Assessment of thermal analysis techniques for determining organic, black, and inorganic carbon contents in urban soils

Although urban greenery is an important area for soil carbon (C) sequestration in national and international policies, there is a lack of studies on its unique soil C status. Especially the contribution of black carbon (BC) and inorganic carbon (IC), which originated from anthropogenic activities, need to be separated from ecosystem-driven organic carbon (OC_eco) to accurately quantify the soil C sequestration in urban ecosystems. However, there is currently no standardized, widely used method to separate various forms of C in this soil. In this study, we suggested a robust and reliable method to discriminate the OC_eco, BC, and IC contents and understand the anthropogenic effects on C in urban soils. To achieve this objective, we tested the accuracy of the “EGA with peak deconvolution approach” that derives a CO₂ thermogram from an evolved CO₂ gas analyzer (EGA) connected to a thermal analyzer and conducts sample-by-sample peak deconvolution. Since we used the model mixtures that had known OC_eco, BC, and IC contents, the absolute accuracy of this approach could be tested. As a result, EGA with peak deconvolution approach showed high accuracy (R² > 0.90), and the regression lines between the known and measured values were close to the 1:1 line.

Using the EGA with peak deconvolution approach, we further investigated the soils in urban greeneries. EGA with peak deconvolution approach was helpful in understanding the impacts of human intervention on the soil C cycle. Surrounding land use significantly altered the soil OC_eco/TC and BC/TC but was not soil IC/TC; the OC_eco/TC tended to increase with green area, while the BC/TC had a positive relationship with impervious area. The suggested method can be used to evaluate the C sequestration rate of SSM practices in the urban area. Without the information on OC_eco and BC, the impacts of human intervention on soil C can be misinterpreted, which overestimates the C sequestration rate.