Intricate Spatial Soil Carbon Distribution in Four Pyrenean Mires

Alpine mires in the Pyrenees are remarkable ecosystems for the singularity of their plant communities and for their occurrence at a southern distribution limit. However, the structure of their organic soils and their contribution to carbon (C) dynamics are largely unknown. In this study, we aimed to map and quantify organic C stocks and peat thickness in Pyrenean mires for the first time. In four mire sites located on the Iberian side of the central Pyrenees, we took more than a hundred soil core samples encompassing the vegetation mosaics found at each site and analysed their organic matter content along the soil profile. We then mapped overall C stock and peat thickness in the sites using interpolation techniques and analysed the relationship between soil properties and vegetation units. Peat thickness and organic C stocks varied greatly within and amongst mires, with average values increasing with elevation from 31.1 to 43.2 cm and from 14 to 21.3 kg m -2 , respectively. Only part of the peat layer in each mire was thicker than 30 cm, and this part increased with elevation. Our results showed that the variation within and among sites is related to the complexity of the alpine physiography, where some drivers (like topoclimatic variables) operate at larger scale and others (such as topography and hydrology) at smaller scales coupled with complex vegetation mosaics. The complexity of the spatial patterns in C stocks observed, along with their relationship to vegetation, highlights the need for future research in mountain peatlands focused on disentangling the distinct factors that contribute to peat formation.