Nitrogen controls on carbon accumulation in coastal system: Not all nitrogen is created equal

Nitrogen (N) concentrations in coastal and estuarine waters are increasing around the globe and much research has been done to understand the effects of nutrient enrichment on coastal ecosystems. Frequently, however, the various forms of N are considered together as ‘reactive nitrogen’ rather than as the individual chemical constituents that compose that reactive N. In coastal waters, this reactive N is most typically in either the oxidized form of nitrate, or the reduced form of ammonium. Both these forms of N can be used to stimulate primary productivity in N-limited coastal waters, but nitrate is unique, in that it can be used both as a nutrient to increase primary productivity and as an electron acceptor to fuel microbial metabolisms in the absence of oxygen. Determining which of these processes dominates is essential, as the former promotes carbon fixation and storage and the latter may decrease the carbon sink capacity of marshes. Meta-analysis results, as well as results from our own long-term nitrate enrichment experiments, suggest that the stimulation of primary production, measured as aboveground biomass accumulation, is lower when N is added in the form of nitrate. Further, controlled flow through experiments, where marsh sediment was exposed to dissolved nitrate under anaerobic conditions, demonstrated greater DIC production than when under nitrate-limiting conditions. Metagenomic analysis, both via short read annotation and through genomic reconstruction, indicate the importance of added nitrate as an electron acceptor for the microbial community. Taken together, our research suggests that the dissolved nitrate that continues to enrich our coastal waters may play an important role as an electron acceptor to fuel microbial decomposition of marsh carbon, which could ultimately affect the carbon storage capacity of these critical blue carbon habitats.