Atmospheric nitrogen deposition alters the biogeochemistry, microbial activity, productivity and biodiversity of alpine freshwaters and vegetation of the US Rocky Mountains.  Now, with record high temperatures, longer snow-free seasons, and retreating glaciers, climate X N interactions stimulate some freshwater biota while depressing some alpine plant species. Global change is reorganizing alpine species assemblages and altering ecosystem processes.

The relative roles of climate and nitrogen deposition in driving  recent ecological change in alpine lakes are unclear. Here we report a 200-year multiproxy palaeolimnological study of two remote, oligotrophic alpine lakes located in the Southeast margin of Tibet, an area sensitive to climate change and loacted within the Southeast-Asian hot spot for reactive nitrogen emission.

The effect of atmospheric nitrogen deposition on phytoplankton nutrient limitation was investigated along a gradient of precipitation in SW Greenland.  Phytoplankton growth was measured over 14 days following one of six nutrient treatments.  A clear response to nutrient supply was found in 95 % of bioassays.  Strong seasonal variation was observed, shifting from P-limitation under ice to co-limitation in summer.

Two decades after application of nitrogen to tundra, effects are still apparent in plant community composition and plant and soil carbon stocks. Nitrogen addition alone resulted in a halving of carbon stocks, seemingly due to increased decomposition. Where phosphorus was also added, carbon stocks have doubled due to increased moss productivity.

Arctic ecosystems are threatened by pollution from both chronic and acute, extreme atmospheric N depositions. Here we report the difference in N (¹⁵N) recovery from the first-ever field simulation of extreme N deposition events (short-term) and snowpack chronic N deposition after 10-years (long-term), within the plant-soil system in the high arctic tundra.

Many nutrient poor fresh waters are nitrogen (N) limited and, in areas that have received enhanced levels of N deposition, lakes may have switched from N to P limitation. Stable isotopes measurements of lake sediments provide a long-term record of biogeochemical change. Here I present results from UK upland lakes that suggest N deposition is causing enrichment.