Carbon Sequestration in Coal Ash Soils: Estimates From Direct Measurements and Indications of Possible Errors in Estimates for Natural Soils.

Reliable estimates of carbon sequestration in terrestrial ecosystems are important, but estimation using routinely available methods presents difficulties. Natural soils contain measurable organic carbon in dynamically inter-active sources and reservoirs not readily resolved by routine analytical methods. A study was undertaken to examine the effectiveness of a direct measurement approach in soils forming under vegetation developing on coal ash as a parent material. Due to its high temperature formation, coal ash is devoid of organic carbon at time of disposal. Two coal ash sites were identified with relatively well documented vegetation and operational histories. Samples collected from the two sites in 2003 were subjected to routine carbon determinations by combustion (loss on ignition) and wet ashing (modified Walkley-Black) procedures. Additional special fresh ash samples were used to verify the ability of the two methods to distinguish amorphous carbon (unburned coal) and organic carbon. Analysis of the data indicated estimates based on direct measurement were reasonable and consistent. Soil carbon sequestration rates per unit depth of soil were equivalent to those reported by other investigators for similar ecosystems on natural soils. However, the sampling depths, rooting depths, or both in the coal ash soils were greater than reported for natural soil situations. Consequently the total carbon sequestration rates per unit land area were substantially greater for the coal ash soils. In 2006 the study was repeated looking at deeper soil under young forest vegetation. Carbon sequestration rates in the 60-180 cm depth equaled or exceeded those in the 0-60 cm. Coal ash soils appear capable of sequestering large amounts of atmospheric carbon. Estimates of carbon sequestration rates should be based on actual rather than arbitrary or conventional assumptions about rooting depths. Actual carbon sequestration rates under temperate forests may be seriously underestimated.