Impact of differentially structured cushion plants on soil organic carbon content under dry high-mountain periglacial conditions

Cushion plants are often viewed as ecosystem engineers, transforming soil conditions and supporting succession processes in dry and periglacial areas. However, the impact of the cushion plants on their habitat and other plant species may vary depending on how stressful the general conditions are. Therefore, to assess how cushion plants influence soil organic carbon (SOC) content under highly stressful environmental conditions, we conducted a study in a model area combining features typical for periglacial and hyperarid regions and characterised by one of the lowest documented organic carbon contents in soil. We selected three cushion plant species differing in morphological structure and place of litter accumulation and compared: (1) the organic carbon content in soils under the cushions against the background organic carbon values in soils outside cushions; (2) the organic carbon content in soils under the three selected species, looking for trends related to their structural differences; and (3) we studied changes in SOC content in the distance from the glacier terminus, along with the expected temperature and moisture gradients. Even under highly demanding environmental conditions, the organic carbon content in soils under the cushions was between 2.4 and 23.7 times higher than in the outside soils. The SOC content was influenced by the cushion structure, including place of litter storage, and environmental factors, including climatic features and soil characteristics. Among the latter, the most important factors were the effects of soil temperature and moisture interplay along the foreland and soil content of inorganic nitrogen. Further aridification projected for the studied area will influence the distribution and coverage of the cushion plants and impact environmental factors shaping organic carbon content under them. Hence, we may expect changes in the role of the cushion plants in nutrient storage in dry high-mountain periglacial areas.