The key to sustaining life with a finite supply of life-giving nutrients on the earth is their constant recycling: the global biogeochemical cycles. While it has been recognized for a long time that the microbial community dominates the biogeochemistry in soils, linking processes to specific organisms or groups has proven elusive. A central problem has been the flawed assumption that the abundance of microorganisms equates to their process contribution. Alternative approaches are emerging, where e.g. microbial growth and activity rates, isotopic tracking of biogeochemical cycling and metabolic modelling are beginning to explicitly link processes to microorganisms. We propose to organize a session focused on new developments to assess the crucial microbial mechanisms that underpin global biogeochemical cycles.