The productivity of most terrestrial ecosystems is limited by the availability of nitrogen (N), but little is known about the factors that control ecosystem N retention and loss. During primary succession, soil microbial communities become progressively more fungal-dominated, and our recent work has shown that soils with fungal-dominated microbial communities, as opposed to those dominated by bacteria, retain N better and reduce N loss. This study is testing the idea that ecosystem N retention increases with succession, and specifically that fungal-dominated soil microbial communities of late-successional series will immobilise, and thus retain, more N than bacterial-dominated microbial communities of early-successional stages. To address this, we are performing a 15N-labelling study across three glacier forelands in Austria and Switzerland to asses how microbial and ecosystem N retention changes as ecosystems develop. This project, which is led by Franciska De Vries, is funded by the British Ecological Society, and involves collaborators at the University of Aberdeen, Innsbruck, Vienna and WSL Zurich.