Availability of labile carbon controls the temperature-dependent response of soil organic matter decomposition in alpine soils

Abstract. Soil organic matter (SOM) decomposition in alpine environments is influenced by multiple factors including temperature and substrate quality. It is important to understand how these factors influence soil carbon dynamics. We incubated subalpine forest and pasture soils at 12.5, 16.5, and 20.5 °C for one year with and without addition of fresh grass litter to assess impacts on total organic carbon (TOC) and lignin dynamics. In the absence of litter, TOC losses were limited, accounting for 6.7 ± 2.4 % in forest soils and 3.3 ± 1.6 % in pasture soils after 360 d, with no consistent temperature effect. In contrast, litter addition strongly increased the decomposition of primary SOM, resulting in TOC losses of 11.8 ± 1.1 % in forest soils and 17.4 ± 1.9 % in pasture soils, which were higher at elevated temperatures. Lignin concentrations declined markedly in forest soils, indicating that warming increases the decomposition of more complex SOM. Pasture soils were dominated by the breakdown of more labile litter-derived C. These results demonstrate that substrate availability is a stronger control for SOM decomposition than temperature. Increasing litter inputs in combination with rising temperatures could accelerate SOM decomposition, potentially shifting subalpine soils from carbon sinks to sources under future climate scenarios, irrespective of vegetation cover.