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. As climate change will have an impact on both factors, it is essential to improve our knowledge, how, e.g., warming will modify carbon cycling in these environments to better prepare soil management for future conditions, even in alpine regions. This study investigates how warming and organic inputs affect SOM decomposition in alpine forest and pasture soils through a one-year laboratory incubation experiment. Soils were exposed to three temperatures (12.5 °C, 16.5 °C and 20.5 °C), with and without the addition of fresh grass litter. While higher temperatures accelerated decomposition, the availability of fresh organic matter played a more decisive role, especially in the lignin-rich forest soil. Without fresh litter, SOM decomposition was limited, suggesting that substrate availability in combination with temperature increase plays a greater role in microbial activity than temperature alone. The forest soil exhibited greater carbon loss than the pasture soil, most likely due to microbial communities that are adapted to lignin decomposition. These results suggest that rising temperatures combined with changes in vegetation and organic inputs could enhance SOM decomposition and potentially transform the alpine soils from carbon sinks to sources.