Controls on terrestrial carbon and nutrient cycling in Arctic permafrost environments

Abstract

High latitude regions are being disproportionately affected by changes induced by climate warming. The direction of global environmental change will largely depend on the response of Arctic ecosystems to positive and negative feedbacks, which has implications for the global climate system and the people that inhabit the Arctic region. Improving our understanding of the biogeochemical cycles that influence Arctic terrestrial systems will help improve models predicting future changes in northern latitudes. This research aims to determine the environmental controls on nutrient status and greenhouse gas exchange by assessing the relationships between environmental conditions, soil nitrogen availability, and greenhouse gas exchange under ambient and experimental conditions. At the Cape Bounty Arctic Watershed Observatory on Melville Island, Nunavut, varying land cover types exist across a naturally occurring moisture gradient, allowing us to investigate these relationships in wet and mesic tundra. Soil moisture, as opposed to soil temperature, was the main driver of inorganic nitrogen availability; ammonium was dominant under wetter conditions, while nitrate prevailed in drier conditions. In the wet tundra, soil moisture and ammonium availability together played an important role in mediating the carbon dioxide balance of the wet sedge meadow and ultimately contributed to the wetland being a net carbon dioxide sink across all three study years. While moisture and ammonium availability also played an important role in methane release, with drier conditions favouring nitrous oxide release via nitrifier denitrification, the strong carbon dioxide uptake offset the methane and nitrous oxide release, allowing the wetland to be a net greenhouse gas sink. The experimentally warmed mesic tundra site demonstrated a sustained warming effect on ammonium availability and ecosystem respiration 10-years after experiment establishment, and these effects were mediated by moisture. An enhanced snowfall and warming interaction increased nitrate availability, which corresponded with higher rates of N2O release. Temperature is often thought to be the key driver of changes in high latitude processes, but this dissertation finds that the future greenhouse gas balance of High Arctic systems will largely depend on the response of soil moisture and subsequent nitrogen availability to climate change.