Direct Measurement of Denitrification Loss from Soils: I. Laboratory Evaluation of Acetylene Inhibition of Nitrous Oxide Reduction

The rate and extent of denitrification in soils incubated under helium atmospheres in the presence of acetylene (C 2 H 2 ) concentrations ranging from 0.1 to 5.0% (vol/vol) was equivalent to that measured in the absence of C 2 H 2 . In the absence of C 2 H 2 , gaseous nitrogen (N) appeared as a mixture of nitrous oxide (N 2 O) and dinitrogen (N 2 ), usually dominated by N 2 , whereas in the presence of C 2 H 2 gaseous N appeared exclusively as N 2 O. An C 2 H 2 concentration of 1.0% was also sufficient to maintain inhibition of N 2 O reduction even when the soil nitrate supply had been exhausted. Overall respiration of incubation systems, measured as carbon dioxide evolution, was also unaffected by the presence of C 2 H 2 . Similar findings were obtained when soils were incubated under argon‐oxygen (4:1) atmospheres containing 1.0% C 2 H 2 . The inhibitory effect of C 2 H 2 was virtually instantaneous when a 1.0% C 2 H 2 concentration was established in an actively denitrifying soil. Diffusion of C 2 H 2 into soils packed to their maximum bulk density and held at 5% air‐filled porosity was also sufficiently rapid to induce essentially complete inhibition of N 2 O reduction during incubation in air using a 15 N‐labeled nitrate addition to the soil. The extent of denitrification varied among soils and ranged from 54 to 154 µg N/g under helium atmospheres, and from 15 to 106 µg N/g under argon‐oxygen atmospheres, after 200 hours incubation. Ratios of N 2 /N 2 O were essentially the same whether measured directly or calculated from the difference in N 2 O production in the presence and absence of C 2 H 2 . The findings are discussed with respect to the use of the C 2 H 2 inhibition technique for direct infield measurement of denitrification.