Acetylene Decomposition in Soils

AcetyIene (C 2 H 2 ) is useful in studies of nitrogen fixation and denitrification in soils. The objective of this study was to determine the soil environmental conditions under which microbial decomposition of acetylene occurs. Samples of Collamer sicl, Timpanogos cl, and Pahokee muck were exposed to carbon‐14 labeled acetylene in the laboratory. Less than 2.5% of the added gas was metabolized during the first 7 d of incubation, whether the soils were incubated aerobically or anaerobically. Adaptation to acetylene metabolism was observed in the two mineral soils during the second 7‐d period of aerobic incubation but adaptation did not occur during 42 d of anaerobic incubation or during a 21 d aerobic incubation of the Pahokee muck. Acetylene metabolism in soil samples adapted to acetylene utilization was stimulated by the addition of 10 mg N kg −1 soil of ammonium, nitrite, or nitrate. Addition of ≥ 100 mg N kg −1 soil of nitrate or nitrite or 1,000 mg N kg −1 soil of ammonium had a substantial inhibitory effect on acetylene utilization. The order of inhibition was ammonium < nitrate < nitrite. The addition of alfalfa, a source of readily available carbon, slowed microbial adaptation to acetylene decomposition and resulted in lower rates of transformation in the Collamer soil. Adaptation of soil microorganisms to acetylene utilization in the Collamer soil occurred in situ at a rate similar to laboratory incubation experiments. The ability of microorganisms in the soil to metabolize acetylene was maintained for at least 42 d after exposure of field plots to the gas was discontinued.