Community‑specific biogeochemical responses to atmospheric nitrogen deposition in subalpine meadow ecosystems of the Cascade Range

Abstract:

Elevated anthropogenic nitrogen (N) emissions result in higher rates of atmospheric N deposition (N_dep) that can saturate sensitive ecosystems. Consequences of increased N_dep include higher emissions of greenhouse gases, eutrophication of watersheds, and deterioration of vegetation communities. Most of the annual N deposition at higher elevations in the Cascades is stored in snowpack until spring snowmelt when it is released as a pulse that can be assimilated by plant and microbial communities, or lost as gaseous emissions or leachate. The relative magnitude of these fluxes is unknown, particularly with accelerated rates of snowpack loss due to climate change. We quantified storage of N_dep in winter snowpack and determined impacts of N_dep on biogeochemical processes in a lush‑herbaceous community characterized by Valeriana sitchensis and Lupinus latifolius, a heath‑shrub community characterized by Phyllodoce empetriformis and Cassiope mertensiana, and a wet‑sedge community dominated by Carex nigricans. These communities were selected to represent early, mid, and late snowmelt vegetation regimes prevalent throughout the Cascades. Ammonium (NH₄⁺) was the dominant form of N_dep in winter snowpack and N_dep rates were higher than anticipated based on nearby National Atmospheric Deposition Program (NADP) measurements. Vegetation N uptake was the dominant N sink in the ecosystem, with the highest growing season uptake occurring in the lush‑herbaceous community, while soil N leaching was the dominant N loss, with the lush‑herbaceous also having the highest rates. Microbial biomass N fluctuated substantially across the growing season, with high biomass N immediately after snowmelt and again 30 days following snow release. Soil nitrous oxide (N₂O) emissions peaked 30 days following snowmelt for all three communities and were greatest in the wet sedge community. These results indicate that subalpine communities have unique responses to N_dep that vary throughout the growing season. Thus, biogeochemical modeling efforts to simulate ecosystem responses to N_dep should be parameterized at a community‑specific level to fully capture this variability. Ultimately, this study will provide insight to land managers on the fate of N emissions and how N_dep affects ecosystem services in high‑elevation ecosystems.