Distinguishing "new" from "old" organic carbon in reclaimed coal mine sites using thermogravimetry: II. Field validation

Thermogravimetry (thermogravimetric and derivative thermogravimetry) was used under laboratory conditions to differentiate "new" and "old" organic carbon (C) by using grass litter, coal, and limestone to represent the different C fractions. Thermogravimetric and derivative thermogravimetry curves showed pyrolysis peaks at distinctively different temperatures, with the peak for litter occurring at 270 to 395 °C, for coal at 415 to 520 °C, and for limestone at 700 to 785 °C. To validate this method in a field setting, we studied four reforested coal mine sites in Kentucky representing a chronosequence since reclamation: 0 and 2 years located at Bent Mountain and 3 and 8 years located at the Starfire mine. A nonmined mature (≈80 years old) stand at Robinson Forest, Kentucky, was selected as a reference location. Results indicated a general peak increase in the 270 to 395 °C region with increased time, signifying an increase in the "new" organic matter (OM) fraction. For the Bent Mountain site, the OM fraction increased from 0.03 to 0.095% between years 0 and 2, whereas the Starfire site showed an increase from 0.095 to 1.47% between years 3 and 8. This equates to a C sequestration rate of 2.92 Mg ha^-1 yr^-1 for "new" OM in the upper 10-cm layer during the 8 years of reclamation on eastern Kentucky reclaimed coal mine sites. Stable isotope results revealed a depletion trend in δ¹³C (-21.78 to -26.7g‰) that approximated values for C₃ plants (-27.0‰) as the tree stands on the reclaimed coal mine sites develop. δ¹³C for an 80-year old undisturbed forest site was 27.89‰. Calcium, magnesium, and manganese decreased over time (from 0 to 8 years), indicating increased weathering due to greater leaching as a result of root development and penetration and soil formation. This suggests that stable isotopes and elemental data can be used as proxy tools for qualifying soil organic C (SOC) changes over time on the reclaimed coal mine sites but cannot be used to determine the exact SOC accumulation rate. However, results suggested that the thermogravimetric and derivative thermogravimetry methods can be used to quantify SOC accumulation and has the potential to be a more reliable, cost-effective, and rapid means to determine the new organic C fraction in mixed geological material, especially in areas dominated by coal and carbonate materials.