Uncoupling of Ammonia Oxidation from Nitrite Oxidation, and Its Impact upon Nitrous Oxide Production in a Grassland Soil.

Ammonia oxidation is typically thought of as the rate limiting step in nitrification. However, in NH₄⁺ supplemented aerobic soil slurry incubations of native grassland soils from semiarid eastern Oregon an uncoupling of NH₄⁺ oxidation from NO₂^- oxidation was observed. The accumulation of NO₂^- was transient, and NO₂^- concentrations increased from 0-12 h, and persisted for 48 h. From 12-48 h, NO₂^- levels declined, while the NH₄⁺ oxidation rates remained linear over 48 h. The increase in NO₂^- oxidizing potential was prevented by bacterial protein synthesis inhibitors, demonstrating that while NH₄⁺ oxidizers are able to oxidize NH₄⁺ to NO₂^- immediately upon addition of NH₄⁺, NO₂^- oxidizers require de novo protein synthesis to achieve their maximum NO₂^- oxidation potential. N₂O production was also measured, and was positively and linearly correlated with the NO₂^- concentration that accumulated over 24 h. Furthermore, our data suggests that N₂O production is directly dependent on nitrification activity. N₂O production was completely inhibited by low acetylene concentrations (0.02%), and was not stimulated by high acetylene concentrations (10%). Even during the period after maximum NO₂^- accumulation, acetylene prevented N₂O production, further suggesting that NH₄⁺ oxidation accounted for N₂O production. Our data demonstrate that NH₄⁺ dependent N₂O production from nitrification can be brought about by the temporary uncoupling of NH₄⁺ oxidation from NO₂^- oxidation. Further studies are underway to identify what factors affect the magnitude of the uncoupling of NH₄⁺ oxidation from NO₂^- oxidation.