Ecotoxicological effects of hexahydro-1,3,5-trinitro-1,3,5-triazine on soil microbial activities

Although hexahydro-1,3,5 -trinitro-1,3,5-triazine (also called RDX or hexog n) is a potentially toxic explosive compound that persists in soil, its eco toxicological effects on soil organisms have rarely been assessed. In this study, two uncontaminated garden soils were spiked with 10 to 12,500 mg RDX /kg dry soil. Soil microbial activities, i.e., potential nitrification, nit rogen fixation, dehydrogenase, basal respiration, and substrate-induced res piration were chosen as bioindicators and were determined after 1-, 4-, and 12-weeks of exposure. Experimental results indicate that RDX showed signif icant inhibition (up to 36% of control) on indigenous soil microbial commun ities over the period of this study. All five bioindicators; responded simi larly to the RDX challenge. The length of exposure also affected the microb ial toxicity of RDX, with 12-week exposure exerting more significant effect s than the shorter exposure periods, suggesting that soil microorganisms mi ght become more vulnerable to RDX when exposure is extended. The estimated lowest observable adverse effect concentration of RDX was 1,235 mg/kg. No b iodegradation products of RDX were detected at all three sampling times. Co mpared with 2,4,6-trinitrotoluene (TNT), RDX is less toxic to microbes, pro bably because of its resistance to biodegradation under aerobic conditions, which precludes metabolic activation of nitro groups.