"The terrestrial C and N cycles are essential for the Earth’s biosphere and intimately linked by microbial activity. Understanding the participants and drivers rendering biologically available N is imperative as N is a limiting factor for primary production. Soils contain the largest pool of C on Earth with cellulose being a major constituent of this C. Therefore understanding this process is essential since it can either promote C sequestration or be a source of atmospheric CO2.My NanoSIMS group in the Department of Microbial Ecology at the University of Vienna focuses on the investigation of two microbial key processes; plant polymeric carbon degradation and nitrogen fixation. Our goal is to understand the active drivers of these processes and their in situ contributions, the degree of efficiency and regulation mechanisms using a multidisciplinary approach combining methods of biogeochemistry, molecular biology, ecology, soil science, and bacterial physiology.It is timely to now apply a functional approach to study the aforementioned processes due to the recent advance in single cell approaches. To identify the active participants of the two processes, we propose to combine stable isotope probing experiments (15N2 gas or 13C-cellulose) with cell identification with FISH/HISH, Raman microspectroscopy and NanoSIMS. These single cell techniques allow comparing the in situ activities of different microbial groups and the analysis of within population heterogeneity. We aim to differentiate the contributions of cellulose-degrading fungi and bacteria, as well as targeted groups within the bacteria. Second, we aim to better characterize participants in soil N2 fixation with particular emphasize on novel groups who are believed to be highly active. The synthesis of stable isotope probing experiments with single cell genomics will allow us to identify novel microorganisms of the targeted function and describe their ecophysiology in the soil."