Effects of fire on dissolved carbon and greenhouse gas production rates and their temperature sensitivity in subalpine wetlands

Abstract

Wetlands comprise only 5–8% of land surface but hold 20–30% of estimated soil carbon globally. However, wetlands are also significant sources of greenhouse gases such as methane (CH₄) and nitrous oxide (N₂O). Disturbances such as wildfires can alter the balance between carbon storage and greenhouse gas production in wetland systems; therefore, it is crucial to understand wetland response and recovery after wildfires. While wildfires are known to significantly impact ecosystem function through changes in soil properties, nutrient cycling, and hydrology, subalpine wetlands remain understudied, with the exception of organic matter-rich peatlands. Though temperature fluctuations regulate microbial processes, it is unclear how seasonal temperature patterns influence wildfire effects. We investigated these interactions in burned subalpine wetland soils in the Medicine Bow National Forest, Wyoming, USA, 1 year after the 2020 Mullen fire. We measured potential rates of carbon dioxide (CO₂), CH₄, N₂O, and DOC production using slurry experiments and flow-through experiments with soil collected from two depths (0–2 and 15–17 cm). Both experiments were conducted at local minimum, mean, and maximum July air temperatures (9, 18, and 27 °C). In situ porewater measurements showed that burned wetland areas had higher dissolved organic carbon (84–105 mg/L vs. 65 mg/L), sulfate (2.8–3.3 mg/L vs. 1.4 mg/L), and nitrate concentrations (1.3–1.9 mg/L vs. 0.5 mg/L) compared to unburned wetland areas, particularly in shallow depths (0–12 cm). Slurry experiments revealed approximately 1.3 times higher potential CO₂ production rates and fivefold higher N₂O production rates, but 2.9 times lower CH₄ production rates in burned compared to unburned wetland soils. Flow-through reactor experiments corroborated these findings, showing higher DOC (2–4 ×), Fe(II) (1.5–2 ×), and DIC (1.3–1.8 ×) potential production rates but lower CH₄ production rates (0.4–0.8 ×) in burned wetland soils. The suppression of methanogenesis and enhancement of Fe(III) reduction in these soils suggest altered redox conditions, potentially resulting from changes in organic matter composition, soil exposure, and hydrology following fire. Temperature sensitivity analysis revealed higher Q₁₀ values for Fe(II) production in burned wetland soils (1.60–2.90 vs. 1.57), indicating that fires enhance the temperature response of Fe(III) reduction pathways. These findings provide insights into post-fire biogeochemistry of sensitive subalpine wetland systems, with implications for the global carbon cycle and drinking water quality.