Abstract
The mountains of southern California receive some of the highest rates of nitrogen (N) deposition in the world (~40 kg ha year). These high rates of deposition have translated into consistently high levels of nitrate (NO3) in some streams of the San Bernardino Mountains. However, not all streams are exhibiting these high levels of NO3. Perennial streams have high NO3 concentrations (~200 μmoles l) while ephemeral streams do not (~20 μmoles l). This difference points to groundwater as the source of the NO3 observed in streams. Furthermore, the evidence indicates a differential impact of N deposition on terrestrial and aquatic systems in Mediterranean climates, with aquatic systems being impacted more quickly. The primary reason for this difference involves the asynchrony between the time that atmospheric deposition occurs (summer), the time period of maximum soil NO3 availability and leaching (winter), and the time of maximum plant N demand (spring). Our results indicate that semiarid Mediterranean climate systems behave differently from more humid systems in that, because of this asynchrony, aquatic systems may not be indicative of changes in terrestrial ecosystem response. These differences lead us to the conclusion that the extrapolation of impacts from humid to Mediterranean climates is problematic and the concept of N saturation may need to be revisited for semiarid and seasonally dry systems.
Highlights
The study of the ecosystem effects of atmospheric nitrogen (N) deposition and its effect on terrestrial aquatic ecosystems has received a great deal of attention in recent years[1,2,3,4]
The N saturation hypothesis as propounded by Stoddard[3] argues that stream chemical composition, NO3 concentrations, is indicative of terrestrial ecosystem processes
Streamwater samples were collected for chemical analysis from eight streamwater sampling sites in Devil Canyon, a catchment heavily impacted by atmospheric deposition from the urban Los Angeles air mass (Fig. 1)
Summary
The study of the ecosystem effects of atmospheric nitrogen (N) deposition and its effect on terrestrial aquatic ecosystems has received a great deal of attention in recent years[1,2,3,4]. These studies have generally identified high levels of nitrate (NO3) observed in stream baseflow in the summer as indicative of the latter stages of N saturation (stage 2). The N saturation hypothesis as propounded by Stoddard[3] argues that stream chemical composition, NO3 concentrations, is indicative of terrestrial ecosystem processes. This lack of consistent work on N concentrations and fluxes in seasonally dry and semiarid systems has not permitted a robust verification of the N-saturation hypothesis outside of the more mesic climates where the hypothesis originated
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