Compounding effect of fire history and freeze thaw cycles on ecosystem resilience in northern temperate peatlands

Rising air temperatures are leading to both an increase in frequency of freeze-thaw cycles (FTCs) during the winter months, and increased fire frequency and severity during the growing season in northern temperate peatlands. Both FTCs and fires have been shown to impact plant and microbial community composition, nutrient availability, and plant-microbe interactions. Ultimately such changes may affect carbon cycling in peatlands. While examples from the vegetation are numerous, no study has assessed the resilience of peatland microbial communities and carbon cycling to the combined disturbance effect of fire and FTCs. The objectives of this study were to (a) determine how peatland microbial community structure and activity are affected following fire, and (b) assess if the impact of FTCs on peatland microbial communities and soil carbon stores is affected by fire history. To address these objectives, we conducted an FTC and incubation experiment using surface peat (2 – 15 cm) from a pristine peatland and nearby peatland that burned in 2006 located in southern Sweden. Peat from both sites was exposed to 15 FTCs, with each FTC consisting of 18 hours at -20 °C followed by 6 hours at +20 °C. Following the termination of the FTCs we incubated the peat at +20 °C for 40 days, measuring peat respiration as the change in headspace greenhouse gases throughout. We measured peat pore water chemistry, hydrolytic enzyme kinetics, and microbial community assembly using metagenomics before and after the FTCs and incubation. Extracellular enzyme kinetics and pore water chemistry data suggest a legacy effect of the fire, whereby the pristine site exhibits greater enzyme degradation and greater lability of dissolved organic matter before and after FTCs. We also saw greater rates of respiration in peat from the pristine site. We found that FTCs in fire affected peat caused an increase of dissolved organic carbon and aromatic compounds in peat pore water, leading to a reduction in extracellular enzyme and respiration rates. We conclude that while FTCs have the potential to disrupt the stability of peatlands, the legacy of fire exerts a greater constrains on biogeochemical processes. This project highlights that growing season disturbances may have a longer lasting impact on peatland resilience.