Low-Intensity Surface Fires and Dissolved Organic Matter: Unraveling Post-Fire Carbon Dynamics in Northern European Boreal Forest Soils

Recent studies highlight a concerning reality: wildfires are becoming more frequent and intense, particularly in northern high-latitude regions where temperatures are rising fastest. Boreal forests, vital carbon (C) reservoirs, play a key role in long-term C storage and climate regulation. However, climate change-driven increases in wildfire frequency, intensity, and severity threaten to turn these soils from C sinks into sources, disrupting soil biogeochemical cycles and hindering forest recovery and ecosystem resilience. Fire significantly alters soil organic matter (SOM) and C cycling processes, particularly impacting soil dissolved organic matter (DOM). In boreal forests of Northern Europe, low-intensity surface fires are common, but their short-term effects on soil DOM dynamics remain poorly understood. We aimed to investigate the short-term effects of a low-intensity surface fire on post-fire DOM properties and dissolved organic carbon (DOC) content in boreal forest soils.
Fieldwork was conducted in a dry Scots pine boreal forest of Eastern Finland (Ruunaa, North Karelia) that underwent a prescribed restoration fire on June 30^th, 2022. The burning resulted in a non-stand replacing surface fire of low intensity and severity. To capture short-term post-fire responses, we compared DOC content, δ¹³C_DOC, and DOM absorbance properties in soil water and throughfall collected from burned and unburned control plots during the first growing season following the burning (from July to October 2022). DOM was analyzed for changes in concentration and isotope composition with a coupled elemental analyzer and mass spectrometer (EA-IRMS), while changes in DOM chemical composition were characterized using UV-visible absorbance spectrophotometry.
Our results indicated that soil DOC contents declined immediately after the fire in burned plots compared to control ones, accompanied by slight enrichment of burned soils DOM in ¹³C. These findings suggest reduced availability of labile C substrates following SOM and biomass combustion, fire-induced reduction of the microbial biomass, and introduction of newly formed pyrogenic carbon (PyC), which has a lower proportion of lignin-derived ¹³C. Additionally, the soil DOM from burned soils showed slightly higher degrees of aromaticity and molecular weights, indicating a shift towards more aromatic and recalcitrant compounds, suggesting the presence of a more stable C pool in the soil water of fire-affected soils.
Our findings emphasize the crucial role of low-intensity surface fires in influencing DOM dynamics and provide vital insights for understanding the post-fire soil C cycling and ecosystem recovery in boreal forests of Northern Europe. Understanding these dynamics is crucial for improving C balance models in these forests and equipping policymakers and forest managers with the tools needed to enhance resilience in one of the planet’s most vital ecosystems.