The coastal sea is perhaps the ultimate common landscape—shared across nations, communities, and generations, yet degraded by a diffuse and collective burden: nitrogen pollution. Human activity has increased reactive nitrogen release up to fifty-fold since 1900, driving harmful algal blooms, hypoxic "dead zones," and billions of dollars in losses to fisheries worldwide. The costs are socialised; the responsibility, diffuse. Here we argue that a natural remedy to this shared problem is hiding in plain sight along our shorelines.
Through a comprehensive global meta-analysis, we establish the first benchmarks for nitrogen removal by mangrove forests, whose sediments convert reactive nitrogen back to inert dinitrogen gas. We find that the world's mangroves remove approximately 870 Gg of nitrogen each year—a purification service worth an estimated $8.7 billion annually, more than twelve times the value of their celebrated "blue carbon" sequestration. Yet this "Blue Nitrogen" service remains invisible to conservation frameworks and markets alike. We further show it is fragile: extreme eutrophication, warming above 22°C, and hypersalinity each suppress it, meaning the sea's capacity to heal itself may falter precisely where and when it is most needed.
We propose that quantifying and pricing Blue Nitrogen transforms an unowned, unvalued ecological process into recognised common infrastructure—a shared asset whose protection can be financed collectively. In dialogue with the conference theme, we ask what it means to govern, value, and belong to a sea that cleans itself, and how a "Blue Nitrogen" commons might reshape the social landscape of coastal stewardship worldwide.

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