Recovery of Carbon and Nitrogen Cycling and Microbial Community Functionality in a Post-Lignite Mining Rehabilitation Chronosequence in East Texas

Surface mining for coal alters the original soil profile characteristics and the associated physical, chemical, and biological conditions. Our objectives were to compare soil characteristics and the distribution of nutrients to 1 m depth over a chronosequence of 40 years to determine when a reclaimed mine soil (RMS) returned to premined conditions. We sampled 5 sites aged 0 to 20 years reclaimed by the crosspit spreader technique (CP) and 3 sites aged 20 to 40 years reclaimed by the mixed overburden technique (MO). An unmined site (UM) served as a control.

Changes in soil texture (sand to clay loam) after mining corresponded with increased macroaggregation (>2 mm) and enhanced C sequestration up to ~250 Mg C ha-1 at the MO20 site. Soil chemical [pH, electrical conductivity (EC), and sodium adsorption ratio (SAR)] and physical properties [bulk density (BD) and texture] met or exceeded reclamation and revegetation standards. Most soil C was associated with organic matter, but a small amount of lignitic C was detected in some samples. Soil organic C and N reached or exceeded premined concentrations after 0 and 10 years, respectively. Soil NO3--N and P did not reach premined conditions, but soil K, Ca, Mg and S exceeded premined conditions and stratified after 10-15 years. Micronutrients exceeded premined concentrations. Soil microbial biomass and mineralization rates recovered after 16 years of reclamation. Bacteria and fungi recovered to premined levels after 20 years. The CP20 site was most closely related to the UM site, but sites 10 years and older were comparable. Dominant phyla (Actinobacteria, Acidobacteria and Proteobacteria; 70% of all sequences) returned to premined levels after 10 years, which correlated with soil quality indicators, suggesting the importance of these phyla in soil health. Community-level physiological profiles did not differ between sites and metabolic diversity peaked at CP15 and CP20. GeoChip showed separation between the UM sites and reclamation sites. Soil microbial functionality appeared to recover faster than taxonomic composition of the soil microbial community. Further analysis of functional genes will expand upon this research so that we may better quantify soil quality in RMS.