| The urbanization and its environmental effects are of much importance to the surface processes research.The urbanization that had direct or potential hazard to urban environment becomes a global concern to the environment.As a very important component of the urban ecosystem,urban soils have been undergoing great impact from human activities.The impacts of human activities on urban soils are usually more extensive than those on natural soils,including sealing,compaction,degradation,land filling,and mixing.Soil sealing,defined here as land covered by impervious surfaces such as housing,roads,or other pavements,is one of the main characteristics of urbanization and changes in urban land cover.Artificial soil sealing in urban area reduces the exchange of material and energy between soil and other environmental compartments,thereby impacting soil functioning and the urban environment.In the Thematic Strategy for Soil Protection,soil sealing is identified as one of the eight most important threats to soil.Soil organic carbon(SOC)is a very important indicator to soil function and quality,and is the most important C pool of terrestrial ecosystems.Thus,study on SOC transformation in the urban soils can promote the studying of the changes in functions and qualities for urban soils,as well as to evaluate the effects of urban soils on the environment.Although the shift to intensive agriculture has been the primary driver of land-use change,at present more than half the global human population resides in cities and towns,a figure projected to rise to 70%by 2050.As a consequence,urban areas are increasing in extent at a greater pace than any other land-use type.The resulting impacts on SOC stocks in urban areas,however,remain poorly characterized,especially for the soils under impervious surfaces.In this study,the impacts of soil sealing,across a range of types of impervious surface,including road pavements,residential areas and squares,on SOC storage and qualities were investigated.Soil samples were collected from the urban areas in Nanjing and Yixing,both of which had rapid urbanization these years.In comparing SOC in sealed soils with that in open soils,the impact of the artificial soil sealing in urban areas on SOC can be easily obtained.The major results are summarized as follows:(1)Great differences in soil properties exist between the sealed and open soils,giving various conditions for the transformation of SOC.Soil bulk densities and clay contents were greater in sealed soils than in open soils due to compaction which was extensively impacted by human activities.Chemical properties were more variable than did physical properties,and they were more variable in urban sealed soils.Concentrations of SOC,water soluble organic carbon(WSOC),total nitrogen(TN),and mineral nitrogen(Nmin)of the sealed soils were significantly lower than those in the open soils,while concentrations of total phosphorus(TP)and Olsen-P were significantly higher in urban sealed soils.In addition,the open soil had higher Cr concentration(88±5 mg kg-1)than did the sealed soil(75×2 mg kg-1,P<0.01),suggesting there was potential heavy metal contamination in urban open soils.The artificial soil sealing was the crucial factor that impacted soil properties,since significant differences in soil properties were found when compared between the sealed and open soils,whereas no significant differences were observed when compared among the sealed soils.Soil microbial organisms were susceptible to soil sealing.Soil sealing in urban areas reduced the amounts of fungi and actinomyces in urban soils,but did not reduce the bacteria's.Basal respiration(0.46×0.06 ?g g-1 h-1)and 35-d accumulated CO2 emissions(108±11 mg kg-1)of urban sealed soils were 35%,33%,respectively,of that of open soils,indicating that the sealing of soil also resulted in decreased soil microbial activities.Unlike soil properties,land use types in the sealed soils could affect soil microbial functional diversity.Substrate use patterns showed that microorganisms in urban sealed soils had higher utilization of polymers but lower use of carbohydrates and amines/amides.(2)Soil organic carbon of urban sealed soils,which had low proportions of labile organic carbon and high aromaticity,was more stable than that in open soils.Urban sealed soils had lower microbial activities than did the open soils,indicating that the SOC transformation in the sealed soils had less driving forces.Soil organic carbon fractions of urban sealed soils varied widely,in which<63?m fraction accounted for the greatest part of SOC.In comparison with open soils,the sealed soils had higher amounts of light fraction,which was mainly consist of black C due to the contamination of bitumen.It can be inferred that using SOC fractionation method that was widely used in natural soil to study urban SOC transformation was ideal.However,using these methods to predict SOC transformation in urban sealed soil based on models(e.g.,RothC)remained uncertain,because of differences in SOC fractions between urban and natural soils.The solid-state 13C nuclear magnetic resonance(NMR)spectra analysis showed that the sealed SOC had lower proportions of carbohydrate but higher proportions of aromatic carbon than did the open soils.Moreover,higher ratios of alkyl C to O-alkyl,and aromatic C to aliphatic C in the sealed soils suggested that SOC in the sealed soils was more decomposed and had more complex structure,thereby being more stable than that in open soils.(3)SOC contents decreased as soil sealing processed,and was highly regulated by soil total nitrogen content and microbial activity.The temporal changes of SOC contents in urban sealed soil were investigated by using the samples collected from Yixing City.The contents of SOC and TN in urban soils decreased after soil sealing,although some large variations were observed in the regression.The reduction of SOC and TN in urban soils after sealing had equilibriums,which was roughly 2.6 kg m-2 for SOC,and about 20-year soil sealing was required before SOC reached the equilibrium.Soil bulk density,TN,cation(Ca+Mg+K),and microbial activity affected SOC transformation in urban soils.The results of canonical correlation analysis showed that WSOC,TN,and 35-d accumulated CO2 emission played crucial roles in regulating SOC transformation of the sealed soil.(4)The sealing of soil in urban areas reduced SOC density,making urban soils to be C sources to the atmosphere.The average SOC density of urban sealed soils at 0-to 20 cm soil layer was 2.35 kg m-2,which was significantly lower than that in the open soils at equivalent depth(4.52 kg m-2,P<0.05).The SOC density that was 2.5-3.0 kg m'2(0-20 cm)might be recommended to calculate SOC storage for urban sealed soils in continental or global scales,given that SOC in the sealed soils was stable and varied little among existing research results around the world.The urban area of Nanjing City increased 489 km2 from 1980 to 2008,mainly at the expense of agricultural land located in.southeast and northwest of the city.The urbanization in Nanjing during that period caused 1.06×1012 g of SOC loss(3.8×1010 g per year).In the continent,the urbanization that had 10790.8 km2 increases in urban areas during 2000 and 2008 resulted in 56×1012 g of SOC loss,with a loss rate of 7×1012 g y-1.Meanwhile,the green in urban areas could accumulate 18.5×1012 g of SOC during that period,which was lower than the loss due to urbanization.Therefore,the sealing of soil in urban areas made urban soils to be C sources to the atmosphere. |
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