Enhanced biodegradation of trinitrotoluene in rhizosphere soil by native grasses

Soil contamination by the munition explosive residues of 2,4,6-trinitrotoluene (TNT) and its metabolites resulting primarily from military operations has been identified as a threat to human health and ecosystems. Biodegradation by native plants to remove this hazardous compound or reduce its toxicity is considered a cost-effective and environmentally sound approach for the cleanup or restoration of TNT-contaminated soils. This study aims to investigate the TNT biodegradation and kinetics by two selected native grasses in the species-specific rhizosphere soils through growth chamber experiments. Native eastern gamma grass (Tripsacum dactyloides) and switchgrass (Panicum virgatum L.) were grown in soil spiked with 14C-TNT for 8 weeks. The 14C-TNT degradation and degradative metabolite profile in the rhizosphere soils were determined by liquid scintillation counter and high-performance liquid chromatography, respectively. The results indicated that both native grass species significantly enhanced the TNT degradation in the rhizosphere soils as compared with the control rhizosphere soils. More than 95% of the applied 14C-TNT was degraded in the first 7 days, and the rate then reached a steady state afterward, but less than 10% of the TNT applied was completely mineralized and transformed into CO2. The degradative reaction was found to follow second-order kinetics. Six major TNT degradative metabolites have been detected and identified in the rhizosphere soils. Overall, switchgrass appeared more effective for biodegrading TNT than eastern gamma grass. This research demonstrated that the native grass species, especially switchgrass, has the potential to mitigate the adverse human health and ecological risks of TNT-contaminated sites and can be considered an environmentally friendly, sustainable approach to safeguarding human health from TNT contamination.