Fire impacts on soil carbon in a non-fire adapted alpine forest

An increase in fire-prone conditions in non-fire adapted regions is rooted in climatic and anthropogenic changes. Such pyrogeographical shifts are observable, for example, in alpine regions. In 2021, Austria, experienced a fire larger than 100 ha for the first time in a century in the Schneeberg-Rax mountain region. In depth understanding of post-fire effects on carbon cycling at such non-fire adapted sites is still scarce. To help close this knowledge gap, post-fire changes were investigated at the abovementioned site, including soil organic matter composition and soil chemical conditions. 

Samples were taken immediately after the fire, 3 months, 6 months and 12 months thereafter from four sampling sites. Selected sites consisted of 1. a pine forest affected by a crown fire, 2. a pine and beech mixed forest affected by a surface fire, and two non-fire affected controls with similar site conditions (vegetation, slope, altitude, and exposition). Samples were analyzed for pH, carbon content, elemental composition, leachable dissolved organic carbon and trace elements, organic matter composition, and environmentally persistent free radical concentrations. 

pH increased after the fire at both sites investigated. This increase was the strongest (up to 1.5 units) immediately after the fire but was still substantial 1 year after the fire. Carbon contents decreased approximately 2fold in the crown fire affected soil compared to the control soil, but remained similar between surface fire affected soil and the respective control. However, aromaticity of bulk carbon and the leachable fraction increased in both fire-affected soils, which can be related to the formation of pyrogenic carbon during the fire. Pyrogenic carbon is a highly aromatic and recalcitrant carbon pool produced during incomplete combustion of biomass. Pyrogenic carbon can also contain substantial amounts of environmentally persistent free radicals (EPFR), which can form reactive oxygen species, which can induce oxidative stress on microbiota. Our EPFR measurements showed an increase by at least 1.5 orders of magnitude of EPFR in fire affected soils. This study suggests that changes in soil carbon cycling can be expected following fires in non-adapted alpine forests.