Impact of biological crusts on soil formation in polar ecosystems

We tested the impact of biological soil crusts (BSC) at different degrees of development on soil formation in Polar ecosystems, specifically in two Arctic sites, Svalbard Island (Norway) and Tarfala (Sweden), and in an Antarctic site, Apostrophe Island (Victoria Land). In each site, slightly developed BSC (thin and made by green algae and cyanobacteria associations - SD-BSC) highly developed BSC (thick and dominated by green algae and cyanobacteria associations with/without mosses and/or lichens - HD-BSC), and moderately developed BSC (MD-BSC) with intermediate characteristics between the slightly and the highly developed, were sampled together with the AC and A horizons immediately under the crust. After separating the organic residue of the biocrust from the mineral phase by density fractionation, in the heavy fractions obtained from HD-BSC, MD-BSC and SD-BSC we determined the amount of soil organic carbon and its radiocarbon natural abundance, total nitrogen content, mineralogical assemblage by x-ray diffraction, and quality of soil organic matter (SOM) by infrared photoacoustic spectroscopy (FTIR-PAS). We found that when BSC were able to develop on stable ice-free surfaces, they modified the soil by supplying new organic substances. These new substances, in addition to diluting the old C inherited from the substrate and darkening the upper mineral horizon, promoted acidification, which is responsible for mineral weathering and neogenesis of clay minerals. With their development, BSC act as ecosystem engineers and promote soil formation in Polar ecosystems by increasing soil stability, organic matter content and nutrient availability, which indirectly improve aggregation, water holding capacity, and soil heating.