Nitrogen-fixing cushion Astragalus siculus modulates soil fertility, microclimate, plant facilitation, bacterial and fungal microbiota along an elevation gradient

Question: Cushion-forming vascular plants are adapted to alpine environments and act as ecosystem engineers by moderating microclimate and creating a fertility island under their canopy. Most of the available studies on cushion ecology have been devoted to plant facilitation, but none used a holistic approach that considers the response of co-existing plant species with other key ecosystem players. Here, we quantified the biogenic effect and facilitation capacity of Astragalus siculus, a spiny nitrogen-fixing cushion plant, and test the hypothesis that the facilitation effect would increase with elevation as stress gradient. Location: Mount Etna, the highest active volcano in the Mediterranean Basin. Methods: We combined chemical characterization of the soil with a description of the soil microbiota by high-throughput sequencing of bacterial and eukaryotic rRNA gene markers carried out at three elevations, that is 1650, 1900 and 2210 m a.s.l. In addition, we analysed the microclimate outside and inside the Astragalus canopy to assess the linkages between above-and below-ground ecosystem compartments and quantified the occurrence of co-existing vascular plants. Results: Astragalus cushions have strong biogenic effects on above-and below-ground conditions by dramatically increasing soil organic carbon and total nitrogen stocks at 1900 and 2210 m elevation. Moreover, a buffering of soil temperature and moisture regimes was recorded, with lower temperatures in summer but higher in winter inside the cushions. Cushions harbour a specific bacterial and fungal microbiota compared to external bare soil. Stem density and cushion height increase significantly with altitude, as does the nurse effect of the cushion on the growth and survival of co-existing vascular plants. Conclusions: As a result of cushion biogenic environmental changes, the effect of Astragalus on some of the co-existing species shifted from negative or null at 1650 m to facilitative along the elevation gradient. Furthermore, our data show a relationship between Astragalus morphological traits and soil biogenic changes related to soil chemistry and microbiota that affect plant interaction and coexistence.