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Urbanization drives convergence in soil profile texture and carbon content
Citation:
Herrmann, D., L. Schifman, AND W. Shuster. Urbanization drives convergence in soil profile texture and carbon content. Environmental Research Letters. IOP Publishing LIMITED, Bristol, Uk, 15(11):114001, (2020). https://doi.org/10.1088/1748-9326/abbb00
Impact/Purpose:
The patterns we found indicate that urbanization shapes urban soils with desirable properties for soils used to support built infrastructures: uniform, high silt content, and low carbon content. From a geotechnical perspective, high organic matter content soils can result in instability as decomposition can result in air pockets. Furthermore, soils that can withstand compaction and are otherwise stable are important characteristics in geotechnical engineering. As a result, soils used as fill material have specific characteristics, set by the International Code Council. Ideally, soils should be evenly graded, include relatively low concentrations of carbon, and fall into texture classifications that include high amounts of silt and fine sands (23–25). Soils that contain too much clay are not desirable, as some types of clays can expand during wetting, which may result in structural damage to structures using the soil as foundation or around its foundations. Sandy soils on the other hand may have high infiltration rates that may lead to preferential flow paths during wetting and result in structural shifts and damage. Ideal soil characteristics for supporting ecological processes contrast with those of structural soils. Soils used in green spaces and green infrastructure have specifications that result in desirable infiltration rates and provide a supportive medium for plant growth, and therefore ideally contain high amounts of organic matter and sand (e.g., 60%). We have illustrated this tension between different objectives for soil uses, and call for more attention to its implications for management.
Description:
The previous and present century are hallmarked by human population growth and rural-to-urban migration. In the process, society is creating – as well as abandoning – urban settlements at an accelerating pace. Conversion to urban land uses can alter the landscape substantially as it typically involves extensive modification to soils through grading, excavation, and filling activities. Urbanization is an indelible transformation, and leaves an imprint onto soils and the landscapes they form within relatively short time frames compared with the formation of the original soils over the course of millennia. We compared soil carbon and texture among urban and best-known reference soil profiles, and across cities representing 10 out of 12 major soil orders. We found that total carbon in urban soil profiles starting at an overall lower level, and its depth distribution was shifted towards a linear compared to an exponential decline observed in reference soils. Differences in the depth distribution of total carbon decreased with depth with values eventually converging at 1-m depth. There was strong evidence that carbon profiles were converging among cities in both amount of total carbon and its depth distribution. We summarized soil particle size distributions with the geometric mean diameter (GMD, mineral fraction only (see Methods), and urban soils overall had a smaller GMD than reference soils, which was consistent to the maximum depth of investigation, which was 1.5 m. Differences were more pronounced than those for total carbon, such that modeled GMD values shifted from fine sand in reference soils to fine silt in urban soils, and for 7 out of 11 cities. Implications of these findings are discussed in terms of green infrastructure and geotechnical objectives.
URLs/Downloads:
DOI: Urbanization drives convergence in soil profile texture and carbon content
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