Enhancement by acetylene of the decomposition of nitric oxide in soil

The acetylene inhibition technique is a widely used method to measure denitrification rates in soil. This technique is based on the inhibition of the N2O reductase with high concentrations of acetylene (about 10%). We tested possible artifacts created by using this technique under oxic conditions. Parts per billion concentrations of NO, an intermediate of the denitrification, were converted to NO2 within seconds when both O2 and acetylene were present. There was no conversion of NO under anoxic conditions or with acetylene concentrations < 0.1%. Propyne and 1-butyne also caused the conversion of NO to NO2, although to a lesser extent. In the absence of soil, the reaction stopped when an equilibrium between NO and NO2 was reached. The NO2 concentration at equilibrium increased with increasing temperature and with increasing acetylene concentrations up to 1%. Addition of small glass beads, quartz sand or sea sand also increased the NO2 concentration. We assume that acetylene (> 0.1%), especially in the presence of surfaces, stimulated the chemical reaction 2NO + O2 leads to 2NO2. In the presence of soil, NO2 accumulated only transiently and was then taken up by the soil. Use of 15NO resulted in 50% recovery of the label in the nitrate and nitrite fractions of the soil, indicating that NO2 was probably chemically converted to nitrate, nitrite and other N compounds. The NO decomposition in the presence of acetylene was observed in all 14 soils tested and occurred in autoclaved and non-sterile soil with the same rate. The rate of NO decomposition increased with increasing acetylene concentration and with an increasing amount of soil, and decreased with increasing soil water content. In general, acetylene (> 0.1%) enhanced the decomposition of NO in soil by factors of 5-557.