Pulses of NO and N2O in Mediterranean ecosystems of the Sierra Nevada (California): importance of gaseous fluxes in annual N budgets

Background/Question/Methods

Recent studies on soil gaseous N emissions in semiarid ecosystems have highlighted the importance of these losses for terrestrial ecosystem budgets.  N Losses tend to be relatively large during seasonal transitions where soil rewetting results in a “hot moment” of increased gaseous N flux.  To better understand chaparral N dynamics, we measured NO and N₂O emissions for one year in a chamise-dominated watershed located in the foothills of the Sierra Nevada (California) whose previous nitrogen budget suggested net N retention (i.e., N inputs from atmospheric deposition > hydrologic outputs).  We also made additional gas flux measurements along an elevational gradient (300 to 2800 m) in Sequoia National Park to determine whether NO fluxes during the hot moment vary across ecosystems (chaparral, mixed conifer, and subalpine) with varying capacity for assimilation of N deposition.

Results/Conclusions

Nitric oxide emissions measured at the chaparral site through the one-year period are weakly related to soil moisture (R²=0.23; P<0.001) resulting in larger NO fluxes when soils are dry, except during the dry-wet seasonal transition in which rewetting results in large pulses of NO emitted from soils (ca. 100 ng N m^-2 s^-1).  Elevated NO fluxes during the hot moment increase instantaneously following wetting but decrease by about half 24 hours post-wetting and the magnitude of the pulse decreases with increasing frequency of rewetting episodes during the winter rainy season.   As with other studies in semiarid ecosystems, NO emissions decreased significantly with decreases in temperature averaging about 0.03 ng N m^-2 s^-1 and sometimes becoming negative during the cool winter.  Measurements of the magnitude of the hot moment along the altitudinal gradient during late summer resulted in larger NO fluxes in subalpine ecosystems (ca. 146 ng N m^-2 s^-1) when compared to mixed conifer (ca. 9 ng N m^-2 s^-1) and chaparral (ca. 4 ng N m^-2 s^-1), and fluxes were higher in south facing slopes when compared to those facing north.  Based on our current data, it is clear that gaseous N fluxes are an important component in N budgets for ecosystems experiencing a strong seasonal transition in soil physico-chemical conditions.