Taking the permafrost’s pulse – Insights from seven years of a simulated permafrost thaw resource pulse.
Dr. Catherine Dieleman, University of Guelph
Northern peatland soils are important long-term stores of both carbon and nitrogen resources, particularly in permafrost underlain systems where cool belowground temperatures can hold soils in stasis for centuries. However, climate change is warming permafrost globally, rendering these vast stores vulnerable to enhanced decomposition. Nitrogen is a limiting nutrient concomitantly released during permafrost thaw events that can stimulate plant and microbial metabolism to either mitigate or intensify carbon release from thawing permafrost soils. Models predict nitrogen release will be both temporally and spatially asynchronous with peak biological productivity, limiting the impact of this permafrost resource pulse on carbon dynamics. We tested these model predictions in situ by injecting 4 g of solid phase urea fertilizer at two soil depths (rooting zone, permafrost thaw front), two seasonal time points (peak biological activity, peak permafrost thaw), and two permafrost thaw conditions (intact permafrost, actively thawing permafrost) from 2017 – 2023 in a northern peatland system in interior Alaska, USA. Over this seven-year period we monitored vegetation community composition, porewater chemistry, and greenhouse gas production to characterize shifts in community structure and function driven by a thaw induced resource pulse. Despite the large nutrient addition we found minimal shifts in ground vegetation composition, porewater carbon and nitrogen content as well as N2O and CO2 cycling, regardless of treatment timing, or depth. Instead, permafrost thaw stage overshadowed resource addition treatments. We did observe a significant initial increase in CH4 fluxes at actively thawing sites with resource pulse treatments, increasing methane production by up to seven-fold. Combined our research demonstrates that lowland microbial communities can overcome temporal and spatial asynchronies in permafrost resource pulses. However, changes in local environmental conditions prompted by thaw are generally a stronger determinant of both ecosystem structure and function in lowland permafrost systems.