Mineral and thermodynamic controls on soil organic carbon stabilisation along a soil moisture gradient 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), in which the sorption of SOC to mineral surfaces protects it from microbial decomposition. Another mechanism is the thermodynamic limitations on microbial activity. When oxygen is limited in the environment, microbial activity is constrained to alternative respiration pathways with lower mineralisation rates. Soil moisture influences both of these mechanisms. However, it remains unclear how the interaction and the relative importance of both mechanisms vary across soil moisture 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. We measured soil pH, texture and total elemental composition, and monitored soil temperature and moisture in situ. 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 drier conditions in the grassland habitat.

 

To assess the potential of SOC stabilisation through MAOC formation along the soil moisture gradient, we quantified the distribution of carbon in three density fractions: the free light fraction, the occluded light fraction and the heavy fraction. To further investigate MAOC formation, we quantified the abundance of reactive Fe and Al phases that preferentially form associations with SOC. 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.

 

We are currently assessing the possibility for SOC stabilisation due to thermodynamic limitations using electron accepting capacity as a proxy for the availability of terminal electron acceptors and electron donating capacity as an indicator for microbial anaerobicity. We expect thermodynamic limitations to be more important in the wetter snowbed, while SOC stabilisation in the MAOC dominates in grassland.