Mediated electrochemical analysis as emerging tool to unravel links between microbial redox cycling of natural organic matter and anoxic nitrogen cycling

Natural organic matter (NOM) is an important redox-active component in soils and aquifers that comprises numerous functional moieties spanning a wide range of redox potentials. Tracking reversible electron transfer from and to NOM in biogeochemical redox processes has been a challenge for decades. Reasons include side reactions of reactants used to determine the redox state of NOM and slow reaction kinetics of reactants or traditional non-mediated electrochemical methods. Furthermore, partially irreversible reactions/methods employed hamper the experimental determination of redox properties of NOM. Recent advances in mediated electrochemical analysis, however, have greatly improved our ability to characterize the redox properties of NOM. Thus, mediated electrochemical analysis may become an important tool in expanding our understanding of NOM-fueled biogeochemical N cycling in anoxic environments. Nonetheless, this technique has rarely been applied to investigate microbial pathways of reversible NOM redox cycling such as its coupling to anoxic nitrogen (N) cycling. Here we advocate for employing mediated electrochemical analysis to address such topics in the future and provide recommendations for a successful experimental application of this method in the presence of reactive N-species. To this end, we review recent applications of mediated electrochemical analysis in studying microbial NOM cycling. We exemplify the potential of mediated electrochemical techniques for biogeochemical research by discussing how microbial NOM redox cycling is linked to anaerobic N cycling. We focus on anaerobic ammonium oxidation (anammox) and reduction of N-oxides that are related to N loss and nitrous oxide (N2O) mitigation. Finally, we present strategies to work around problems arising from electroactive intermediates that hamper the application of mediated electrochemical analysis in microbial experiments.