High Diversity of Diazotrophs in the Forefield of a Receding Alpine Glacier

Abstract

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.

Abundances and potential activities of nitrogen cycling microbial communities along a chronosequence of a glacier forefield

“…Interestingly, our Abundances and activities of nitrogen cycling microbes R Brankatschk et al data revealed no potential N-fixation activity and the lowest nifH gene copy numbers per gram soil in the 10a soil. This is supported by the studies of Duc et al (2009) and Nemergut et al (2007) in which the lowest N fixation rate was measured in the bulk soil of the initial stage of the Damma glacier (Switzerland) and Puca glacier (Peru) forefield, respectively. In contrast, mineralization of organic material seemed to be the driver for N cycling at the initial stage of ecosystem development, indicated by the highest potential activities for proteases combined with highest relative aprA gene copy numbers (based on the amount of extracted DNA).…”

“…Input of total N is in the range between 10-15 kg N ha -1 a -1 . To estimate the input of organically bound N and C to the forefield of the Damma glacier, we sampled snow patches at the end of the winter season (o1 m snow depth) in the forefields Duc et al, 2009). The distance from the glacier terminus was 100 m for the 10a site and 650 m for the 120a site.…”

“…This observation is in agreement with studies reporting highest diversity of functional groups, such as nitrate reducers and N-fixing microbes. Duc et al, 2009;Schmalenberger and Noll, 2009). Moreover, initial sites are often dominated by microbes belonging to the r-strategists being able to respond quickly to changing environmental conditions, whereas with continuing succession a shift to k-strategists occurs that rather pursue the strategy of maintenance .…”

“…In the last decades, different studies at glacier forefields focused on the dynamic of selected processes and transformation steps of the N cycle Kandeler et al, 2006;Duc et al, 2009;Towe et al, 2010a) because N as a macronutrient is essential for ecosystem development, but most bedrocks do not contain any N.…”

A review of soil NO transformation: Associated processes and possible physiological significance on organisms

“…Alcaligenes, Thiosphaera, Pseudomonas spp., described by Kuenen and 357 Robertson (1994), Moir et al (1996), Daum et al (1998), Nishio et al 358 (1998)) and fungi (e.g. Ascomycota and Basidiomycota There is a need for more continuous NO flux measurements during cold 551 periods and winter/spring transition periods, in order to improve our 552 estimates of annual flux rates.…”