Surges in global N2O fluxes from saltmarshes are driven by increasing porewater nitrate and ammonium concentrations

Saltmarshes have the potential to sequester large amounts of carbon, however, the value of stored carbon may be partially offset by emissions of the potent greenhouse gas nitrous oxide (N₂O). Increased nutrients [NO₃^- and NH₄⁺] have been shown to increase N₂O emissions from saltmarshes, however, a global-scale analysis of this relationship has not been performed. Here, we present a global meta-analysis to investigate the relationship between N₂O fluxes and porewater nitrogen and determine the relative importance of porewater NO₃^- and NH₄⁺ as key drivers of enhanced saltmarsh N₂O fluxes. Both porewater NO₃^- and NH₄⁺ were significantly, positively correlated with N₂O fluxes (p < 0.01), explaining 25 and 18% of the variation in fluxes, respectively. We estimate a global saltmarsh N₂O flux of 0.012 Tg N₂O yr^-1, which is six times higher than the current estimate (0.0021 Tg N₂O yr^-1), representing an offset of 19% of the estimated global saltmarsh carbon burial. Using predicted future increases in riverine DIN export, our meta-analysis suggests that 17-31% of the estimated global saltmarsh carbon burial could be offset by a surge in N₂O emissions under chronic mineral N pollution. This meta-analysis indicates the importance of reducing nutrient inputs into saltmarshes to reduce N₂O fluxes and maximise their negative radiative forcing.