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, SOCISA) are key for understanding the urbanization impact on carbon pools, but previous studies either ignored the SOCISA 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 SOCISA stock of 1,016 Tg to 100-cm depth, with a mean density of 6.21 ± 3.90 kg m−2. Comparison between SOCISA and the SOC of background soil (SOCbackground) indicates ∼19% SOC loss due to ISA conversion, similar to the effect of cropland conversion. Unlike the vertical pattern of SOCbackground, which declined faster in the upper soil layers, SOCISA decreased linearly with depth. Moreover, the SOCISA is uncorrelated with SOCbackground and decreases with precipitation. These unique patterns indicate the SOC loss mainly caused by topsoil removal during land conversion. The fact that the SOCISA 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 SOCISA and SOCPSA (SOC of the adjacent pervious surfaces) and the converging SOC densities in urban soils showed strong influences from urban greenspace on SOCISA. 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 SOCISA is a major component of the urban carbon pool. It has a unique spatial pattern and cannot be estimated using surrogate data such as SOCbackground or SOCPSA.