Reduction in denitrification activity in field soils exposed to long term contamination by 2,4,6-trinitrotoluene (TNT)


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Terrestrial sites contaminated with 2,4,6-trinitrotoluene (TNT) are a widespread and persistent problem and often contain non-vegetated areas with TNT concentrations well in excess of 1000 mg kg−1. In this study, we examined the effect of TNT on denitrification activity in field soils, and compared the sensitivity of denitrifying enzymes to TNT. DNA probes assessed the prevalence of nirS, nirK and nosZ (encoding cd1 or copper nitrite reductase and nitrous oxide reductase, respectively), denitrifying genotypes in the culturable and total microbial community. The nitrate (NaR), nitrite (NiR) and nitrous oxide (N2OR) reductase activities in field soil and in isolates were assessed by gas chromatography. The relative occurrence of the nirK, nirS or nosZ genotypes increased in the cultured community and in total uncultured community DNA as nitroaromatic concentrations increased. However, denitrifying activity decreased in response to increasing TNT concentrations, with an IC50 for NaR+NiR+nitric oxide reductase (NOR) of 400 mg TNT kg−1 soil and for N2OR of 26 mg TNT kg−1 soil. The denitrifying activity of four soil isolates also decreased in response to TNT, with N2OR activity being three times more sensitive to TNT than NaR+NiR+NOR activity. Interestingly, there were 118 times more nirK isolates than nirS isolates in uncontaminated soil but only 1.5 times more in soil containing 17 400 mg kg−1 TNT. The results from this study indicated that TNT reduced denitrification activity in field soils, and N2OR was much more sensitive to TNT than NaR+NiR+NOR.