Resource Limitations to Nitric Oxide Emissions From a Sagebrush-Steppe Ecosystem

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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, NH+4, NO3 and sucrose. Despite low levels of soil NH+4 (5.60±0.95 mg NH+4-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−1soil, and soil temperature was greater than or equal to 20 °C. Wetting treatments with 20 mg NH+4-N kg−1soil 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−1soil, 240 mg sucrose-C kg−1soil, or NO3plus 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 NH+4 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.

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