What happens under the snow in alpine topsoils? A seasonal study of soil organic carbon pools and microbial activity

Alpine soils store substantial amounts of soil organic carbon (SOC), much of which is
preserved under cold conditions so is highly vulnerable to climate change. As
snowpack regimes shift and the length of the growing season increases, these alpine
ecosystems may become hotspots for accelerated carbon turnover. The seasonal
dynamics of SOC, along with related microbial activity, remain poorly understood,
particularly during winter. This study examined how local hydrological gradients
influence SOC fractions and extracellular enzymatic activity (EEA) across
ridge-to-snowbed transects in the Swiss Alps, with seasonal sampling in summer,
autumn, and winter. Soil organic matter density fractionation and enzyme assays
were conducted on alpine soils. Hydrology-driven differences in soil moisture were
hypothesized to drive SOC concentrations and soil function. Contrary to
expectations, hydrological position did not significantly affect SOC fractions. Instead,
strong seasonal shifts were observed in labile SOC pools (free POM-C and WEOC),
and unexpectedly, mineral-associated organic matter (MAOM-C), which is typically
viewed as stable, also showed significant seasonal variability. Microbial activity
showed a stronger influence from topography than season, with enzymatic activity
consistently suppressed in snowbed soils. Notably, alanine-aminopeptidase activity
peaked in winter, potentially reflecting microbial use of frost-resistant substrates.
These findings challenge assumptions about MAOM stability and the controlling role
of hydrology, highlighting the importance of fine-scale microclimate and winter
processes in regulating alpine SOC dynamics under changing snow regimes.