Microbial nitrogen cycling in soils at pioneer sites

Pioneer sites such as glaciers forefelds, sand dunes, and volcanic lava? ows, represent ideal environments to study the development of terrestrial ecosystems. Nitrogen (N) is virtually absent in soils at pioneer sites. Nevertheless, over the years N accumulates and plants colonise the sesystems, indicating a rapid development of the Ncycle. Two contrasting pioneer sites were chosen to study the development of the N cycle in soil; the forefield of the Damma Glacier in canton Uri, Switzerland and a mobile sand dune near the town of Lieberose, Germany. At both sites, different phases of ecosystem development were in vestigated. Four processes dominating the terrestrial N cycle were quantified by enzyme activity and gene abundance. Latter was measured by real-time PCR, targeting the marker genes of each of the four processes: N fixation, mineralisation, nitrification, and denitrification. The N cycle developed in a similar pattern at the glacier forefield and the sand dune. As a general rule, three stages of N cycle development can be distinguished. First, the heterotrophic stage is dominated by a community of opportunistic microorganisms and the process of mineralisation. Second, in the transition stage primary producers colonise the system and stabilise the soilsurface; N starts to accumulate. Last, during the developed stage nitrification and denitrification gain importance and a closed Ncycle, within which all processes are active, is established.