| Summary: | Atmospheric carbon dioxide (CO 2 ) concentrations continue to rise as a result of fossil fuel burning and land-use changes, thereby contributing to increases in global temperature, ocean acidification, and sea level rise. Sequestering some of this excess CO 2 in blue carbon habitats, such as salt marshes, mangroves, and seagrasses, has been proposed as a mitigation strategy due to their ability to efficiently store carbon. Salt marshes, in particular, store carbon at rates that are orders of magnitude greater than terrestrial forests due to large inputs of organic matter (OM) from primary production concurrent with slow decomposition rates; the balance between the two ultimately determines the burial of OM and carbon storage over time. As nitrogen loading to coastal waters continues to rise, primarily in the form of nitrate (NO 3 -), it is unclear what effect it will have on carbon storage capacity of these systems. This uncertainty is largely driven by the dual role NO 3 - can play in biological processes, where it can either serve as a nutrient for primary production or a powerful electron acceptor fueling heterotrophic microbial metabolism. Distinguishing between the two is critical, since the former could promote carbon storage by enhancing fixation, while the latter could potentially deplete this service by stimulating microbial decomposition.
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