Effects of Wildfire on Nitrogen and Phosphorus Availability in Arctic Tundra Soils

The Arctic is warming faster than the rest of the globe, leading to intense disturbances including permafrost thaw, thermokarst development, and wildfire. These disturbances alter rates of biogeochemical cycling of nitrogen (N) and phosphorus (P), further altering the ecology and permafrost dynamics of the region. My goal is to understand the effects of wildfires on soils and plants in the western Canadian Arctic tundra. Ash is high in available nutrients which promote plant growth immediately after fire, but may increase N and P loss after fire. In tundra ecosystems, nitrate and ammonium concentrations are particularly low, meaning that an increase in concentrations of these nutrients after a wildfire can have a particularly large effect on biological activity. Not all plant species are equally able to take advantage of the pulse of nutrients available after a wildfire; some will have an advantage over others. Large woody shrubs can outcompete smaller evergreen shrubs, lichens, and mosses during the recovery period, which  increases the amount of above-ground biomass and thus the risk of future wildfires. This may result in a positive feedback loop where wildfire increases shrub growth and shrub growth increases wildfire risk, leading to major changes in plant species and biogeochemical cycles in the ecosystem. I examine whether wildfire can be a realistic mechanism for providing the nutrients necessary for shrub growth, leading to permanent changes in the tundra ecosystem.

To study the effects of fire on plants and soil nutrients, I collected soil and vegetation samples from five burned sites and one unburned control site near Inuvik, Northwest Territories, during the summer of 2024. Tunda fires occurred in 1968, 1983, 2003, 2012, and 2023. In the field, I measured the active layer depth, soil temperature, soil moisture, and plant community composition. In the laboratory, I measured available phosphate, nitrate, and ammonium concentrations in soils through wet chemical extraction, measured heavy metal concentrations using X-ray fluorescence (XRF), measured microbial phosphate concentrations, performed sequential phosphate fractionation to measure a gradient of phosphorus availability, and measured basic soil parameters such as soil texture, gravimetric soil moisture, soil organic carbon content, and soil pH.

This work is necessary to understand the future of tundra ecosystems in a changing climate. At present, carbon emissions due to permafrost thaw and potential carbon uptake by increased plant growth in the tundra are poorly understood and must be quantified if we are to understand the carbon budget of the circumpolar Arctic-boreal region. As such, my work will inform terrestrial biosphere models. Of greater local importance is the future of culturally relevant tundra plant species and the future of ecosystem services that determine the identity and livelihoods of local communities.