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Mangroves, long recognised for absorbing large amounts of carbon dioxide (“blue carbon”, referring to carbon stored in marine and coastal environments), have been regarded as a key focus of global climate change mitigation, with their carbon sequestration function forming a cornerstone of international conservation efforts. A new study by the Department of Earth and Environmental Sciences at The Chinese University of Hong Kong (CUHK) has revealed that mangroves also possess another important but long-underestimated function – the highly efficient removal of nitrogen pollution, known as “blue nitrogen”. The study has found that although mangroves cover less than 0.1% of the world’s land area, they remove up to 870,000 tonnes of nitrogen pollutants from coastal waters each year, with a nitrogen removal efficiency nearly twice that of terrestrial soils. The annual ecological value of this nitrogen purification service is worth more than US$8.5 billion, more than 12 times the value of mangroves’ carbon sequestration credits. The study has been published in the international journal Earth’s Future.

Nitrogen pollution is one of the world’s most pressing environmental crises. Over the past century, human nitrogen emissions have surged 50-fold, triggering harmful algal blooms, expanding coastal dead zones, damaging fisheries and threatening drinking water safety. Mangroves offer a natural solution: microbes in their waterlogged sediments convert harmful reactive nitrogen into harmless nitrogen gas, permanently removing it from the ecosystem.

The research, conducted by Professor Benoit Thibodeau, Assistant Professor of the Department of Earth and Environmental Sciences and the School of Life Sciences, and PhD candidate Wang Ziyan, has revealed that mangrove forests remove nearly 870,000 tonnes of nitrogen pollutants from coastal waters each year. According to current nitrogen credit market valuations, this ecosystem service is worth more than US$8.5 billion annually, more than 12 times the value of mangroves’ annual carbon sequestration credits. The research team performed the most comprehensive global analysis to date, synthesising 51 published studies and unpublished datasets covering 42 mangrove sites worldwide. It found that under optimal conditions, mangroves can remove up to five million tonnes of nitrogen annually, offsetting about 3% of global human-generated reactive nitrogen.

The study emphasised that blue nitrogen is not a replacement for blue carbon but an important complement to it. Together, they provide a more complete picture of the true value of mangroves. Professor Thibodeau said: “Blue carbon showed the world mangroves’ power against climate change. Blue nitrogen reveals they are also powerful water purifiers, a role we have long underestimated. The former changed how we talk about mangroves; the latter completes the picture. Together, they provide a stronger scientific basis for the protection of these vital coastal ecosystems.”

First author Ms Wang said: “Every mangrove forest performs double duty – sequestering carbon and purifying water. This gives us even stronger reasons to protect them. Blue carbon gave us a successful model for valuing ecosystem services, and blue nitrogen builds on that foundation to help us better assess and plan coastal conservation.”

The research team recommended integrating blue nitrogen credits into existing ecosystem service frameworks and establishing a dual carbon-nitrogen credit mechanism. This would allow mangroves’ water purification function to generate economic returns, opening new funding channels for coastal conservation. The Greater Bay Area, with its extensive coastal development and significant mangrove coverage, is well-positioned to serve as an international pilot site. At a time when coastal nations worldwide are facing severe nitrogen pollution and ageing water infrastructure, creating an urgent need for nature-based solutions, blue nitrogen offers a rigorous scientific framework to recognise and protect mangroves’ natural water purification role, while building upon the established foundation of blue carbon.

The study also noted that current nitrogen removal by mangroves is only about 15% of their theoretical maximum potential, indicating substantial room for improvement. It further found that even nutrient-poor, high-salinity mangrove systems exhibit strong nitrogen removal capacity. Understanding these mechanisms will provide valuable guidance for mangrove restoration and management. The research team stated that the ecological functions of mangroves still hold potential for further study. The findings will help optimise conservation investment, achieving dual benefits of carbon sequestration and water purification.

The full research article is available here: https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2025EF007772