Effect of Air‐filled Porosity, Nitrate Concentration, and Time on the Ratio of N2O/N2 Evolution During Denitrification

Abstract

Nitrous oxide evolution from soils may contribute to partial destruction of the ozone layer in the stratosphere. Knowledge on factors affecting the N₂O/N₂ evolution ratio from soils is important in properly accessing the hazard associated with N₂O evolution. Soils were wet to various air‐filled porosities with 400 ppm ¹⁵N‐labeled NaNO₃ solution or to 5% air‐filled porosity with various nitrate concentrations. The treated soils were incubated in closed containers in the laboratory and periodically checked for N₂O and N₂ evolution. The incubation flasks were opened and brought to equilibrium with the atmosphere after each gas sampling. All samples which were sufficiently wet to cause denitrification had an initial high N₂O evolution rate which decreased and approached zero with time. Dinitrogen gas evolution rate was initially low but increased after 3 to 4 days. The N₂O/N₂ ratio was initially high (infinity in some cases) and decreased rapidly with time. Except for the three lowest initial nitrate treatments, there was relatively high nitrate concentration throughout the incubation period; thus the decrease in the ratio could not be attributed to low nitrate concentration during the latter part of incubation. At a given time during the early stages of denitrification, the N₂O/N₂ evolution ratio increased with increased initial nitrate concentration. The ratio eventually became zero even under highest nitrate treatment. It is proposed that the enzyme dissimilatory nitrate reductase develops rapidly after anoxic conditions are initiated, and that the enzyme dissimilatory nitrous oxide reductase develops only after a period of time following anoxic conditions.