbon pool content in the center of the study area showed a significant downward trend.In the sprawl scenarios,the annual NPP,soil organic carbon density and soil organic carbon pool content will be lower than those in the densification scenarios,except the center of the city.In the case of priority protection of ecological land,the degree of carbon cycle change in each district of the study area will be higher than that of priority protection of cultivated land.Based on the needs of urban development and ecosystem protection in the future,it is suggested that the urban development of Nanjing should choose the model of urban densification and the priority protection of cultivated land in the next 10 years instead of the urban sprawl and ecological land priority protection model.