Resource limitations to nitric oxide emissions from a sagebrush-steppe ecosystem

We monitored soil emissions of NO, NO₂, N₂O, and CO₂ throughout the summer dry season at a remote North American sagebrush-steppe ecosystem following application of several resources, including water, NH₄⁺, NO₃^– and sucrose. Despite low levels of soil NH₄^– (5.60±0.95 mg NH₄^–-N per kg soil, mean ± S.E.), and NO₃^–-N (1.34±0.20 mg NO₃^–-N per kg soil), NO emissions ranged from about 0.2 to 2.8 ng NO-N m^–2 s^–1, comparable to rates measured from many agricultural, tropical, and other undisturbed ecosystems. Soil wetting increased NO emissions as much as 400-fold when initial gravimetric soil moisture contents were less than about 50 mg kg_soil^–1 and soil temperature was greater than or equal to 20 °C. Wetting treatments with 20 mg NH₄⁺-N kg_soil^–1 raised NO emission rates to a level that was nearly an order of magnitude higher than that observed after water addition alone. Wetting treatments with 20 mg NO₃^–-N kg_soil^–1, 240 mg sucrose-C kg_soil^–1, or NO₃^– plus sucrose had no statistically significant effect upon NO emissions. Soil denitrifying enzyme activity was low at this site, and N₂O emissions in the field were below detection limits. Soil nitrifying enzyme activity was extremely high at this site, indicating that the NH₄⁺ released by ammonification would be consumed at least once every 1.7 days. These observations indicate that NO emissions from this undisturbed ecosystem were likely a consequence of high nitrification activity, and that sagebrush-steppe ecosystems may be a more important NO source than has been previously assumed.