Urban Land Conversion Reduces Soil Organic Carbon Density Under Impervious Surfaces

Abstract Accurate assessments of soil organic carbon (SOC) storage beneath impervious surface areas (ISAs, SOC ISA ) are key for understanding the urbanization impact on carbon pools, but previous studies either ignored the SOC ISA pool or overlooked the urbanization impacts on the SOC pool. Based on observations from 152 sampling sites in 43 cities, we show that mainland China has a SOC ISA stock of 1,016 Tg to 100‐cm depth, with a mean density of 6.21 ± 3.90 kg m −2 . Comparison between SOC ISA and the SOC of background soil (SOC background ) indicates ∼19% SOC loss due to ISA conversion, similar to the effect of cropland conversion. Unlike the vertical pattern of SOC background , which declined faster in the upper soil layers, SOC ISA decreased linearly with depth. Moreover, the SOC ISA is uncorrelated with SOC background and decreases with precipitation. These unique patterns indicate the SOC loss mainly caused by topsoil removal during land conversion. The fact that the SOC ISA of older ISA was not lower than that of newly converted ISA further confirmed that soil sealing preserved SOC. Finally, both the high correlation between SOC ISA and SOC PSA (SOC of the adjacent pervious surfaces) and the converging SOC densities in urban soils showed strong influences from urban greenspace on SOC ISA . Based on the findings, we recommend improving greenspace management to increase the presealing SOC stock, preventing ISA construction in wet/warm seasons to mitigate SOC loss, and developing technology to seal carbon stock under ISA. We conclude that SOC ISA is a major component of the urban carbon pool. It has a unique spatial pattern and cannot be estimated using surrogate data such as SOC background or SOC PSA .