Geospatial and Statistical Basis for Mine Soil Sampling: Where, How Deep, and When to Dig for C Credits.

Across the United States, there are hundreds of thousands of hectares of land disturbed by surface coal-mining, ranging from abandoned grasslands to brushlands and forestlands. If reclaimed and managed properly, these lands have the potential to sequester significant amounts of carbon in the soil. To aid in formulating mined land-specific soil organic carbon (SOC) inventory guidelines, the objectives of this work were to determine the horizontal and vertical distributions and variability of sequestered SOC, and identify the maximum cost-effective depth (MCD) of mine soil sampling, and the minimum detectable difference (MDD) of measured SOC between two consecutive carbon inventories.

Soil samples from topsoil and mine spoil materials from shallow (to 30 cm depth) and deep (to 200 cm depth) soil pits were collected from mined sites of different ages reclaimed to grasslands in Ohio (OH, 9 yr), West Virginia (WV, 4 yr), and Virginia (VA, 1 yr). The volume of excavated mine soil was transformed in terms of costs of excavation, sample collection, preparation, and carbon analysis, and the measured SOC content was transformed in terms of profits from carbon credits. These cost and profit estimates were used to estimate the MCD. The SOC content (0-10 cm layer) in OH (11.2 Mg ha^-1) was significantly higher compared to the sites in WV (4.3 Mg ha^-1) and VA (1.5 Mg ha^-1). The MCD for the OH2 site was 18 cm, at sampling intensity of 17 plots ha^-1, and the MDD was about 54 Mg C ha^-1, which would become statistically significant after >32 yr. Due to the lower horizontal variability of SOC on OH1 and OH3 sites, the MCD exceeded 200 cm. The overall findings from this study supported the understanding that establishing carbon sequestration projects on mined lands could be a profitable undertaking for mined land reclamation.