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

Abstract

The rate and extent of denitrification in soils incubated under helium atmospheres in the presence of acetylene (C₂H₂) concentrations ranging from 0.1 to 5.0% (vol/vol) was equivalent to that measured in the absence of C₂H₂. In the absence of C₂H₂, gaseous nitrogen (N) appeared as a mixture of nitrous oxide (N₂O) and dinitrogen (N₂), usually dominated by N₂, whereas in the presence of C₂H₂ gaseous N appeared exclusively as N₂O. An C₂H₂ concentration of 1.0% was also sufficient to maintain inhibition of N₂O 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 C₂H₂. Similar findings were obtained when soils were incubated under argon‐oxygen (4:1) atmospheres containing 1.0% C₂H₂. The inhibitory effect of C₂H₂ was virtually instantaneous when a 1.0% C₂H₂ concentration was established in an actively denitrifying soil. Diffusion of C₂H₂ 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 N₂O reduction during incubation in air using a ¹⁵N‐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 N₂/N₂O were essentially the same whether measured directly or calculated from the difference in N₂O production in the presence and absence of C₂H₂. The findings are discussed with respect to the use of the C₂H₂ inhibition technique for direct infield measurement of denitrification.