Using isotopic tracers and ecological modeling to examine partitioning of atmospheric nitrogen deposition in subalpine ecosystems

Background/Question/Methods

Elevated anthropogenic nitrogen (N) emissions contribute to higher rates of atmospheric N deposition (N_dep) that can saturate sensitive ecosystems, such as the subalpine meadows of the Cascade Range.  Consequences of increased N_dep include greater emissions of greenhouse gases, such as nitrous oxide (N₂O), deterioration of vegetation communities, and N leaching to watersheds, which contributes to eutrophication. Most of the annual N deposition in the Cascades is stored in snowpack until spring snow release. The capacity of subalpine vegetation communities to assimilate N_dep is unknown, particularly with climate‑induced snowpack loss. Therefore, we evaluated the impacts of warming and increased N_dep on ecosystem partitioning through a combination of field manipulations and ecosystem modeling using the Regional Hydro-Ecologic Simulation System (RHESSys) model. We hypothesized that increased N_dep would cause elevated N loss in subalpine ecosystems. To test this hypothesis, we applied experimental rates of ¹⁵N-labeled (10-at%) N_dep (additional 0, 3, 5, and 10 kg N ha^-1 yr^-1) to determine N partitioning between N₂O emissions, inorganic N leaching, microbial immobilization, and plant uptake in subalpine vegetation communities. Three communities—lush‑herbaceous, heath‑shrub, and wet sedge—were selected to represent early, mid, and late snow release vegetation regimes prevalent throughout the Cascades. We also parameterized the RHESSys model with baseline field measurements to capture the magnitude and trends of plant N uptake, N leaching, and soil N₂O emissions throughout the growing season under ambient conditions.

Results/Conclusions

Soil N₂O emissions were highest in the lush-herbaceous and wet sedge communities under 10 kg N ha^‑1 yr^‑1 N_dep rates, with the peak emissions occurring immediately after snow release. The applied N_dep only increased soil N₂O emissions for 10 days after snow release and had no appreciable impact on soil N₂ fluxes. Preliminary results suggest that vegetation N uptake was the dominant N sink in the ecosystem, with the highest growing season uptake occurring in the lush‑herbaceous community. Leaching rates were highest in the wet sedge community, particularly under the 10 kg N ha^-1 yr^-1 N_dep application. RHESSys simulations of ecosystem response to N_dep under warming conditions for RCP2.6 and 8.5 (1.0 and 2.0 ^oC, respectively) indicated that winter snowpack was severely decreased, with modeled snow release occurring 40 days earlier than observed in 2013. This earlier snow release contributed to increased inorganic N leaching, higher plant N uptake, and higher N₂O emissions. These results indicate that higher N_dep rates and warming conditions will increase N loss fluxes in subalpine ecosystems.