Enhancement by acetylene of the decomposition of nitric oxide in soil

Abstract

The acetylene inhibition technique is a widely used method to measure denitrification rates in soil. This technique is based on the inhibition of the N 2O 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 NO 2 within seconds when both O 2 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 NO 2, although to a lesser extent. In the absence of soil, the reaction stopped when an equilibrium between NO and NO 2 was reached. The NO 2 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 NO 2 concentration. We assume that acetylene (> 0.1%), especially in the presence of surfaces, stimulated the chemical reaction 2NO + O 2 → 2NO 2. In the presence of soil, NO 2 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 NO 2 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.