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 denitrification rates, the rates of gross production or net release of NO by denitrification (NOD), and the rates of net release of N2O by denitrification (N2OD) in 29 different soils. The denitrification rates were measured by accumulation of N2O in the presence of 10 kPa acetylene. The rates of NOD and N2OD were measured in the presence of only 10Pa acetylene, which inhibits nitrification, but not denitrification. We assumed that the residual rates of NO and N2O production were due to denitrification. Most of the soils (24 out of 29) showed NOD rates that were higher 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 increased with increasing NOD rates. The discrepancy was highest at a soil moisture content of 70% of the water holding capacity and decreased at higher soil moisture. We have shown that the oxidation of NO to NO2 was enhanced by the presence of acetylene at concentrations >0.1% (>0.1 kPa). The resulting NO2 was taken up by soil. We therefore interpret the observed discrepancies between NOD rates and denitrification rates as an artifact created by the acetylene (10 kPa) used in the denitrification assay. The acetylene probably resulted in scavenging of part of the NO that was produced as intermediate in the denitrification sequence and thus could not be further reduced to N2O. Consequently, the denitrification rates were underestimated.