Urbanization induced changes in the accumulation mode of organic carbon in the surface soil of subtropical forests

• SOC content and fractions were studied in surface soil of urban and suburban forests. • Urban forests contained lower SOC content than suburban forests. • Both labile and non-labile SOC contents were lower in urban forests. • Urbanization may have altered the SOC accumulation mode of forest soil. Forests cover a considerable proportion of the land area and function importantly to regulate the global climate system as carbon (C) sinks. Rapid urbanization process may be changing the C sink function of forests worldwide, but its potential effects on the accumulation of soil organic C (SOC) remain inconclusive. In this study, we collected litter and soil samples in the surface layer of six urban and suburban forests to fractionate SOC by combining physical, chemical and biological methods and quantify the contents of these SOC fractions. The results showed that urban forests contained significantly lower contents of labile and non-labile SOC fractions, therefore contributing to significantly lower total SOC content in urban than in suburban forests. Interestingly, we observed that the SOC content was significantly correlated with the litter C stock in urban forests but not in suburban forests, while it was significantly correlated with the soil microbial biomass in suburban forests but not in urban forests. These scenarios suggest that urbanization could have substantially regulated the SOC accumulation mode in the surface soil of subtropical forests, e.g., from microbial filtering pathways to abiotic ones, therefore influencing the stabilization of SOC in these subtropical forests.