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Effect of Preincubation Treatments on the Ratio of N₂O/N₂ Evolution¹

ABSTRACT

Previous investigations demonstrated that air-dry soils incubated in the laboratory under conditions to cause denitrification had an initial high N₂O/N₂ evolution ratio which rapidly decreased and approached zero. Two experiments were conducted to determine if the observed ratios were strictly related to using air-dry soils. Three preincubation treatments were used in the first experiment: i) air-dry, ii) moist to promote microbial activity under aerobic conditions, and iii) saturated to impose anoxic conditions. All soils had ¹⁵NO₃ added and were incubated under saturated conditions following 1 week of the preincubation. Almost no N₂O evolved during incubation of the soil preincubated under saturated conditions. Nitrous oxide evolution was generally highest during the first day of incubation and decreased thereafter for soils preincubated under air-dry and moist conditions. During the second experiment, soils were preincubated under air-dry and moist conditions and incubated under 0, 5, and 10% air-filled porosity. The N₂O and N₂ evolution patterns were similar for the two preincubation treatments except that there was a higher rate of gas evolution from the air-dry soils presumably because of a greater energy source. The results support a previous proposal that dissimilatory NO₃ reductase develops rapidly and that dissimilatory N₂O reductase develops more slowly following anoxic conditions. Our data provide indirect evidence that soils may act as a sink for N₂O even in the presence of NO₃ under appropriate conditions.

Key Words: nitrate • nitrogen transformations • denitrification

¹ Contribution of the Dep. of Soil and Environ. Sci., Univ. of California, Riverside, CA 92521. The research was supported by Grant GI-34733X of RANN of the Natl. Sci. Found.

² Professor of Soil Physics, Post-graduate Research Soil Scientist, Staff Research Associate and Associate Professor of Soil Science, respectively. (Aviva Hadas was on leave from Inst. of Soils and Water, ARO, The Volcani Center, Bet Dagan, Israel.)

Received for publication June 19, 1978.