High diversity of diazotrophs in the forefield of a receding alpine glacier.

Forefields of receding glaciers are unique and sensitive environments representing natural chronosequences. In such habitats, microbial nitrogen fixation is of particular interest since the low concentration of bioavailable nitrogen is one of the key limitations for growth of plants and soil microorganisms. Asymbiotic nitrogen fixation in the Damma glacier (Swiss Central Alps) forefield soils was assessed using the acetylene reduction assay. Free-living diazotrophic diversity and population structure were resolved by assembling four NifH sequence libraries for bulk and rhizosphere soils at two soil age classes (8- and 70-year ice-free forefield). A total of 318 NifH sequences were analyzed and grouped into 45 unique phylotypes. Phylogenetic analyses revealed a higher diversity as well as a broader distribution of NifH sequences among phylogenetic clusters than formerly observed in other environments. This illustrates the importance of free-living diazotrophs and their potential contribution to the global nitrogen input in this nutrient-poor environment. NifH diversity in bulk soils was higher than in rhizosphere soils. Moreover, the four libraries displayed low similarity values. This indicated that both soil age and the presence of pioneer plants influence diversification and population structure of free-living diazotrophs.