Iron Biogeochemistry of Acid Mine Drainage Sediments Associated with Vascular Plants and Biological Soil Crusts.

Secondary iron oxide minerals precipitating out from Acid Mine Drainage (AMD) are a persistent legacy of coal mining in the Appalachian region of the USA. Ferric oxides coat soil surfaces as AMD flows overland, leaving behind barren areas. Experimental plots in a 100-yr-old AMD impacted site in central Pennsylvania were reclaimed with a one-time addition of compost, lime, and annual nurse crop. Barren surfaces were characterized by iron oxide deposits up to 35 cm thick and pH values near 2.5. Although reclamation resulted in high vegetative cover, one concern was that organic matter mineralization in the new rooting zone would result in increased ferric iron reduction and greater mobility of ferrous iron, thereby generating additional acidity in receiving waters. Our research objective was to compare Fe partitioning in root-associated sediments with Fe partitioning in underlying, less organic-rich sediments. Fe fractions measured in air-dried sediments included dithionite-extractable Fe (crystalline Fe), oxalate-extractable Fe (amorphous Fe) pyrophosphate-extractable Fe (organically bound Fe), and total Fe (6 N HCl) by ICP-AES. Ferrous iron in fresh samples was quantified by UV-spectrophotometry using the ferrozine method. Since unamended areas of the research site supported biological crusts, we also made a comparison of natural Fe partitioning between crust-attached sediments and unattached sediments of lower organic carbon content.  We expected to observe higher proportions of organic-bonded iron in sediments with higher organic carbon contents (root- and crust-associated) than in less organic-rich sediments. These findings will help evaluate Fe mobilization by reclamation of AMD barrens.