Vulnerability of carbon in subalpine soils in the face of warmer temperatures

Abstract. Alpine and subalpine soils are significant reservoirs of labile carbon (C) and are highly sensitive to warming, yet the mechanistic interactions between temperature and organic inputs are poorly understood. A one-year laboratory incubation was conducted with mineral surface soils from a subalpine pasture and an adjacent coniferous forest site. Soil samples were incubated in closed jars at three different temperatures: Current growing season temperature (12.5 °C), and two increased temperature treatments (16.5 °C and 20.5 °C). To assess decomposition differences between litter and native soil organic matter (SOM), ¹³C-labelled plant litter was added to a subset of the jars. CO₂ production, δ¹³C partitioning, and phospholipid fatty acid (PLFA) profiles were used to quantify soil organic matter (SOM) and litter decomposition, and to assess microbial dynamics. Warming increased total CO₂ respiration by 15–37 % in pasture and 12–33 % in forest soils, with strongest stimulation in litter-amended soils. Positive priming of native soil organic matter (SOM) peaked within one week (up to +77 % over controls) and declined to near zero after one month. Cumulative litter-induced respiration (LIR) was highest at 16.5 °C (+6–10 % vs. 12.5 °C) in both soils, coinciding with maximum microbial biomass; 20.5 °C reduced microbial biomass by up to 25 % and accelerated ¹³C label loss. The response of pasture soils was more rapid and pronounced compared to forest soils, which exhibited slower, more sustained responses. PLFA profiles revealed warming-induced declines in Gram⁺ and Gram^- bacteria and increased cyclopropyl markers at high temperature. These findings show that even moderate warming can accelerate C loss from subalpine soils, with vegetation history and microbial traits modulating both rate and timing of decomposition.