Rapid production of nitric oxide in seasonally-dry catchments

Background/Question/Methods

In seasonally-dry ecosystems, soil wet-up typically induces large and rapid pulses in nitric oxide (NO) emissions.  Because NO emissions increase within seconds after wet-up, we suspect this initial pulse is less influenced by biological nitrification and/or denitrification, but more so by chemical mechanisms.  In particular, if abiotic processes are important, what is their contribution at the ecosystem scale and how do these emissions vary spatially and temporally?  To answer these questions, we measured NO emissions after artificially wetting summer-dry soils along an elevation gradient in the Sierra Nevada, California.  Our sites included a low-elevation chaparral, mid-elevation mixed-conifer forest, and high-elevation subalpine meadow.

Results/Conclusions

At all three sites, NO emissions increased immediately after wet-up (≤ 30 seconds).  Chaparral soils exhibited the strongest response to water addition, where NO emissions increased from 1.3 to as high as 124 ng NO-N m^-2 sec^-1 within seconds of soil wet-up.  At the subalpine site, NO emissions increased in response to natural rainfall to as high as 116 ng NO-N m^-2 sec^-1.  Soil NO emissions were lowest at the mixed-conifer (0.4 ng NO-N m^-2 sec^-1), and had a modest response to water addition (2.6 ng NO-N m^-2 sec^-1).  At the chaparral site, NO fluxes decreased to background levels within 9 days after artificial wetting.  Ongoing laboratory soil incubations are being used to understand N sources for NO production and to differentiate between biotic and abiotic mechanisms.