Impacts of nitrogen deposition on alpine ecosystem structure and function

Background/Question/Methods

The deposition of reactive nitrogen (N) has increased substantially due to agricultural and industrial activity.  Anthropogenic N deposition has been implicated in a host of ecological changes, including both increases and decreases in NPP, loss of diversity and increased mortality of plants, and acidification of soils and surface waters.  Alpine ecosystems with acidic soil parent material are considered particularly sensitive to N deposition effects, due to low rates of biological activity, thin poorly weathered soils with inherently low buffering capacity, and enhancement of deposition rates due to orographic enhancement of precipitation.  Research in the Colorado Front Range and other sites worldwide have provided a picture of the probable temporal trajectory of ecological changes that accompany chronic N deposition in terrestrial alpine ecosystems.
Results/Conclusions

Although NPP in alpine systems has been found to respond to N fertilization, greater plant growth is often the result of increases in the abundance of more nitrophilic species. Experiments on Niwot Ridge and Rocky Mountain National Park, subjected to 4-8 kg N/ha/year ambient deposition, indicate that realistic low-level inputs of N (5-30 kg N/ha/yr) elicit no or very small increases in NPP, lower in magnitude than interannual variation.  Plant diversity has increased due to increased abundance of non-dominant species, but species richness has not changed.  The potential for loss of rare species is high, although it has not been observed in monitoring or experimental plots.  Soil cation concentrations, buffering capacity, and pH decreased, and soil aluminum and manganese concentrations increased in plots on Niwot Ridge after a decade of simulated N deposition above 20 kg/ha/yr. In the Western Tatra mountains of Slovakia, where chronic deposition > 10 kg N/ha/yr has occurred for decades, increased N deposition decreases NPP, soil pH (< 3.4), and soil aluminum concentrations, but increased soluble soil iron concentrations, signaling major shifts in soil buffering capacity and ecosystem response to N deposition. The picture emerging from these studies indicates only modest “eutrophication” of alpine terrestrial ecosystems occurs due to N deposition, and thus low biological buffering of soils.  The transition to an “acidification” stage may occur more quickly than in forested ecosystems, and although the environmental consequences for this are largely unknown, may lead to greater aluminum toxicity of terrestrial and aquatic organisms.  Warming of alpine regions has the potential to both increase (greater NPP) and decrease (more rapid N cycling) N sequestration, and remains an important wild-card.