EFFECTS OF TOLUENE ON MICROBIALLY-MEDIATED PROCESSES INVOLVED IN THE SOIL-NITROGEN CYCLE

The effects of toluene on indigenous microbial populations involved in the soil nitrogen cycle were examined. Ammonia oxidation potential (A OP) and nitrite oxidation potential (NOP) were both reduced after incu bation with high toluene concentrations for 45 days, with the former a ctivity showing greater sensitivity. KCl-extractable ammonium (NH4ext) levels increased dramatically in soil exposed to high toluene levels , and arginine ammonification was not significantly affected. Alfalfa- amended soil incubated in the presence of 200 mu g toluene ml(-1) show ed progressive accumulation of NH4ext+ over 37 days, indicating that m ineralization of plant-associated nitrogen was not hindered by toluene . AOP in treated soil was much less than in control soil on days 7 and 37, but the MPN of ammonia oxidizers in control and exposed soil were not significantly different. Soil incubated with 100 mu g toluene ml( -1) for 28 days, vented and allowed to incubate for an additional 7 to 30 days, exhibited only slight increases in AOP and NOP, while NH4ext + returned to control levels within a week. Soil exposed to 200 mu g t oluene ml(-1) and treated in the same manner showed no increases in ei ther AOP or NOP, and NH4ext+ remained elevated for the duration of the experiment, indicating more longterm effects on soil nitrogen cycling had occurred. Ammonia oxidizer levels in control soil and soil incuba ted with 100 mu g toluene ml(-1) were not appreciably different, where as levels of ammonia oxidizers were very low in soil exposed to 200 mu g toluene ml(-1) and increased only slightly by 30 days post vent. Ex periments to determine how toluene affects the AOP of soil indicated a competitive inhibition mechanism, with an effective concentration cau sing 50% reduction in activity (EC(50)) of 11 mu M toluene, and a comp etitive inhibition constant (K-i) of 0.1 +/- 0.05 mu M toluene. These results indicate the potential for toluene to adversely impact nitroge n cycling in the terrestrial ecosystem by affecting indigenous soil ni trifiers, which are sensitive to lower levels of toluene than has been previously reported.