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 inhibiti on of the N2O reductase with high concentrations of acetylene (about 1 0%). We tested possible artifacts created by using this technique unde r oxic conditions. Parts per billion concentrations of NO, an intermed iate of the denitrification, were converted to NO2 within seconds when both O-2 and acetylene were present. There was no conversion of NO un der anoxic conditions or with acetylene concentrations <0.1%. Propyne and 1-butyne also caused the conversion of NO to NO2, although to a le sser extent. In the absence of soil, the reaction stopped when an equi librium between NO and NO2 was reached. The NO2 concentration at equil ibrium increased with increasing temperature and with increasing acety lene concentrations up to 1%. Addition of small glass beads, quartz sa nd or sea sand also increased the NO2 concentration. We assume that ac etylene (>0.1%), especially in the presence of surfaces, stimulated th e chemical reaction 2NO + O-2 --> 2NO(2). In the presence of soil. NO2 accumulated only transiently and was then taken up by the soil. Use o f (NO)-N-15 resulted in 50% recovery of the label in the nitrate and n itrite fractions of the soil, indicating that NO2 was probably chemica lly converted to nitrate, nitrite and other N compounds. The NO decomp osition in the presence of acetylene was observed in all 14 soils test ed and occurred in autoclaved and non-sterile soil with the same rate. The rate of NO decomposition increased with increasing acetylene conc entration and with an increasing amount of soil, and decreased with in creasing soil water content. In general, acetylene (>0.1%) enhanced th e decomposition of NO in soil by factors of 5-557. (C) 1997 Elsevier S cience Ltd.