Consistent patterns of 15N distribution through soil profiles in diverse alpine and tundra ecosystems

We studied the vertical patterns of δ15nitrogen in total N and exchangeable NH4+-N through soil profiles in diverse alpine and tundra ecosystems. Soil samples were analyzed from 11 sites located in three mountain areas: NW Caucasus (Russia), the Khibiny Mountains (NW Russia) and Abisko region (N Sweden). Despite differences in the profile patterns of organic matter, nitrogen accumulation and nitrogen availability, we found consistent patterns of 15N distribution through all studied soil profiles. The δ15N values of total N were in general about zero or positive in the surface horizon and increased with soil depth. In contrast with total N, the δ15N values of exchangeable NH4+-N were in general about zero or negative in the surface horizons and decreased with soil depth. NH4+-N was significantly 15N-depleted compared with total N in all mineral horizons, while in the surface organic horizons differences between isotopic composition of total N and NH4+-N were mostly not significant. We do not know the exact mechanism responsible for 15N depletion of NH4+-N with soil depth and further research needs to evaluate the contributions of natural processes (higher nitrification activity and biological immobilization of “lighter” NH4+-N near the soil surface) or artifacts of methodological procedure (contribution of the 15N-enriched microbial N and dissolved organic N near the soil surface). Nevertheless, our finding gives a new possibility to interpret variability in foliar δ15N values of plant species with different rooting depth in alpine and tundra ecosystems, because plants with deeper root systems can probably consume “lighter” rather than “heavier” NH4+-N.