Effects Of Urbanization On Soil Organic Carbon Fractions In The Remnant South Subtropical Evergreen Broad-leaved Forests

With the continuous promotion of urbanization process,the environmental factors of forest ecosystem have changed and then affect the carbon accumulation of forest soil.The change of forest soil organic carbon fractions is one of the key in the study of soil carbon accumulation.Understanding the relationship between soil organic carbon fractions and environmental changes is the basis on which we can predict forest carbon sequestration potential under the background of urbanization.Therefore,with an emphasis on soil organic carbon fractions,this study investigates on the soil organic carbon of evergreen broad-leaved forest,a zonal vegetation in the highly urbanized Pearl River Delta.We collected soil samples from urban sites?BY,XT,DL forests?and suburban sites?DH,XT,SM forests?to study the changes of forest soil organic carbon?SOC?and its fractions using chemical fractionation,density fractionation,and solid-state ¹³C NMR analyses.In combination with the study of the changes of environmental factors between urban and suburban forest,the thesis will conclude the changes of soil organic carbon accumulation mechanism in evergreen broad-leaved forest induced by urbanization.Our results included that:First,the trend of changes in SOC and its fractions were consistent in all the soil layers.The labile SOC?ROC and LF?content was lower than those in suburban forests without significant difference but the non-labile SOC?NROC and HF?content has significantly reduced in the urban forests.What's more,the changes of the total amounts of SOC and its chemical structure are affected by the decrease in non-labile SOC fractions more obviously.Thus,the deficiency of non-labile SOC fraction accumulation induced by urbanization is an important driving factor of the change in SOC contents.Soil C lability was calculated base on the distribution of labile SOC and non-labile SOC fractions and it was found to increase in the urban forests,which means that SOC would be decomposed more easier in the urban forests.Second,the ¹³C NMR spectroscopy of whole soil among the study remnant forests showed close similarities in overall molecular structure:O-alkyl C and Alkyl C dominated organic C,followed by Aromatic C,and the smallest intensity was found in the Carbonyl C.Comparing with suburban forests,in urban forests the Alkly C proportion significantly lower especially in the surface soil,and the proportion of O-alkly C and Aromatic C tended to increase,but the Carbonyl C proportion had no obvious changes.The analysis of SOC chemical structure showed that the ratio of Alkly C to O-alkly C had a sufficient reduction in the urban forests,and the aromaticity tended to increase while the recalcitrance index tended to reduce.These trends,especially in the surface soil,are considered to be an indication of decrease in SOC stability.Third,We have examined the variety of soil physical and chemical properties and the soil microbial community changes in the urban forests.There are higher soil temperature,more serious soil drought,less nitrogen contents but higher phosphorus contents in the urban forests.The decrease of soil pH was found to have close relationship with air pollution and soil heavy metal pollution in the sites.Environmental changes cause the reduction of soil microbial biomass and effect the microbial community.However,it is not beneficial to soil carbon sequestration.The effect of urbanization on the accumulation mechanism of forest organic carbon soil is mainly reflected in two aspects:the change of external environment and the internal change of organic carbon in its fractions and structure.The results indicate that the effects of the environmental changes induced by urbanization on soil C sequestration are largely affected by the soil nitrogen contents,soil moisture,soil temperature and soil microbial community,it is an significant contributor to the decrease of SOC content in the urban forests.On the other hand,it is found that the difference of SOC fractions and structure between urban forests and suburban forests will lead to a change in SOC stability.Influenced by urbanization,the soil organic carbon in the urban forest is more easily mineralized and the carbon sequestration capacity continued to decline,which go against the accumulation of soil organic carbon.