Characterization of Atmospheric Nitrate Dynamics in a Sub-Alpine Watershed Using Δ17O and δ15N

Abstract:

Remote subalpine ecosystems are usually characterized by nutrient-poor soils (Körner, 2004; Seastedt et al., 2004), making them particularly susceptible to undergo changes due to increased atmospheric N deposition (Vitousek et al., 1997; Preunkert et al., 2003). Using Δ¹⁷O, a conserved tracer of atmospheric nitrate (NO₃ atm) (Michalski et al., 2004; Tsunogai et al., 2010), and δ¹⁵N, indicator of NO₃ biological sources (Kendall, 1998; Casciotti et al., 2009), we measured the seasonal variations of NO₃ atm stable isotopic composition and concentration in several streams and soils originating from two sub-alpine watersheds in the French Alps. Our objective was to investigate whether or not NO₃ atm impacts the soil N biogeochemical cycle by increasing nutrients availability for plants and bacteria. We coupled streams and soils measurements with snow-pits sampling and aerosols collection at the Lautaret Pass, to better emphasize the correlation between atmospheric deposition, soil retention and watersheds effluents response.

Our results reveal that different temporal dynamics govern our study site: stream measurements show that in spring, snowmelt results in a NO₃ atm impulse, accounting for ca. 31 % of the total stream NO₃ budget; on the opposite in autumn, NO₃ atm accounts only for ca. 3 % of the total stream NO₃ budget, highlighting the presence of a NO₃ bacterial pool (nitrification). We also inferred from the observed Δ¹⁷O variations two distinct phenomena in the spring/summer season: a fast snow run-off and a slower snow-water percolation. The later is believed to affect most the soil N cycle as it directly increases available NO₃. Measured soil leachates and extracts confirm this hypothesis and point out the potential importance of anthropogenic N deposition as on average 7 to 10 % of the soil solutions NO₃ derives directly from the atmosphere.