Blue nitrogen follows the fate of tidal wetlands

Tidal wetlands are hotspots of soil accumulation due to high sedimentation rates and low soil oxygen concentrations that inhibit organic matter decomposition1. Accordingly, tidal wetlands can sequester “blue carbon” at much higher rates than other ecosystems2,3 helping to offset human emissions. Organic carbon burial is tightly linked to the cycling of nitrogen, which is a key pollutant and limiting nutrient for many ecosystems4–6. Yet, the current global burial rate of “blue nitrogen” and how it may respond to future change remain uncertain. We assembled a global database of 8012 soil nitrogen measurements from 255 tidal wetland sites and found that the relationship between soil carbon and nitrogen concentrations was strikingly consistent but differed between the two dominant types of emergent tidal wetlands – marshes and mangroves. Leveraging extensive knowledge of blue carbon accumulation, we estimated that tidal wetlands, which cover less than 0.1% of the Earth’s surface, bury 3.2 Tg N yr-1, representing 13-15% of marine nitrogen burial. This rate could more than triple globally by 2100 if wetland elevation increases with accelerating sea-level rise. Alternatively, if wetlands become submerged and shorelines erode, nitrogen sinks could reverse and become sources, exacerbating coastal pollution.