Soil organic carbon stabilisation mechanisms along short environmental gradients in the Swiss Alps

Soil organic carbon (SOC) is a key component in the soil ecosystem. SOC can be stabilised against microbial mineralisation by several mechanisms. One mechanism is the formation of mineral-associated organic carbon (MAOC) by adsorption of SOC onto mineral surfaces, which prevents accessibility to microorganisms (Kleber 2015). Another mechanism is the thermodynamic limitations on microbial activity arising from lower energetic yields of the reduction of alternative electron acceptors (Keiluweit et al., 2016). Both mechanisms are influenced by environmental conditions, especially soil moisture. However, it remains unclear how the relative importance of both mechanisms varies across short environmental gradients. To study the mechanisms behind SOC stabilisation in alpine environments, we selected three transects covering a moisture gradient highlighted by vegetation type changes from snowbed to grassland habitats. We assessed SOC content and thermal stability with RockEval analysis (Sebag et al., 2016). We measured soil pH, texture and total elemental composition, and monitored soil temperature and soil moisture. From preliminary data analysis, we found that SOC content increased from the snowbed to the grassland habitat. Soil moisture monitoring revealed a strong gradient with wet to potentially waterlogged conditions in the snowbed habitat and overall dry conditions in the grassland habitat. As a next step, we will indirectly assess SOC stabilisation mechanisms. The possibility of stabilisation through MAOC formation increases with the concentration of mineral reactive phases (Porras et al., 2017). We will quantify reactive Fe, Al and Ca phases with selective dissolution. The possibility for SOC stabilisation due to thermodynamic limitations will be assessed using soil redox state as a proxy for the availability of terminal electron acceptors (Aeppli et al., 2022). If electron donating capacity is high, we assume that alternative respiration pathways are active and that microbial respiration might be thermodynamically limited. We expect thermodynamic limitations to be more important in the wetter snowbed, while SOC stabilisation in the MAOC dominates in grassland. The distribution of SOC within particulate organic carbon and mineral-associated organic carbon pools gives additional information on the stabilisation mechanisms at play. We will assess the distribution of carbon in the free light fraction, the occluded light fraction and the heavy fraction pools with density fractionation (Viret and Grand, 2019). We expect the relative proportion of carbon in the free light fraction and the occluded light fraction to be higher in the snowbed where thermodynamic limitations are stronger. Inversely, we expect the relative proportion of carbon in the heavy fraction to be higher in the grassland where MAOC formation dominates.