Microbial community structure as influenced by grasses in soils contaminated with TNT

Plants in association with rhizosphere microorganisms can be used to bioremediate some TNT (2,4,6-trinitrotoluene)-contaminated soils as an alternative to incineration or composting. Greenhouse experiments were conducted to determine the tolerance of warm-season grasses (big bluestem and switchgrass) and cool-season grasses (smooth bromegrass and tall fescue) to TNT in soil. Changes in microbial community structure due to TNT and the presence of grasses were assessed using fingerprints of fatty acid methyl esters (FAMEs) derived from the soil microbial cells. A site survey was conducted to assess plant distribution and microbial species composition in response to soil concentrations of TNT at the former Nebraska Ordnance Plant (NOP) near Mead, Nebraska. Although germination and height of the warm-season grasses were sensitive to increasing TNT concentrations, shoot and root growth of the cool-season grasses were reduced at lower concentrations of water-extractable TNT than the warm-season grasses. Results indicated that the warm-season grasses can be established in soils containing less than 86 mg of water-extractable TNT kg-1 and in uncontaminated soil overlying TNT-contaminated soil. FAME analysis indicated a shift in microbial community structure in TNT-contaminated soil that was greatly influenced by the presence of grasses. As TNT concentration increased, fatty acids C18:1 (cis 9) and C18:1 (cis 11) decreased in extracts of soil with cool-season grasses, while C16: 1 (cis 11) was lower in soil with warm-season grasses. Microbial biomass, measured as total FAMEs, decreased with soil depth and in the presence of TNT at the NOP site. Unsaturated fatty acids (typical of grain-negative eubacteria) were associated with soils containing plants, while saturated, cyclopropyl, iso and anteiso, branched fatty acids (typical of gram-positive and anaerobic gram-negative) were associated with TNT-contaminated soils. The beneficial effects of plants in restoring the soil microbial community was indicated by the shift in the FAMEs profile of contaminated NOP soils with plants away from that of contaminated soils without plants towards uncontaminated soil with growing plants