Installation of impervious surfaces in urban areas has attracted increasing attention due to its potential hazard to urban ecosystems. Urban soils are suggested to have robust carbon (C) sequestration capacity; however, the C stocks and dynamics in the soils covered by impervious surfaces that dominate urban areas are still not well characterized. We compared soil organic C (SOC) densities and their stabilities under impervious surface, determined by a 28-d incubation experiment, with those in open areas in Yixing City, China. The SOC density (0–20 cm) under impervious surfaces was, on average, 68% lower than that in open areas. Furthermore, there was a significantly (P<0.05) positive correlation between the densities of SOC and total nitrogen (N) in the open soils, whereas the correlation was not apparent for the impervious-covered soils, suggesting that the artificial soil sealing in urban areas decoupled the cycle of C and N. Cumulative CO₂-C evolved during the 28-d incubation was lower from the impervious-covered soils than from the open soils, and agreed well with a first-order decay model (C_t = C₁+C₀(1-e^-kt)). The model results indicated that the SOC underlying capped surfaces had weaker decomposability and lower turnover rate. Our results confirm the unique character of urban SOC, especially that beneath impervious surface, and suggest that scientific and management views on regional SOC assessment may need to consider the role of urban carbon stocks.

Artificial soil sealing in urban areas has attracted increasing attention due to its potential hazard to urban ecosystems. It has negative impacts on soil function and the urban environment, since the impervious surface can hamper the exchange of material and energy between the soil and other environmental compartments. However, information about the effects of artificial soil sealing in urban area on soil quality and properties, especially the microbiological components, is still limited. Ten plots which differed in land use were selected from Nanjing City, China, to investigate the effects of impervious surfaces on microbiological characteristics in urban soil. Plot types were paved road, residential paved square, residential paved alley, and grassed area. Soil microbial biomass carbon (Cmic) and nitrogen (Nmic), and activities were analysed, and the microbial functional diversity of fine earth (<2 mm material) was characterised by the Biolog EcoPlate technique. Mean concentrations of soil organic carbon (SOC), Cmic, and Nmic in fine earth from the impervious areas (0–20 cm) were, respectively, 6.5 g kg–1, 55.8 mg kg–1, and 12.2 mg kg–1, which were significantly lower than concentrations from grass areas. Urban sealing also resulted in decreases in soil microbial activity and functional diversity, but the influences on soil microbial diversity varied among land uses, with road pavement having the most negative effect. Substrate use patterns showed that microorganisms in urban sealed soils had higher utilisation of polymers (P < 0.05) but lower use of carbohydrates and amines/amides (P < 0.05). These findings demonstrate that SOC in the sealed soils was more stable than in open soils, and the installation of impervious surfaces such as asphalt and concrete, which are very common in urban areas, can result in decreases in SOC content, soil microbial activity, and diversity in urban soil.