| The development of human society and the acceleration of urbanization have a huge impact on the ecological environment,more and more agricultural land and natural land into urban land.Changes in urban vegetation and landscape patterns affect soil organic carbon content,and soil organic carbon pool influences global climate change and global carbon balance.The study of urban soil organic carbon content is essential to understand the human impact on soil carbon storage.Urban development is often restricted by many factors.scientific quantification of the“City-suburb-countryside” gradient provides a basis for the study of urbanization.Based on the remote sensing satellite image,this paper uses the grid method to evaluate the urban physical index and Landscape Pattern Index.Based on principal component analysis(PCA),a gradient model of “Urban-suburban-rural” in Hefei was constructed by using the factor analysis method,the results show the spatial heterogeneity of the urban-suburban-rural landscape pattern in Hefei,and lay a foundation for further research.The soil organic carbon content,soil bulk density,p H value,total nitrogen,total phosphorus and total potassium content were measured through field investigation and sampling,the gradient distribution characteristics of soil physical and chemical properties under different urbanization degree were analyzed,and the relationship between each other was explored.The gradient effect of urbanization process on the spatial distribution of organic carbon was analyzed from two aspects of physical and landscape factors,and the relevant models were constructed.The study concluded the following:(1)Using three urban physical indexes(ROAD,DISTANCE,PLAND2)and three urban landscape indexes(LSI,SHDI,PAFRAC)to construct the urbanized index UI model can reflect the urban landscape characteristics under the urbanization development of Hefei.The areas of-1.320<UI ≤-0.492 are defined as villages,-0.492<UI≤0.310 are defined as outer suburbs,0.310<UI≤1.357 are defined as inner suburbs,and 1.357 <UI<2.691 are defined as urban areas,the value of UI increases with the gradient from country to outer suburbs,inner suburbs,and then to urban areas.The proportion of land gradient in Hefei,rural area(43.925%),exurb area(27.503%),urban area(14.553%),suburb area(14.019%),the spatial form is fanshaped,three-wing urban structure.(2)The urban green space(forest,grassland and water area)can effectively reduce the urbanization index value,but the large area artificial construction patch(road,building,etc.)can cause the urbanization index to increase.As the UI value of the urbanization index increases,the changes of patch density in the "urban-suburbanrural" gradient transect in Hefei showed the following characteristics:(1)The overall change trend of patch density in urban area was larger than that in suburb and far suburb.(2)In the rural and exurb areas where the agricultural patch is more dominant,and in the urban areas where the artificial construction patch is more dominant,the patch shape is less complex,and the patch shape is more complex in the peri-urban areas where the urbanization degree is moderate.(3)The average patch size in rural and remote suburbs was larger than that in urban areas.(4)The landscape connection degree of the whole village is larger than that of the outer suburb than that of the inner suburb.(3)The high value of UI and the low value of UI tend to have the high value of organic carbon,and the low value of organic carbon is concentrated between-1 and 1.The UI of urbanization index can reflect the spatial distribution of 15.9% organic carbon.The organic carbon content in the transect is log-normal distribution,and the average value is on the gradient of "urban-suburban-rural",which shows the character of the outer suburb,the outer suburb,the outer suburb and the outer suburb.(4)There was a significant correlation between soil bulk density,total nitrogen and total phosphorus content and organic carbon content,while there was no correlation between total potassium and soil p H value and organic carbon content,human input and urban construction activities are the main reasons for the spatial heterogeneity of soil physical and chemical properties.Both total nitrogen and total phosphorus had significant positive correlation with soil organic carbon content,which could increase the accumulation of organic carbon in soil surface layer.Soil bulk density has negative correlation with soil organic carbon content.Higher soil bulk density usually indicates low soil porosity and poor water permeability,which is disadvantageous to the accumulation of soil surface organic carbon.Moderate soil p H was the most suitable for the accumulation of organic carbon,acidic soil was the least suitable for the accumulation of organic carbon.(5)The results showed that the most significant factors affecting soil surface organic carbon content in Hefei were population concentration and the change of landscape patch shape during urbanization.The most significant factors influencing the spatial distribution of organic carbon content were population density(POPD)and area-weighted average patch SHAPE index(SHAPE).Population density(POPD)mainly reflected the influence of social factors such as population concentration and urban construction on organic carbon content,and positively correlated with organic carbon content,while SHAPE index(SHAPE)mainly reflected the influence of landscape patch SHAPE on organic carbon content and negatively correlated with organic carbon content.In the traditional linear regression model,POPD and SHAPE index could explain 36.0% and 10.8% of soil surface organic carbon gradient spatial distribution respectively.(6)Compared with the traditional regression model,the geographical weighted regression model can reflect the spatial distribution characteristics of organic carbon content more accurately.The influence of population density on organic carbon content was similar and positively correlated in the urbanized suburbs and urban areas,and the influence of landscape patch shape was spatial heterogeneity,some units have a large influence on the grid,and some units have a small influence on the grid,all show negative correlation.In the less urbanized exurban and rural areas,the population density in the western transect has a greater influence on the organic carbon content than that in the eastern transect,and the organic carbon content increases with population density.The organic carbon content was negatively correlated with population density in the low urbanization area with better natural ecological environment.The population density has less influence on the spatial distribution of organic carbon in the region with higher concentration of organic carbon.The landscape patch shape on the western transect had little effect on the organic carbon content,and showed a negative correlation.However,in the rural and exurb areas where the main agricultural land was used,the organic carbon content was positively correlated with the area-weighted mean patch shape index,and generally decreased with the increase of the complexity of the patch shape.In the region where the organic carbon is more abundant,the patch shape has a greater influence on the spatial distribution of organic carbon.Figure [44] table [18] reference [89]... |
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