COUPLED ECOSYSTEM CARBON AND NUTRIENT CYCLING IN A HIGH ARCTIC ECOSYSTEM ARE ALTERED BY WARMING AND HIGHER RAINFALL

The rapid changes in temperature and precipitation in High Arctic tundra ecosystems are altering the biogeochemical cycles of nitrogen (N) and carbon (C), but in ways that are difficult to anticipate. Understanding the processes that are leading to changes in High Arctic biogeochemical processes are especially important today as soil organic C pools in the High Arctic are up to 6 times greater than previously estimated, and are sensitive to being oxidized to the atmosphere through changes in microbial decomposition associated with warmer and wetter conditions. We used a combination of experimental manipulation and natural vegetation gradients to determine the impact of interactions between temperature, water availability, and microbial metabolism on the cycling of C and plant-available N in High Arctic tundra soil. We find that water availability plays a critical role in these cycles in High Arctic tundra, over and above that from temperature increases. On seasonal time scales, we observed greater net N mineralization under both global change scenarios, yet water addition also significantly increased net nitrification rates, loss of NO₃^--N via leaching from surface soil layers, and lowered rates of labile organic C and N production. We also expected the chronic warming and watering would lead to long-term changes in soil N-cycling that would be reflected in soil δ¹⁵N values. However, we found that soil δ¹⁵N decreased under the different climate change scenarios. Our findings indicate that warmer, wetter High Arctic tundra will be cycling N and C in ways that may transform these landscapes in part leading to greater C sequestration, but simultaneously, N losses from the upper soil profile that may be transported to depth dissolved in water and or transported off site in lateral flow. While we do not have a complete picture of how soil mechanistic processes interact with moisture and temperature to regulate the fate of C and N, we can improve the ability of our ecosystem process models to predict the future trajectory of these ecosystems in the face of climate change by identifying the critical feedback linkages between N and C cycles.