ACETYLENE BLOCKAGE TECHNIQUE LEADS TO UNDERESTIMATION OF DENITRIFICATION RATES IN OXIC SOILS DUE TO SCAVENGING OF INTERMEDIATE NITRIC-OXIDE

We used the acetylene inhibition technique to measure the denitrificat ion rates, the rates of gross production or net release of NO by denit rification (NOD), and the rates of net release of N2O by denitrificati on (N2OD) in 29 different soils. The denitrification rates were measur ed by accumulation of N2O in the presence of 10 kPa acetylene. The rat es of NOD and N2OD were measured in the presence of only 10 Pa acetyle ne, which inhibits nitrification, but not denitrification. We assumed that the residual rates of NO and N2O production were due to denitrifi cation. Most of the soils (24 out of 29) showed NOD rates that were hi gher than denitrification rates themselves. Only five soils with very low NOD rates had denitrification rates that were higher than the NOD rates. The discrepancy between the NOD and the denitrification rates i ncreased with increasing NOD rates. The discrepancy was highest at a s oil moisture content of 70% of the water holding capacity and decrease d at higher soil moisture. We have shown that the oxidation of NO to N O2 was enhanced by the presence of acetylene at concentrations >0.1% ( >0.1 kPa). The resulting NO2 was taken up by soil. We therefore interp ret the observed discrepancies between NOD rates and denitrification r ates as an artifact created by the acetylene (10 kPa) used in the deni trification assay. The acetylene probably resulted in scavenging of pa rt of the NO that was produced as intermediate in the denitrification sequence and thus could not be further reduced to N2O. Consequently, t he denitrification rates were underestimated. (C) 1997 Elsevier Scienc e Ltd.