Mangrove Forests Mitigate Nitrogen Pollution by Trapping Sediment that Contains Anaerobic Microbes That Break It Down in Water: The Potential for Nitrogen Removal Credit Markets

     A recent study published in the journal Earth’s Future estimates that mangrove forests provide $8.7 billion per year in benefits through mitigating coastal nitrogen pollution. That adds to their ability to sequester carbon, to curb coastal erosion, and to protect against storm surges, resulting in an impressive ecosystem services portfolio for the species.

     According to Live Science:

“Mangroves are salt-tolerant plants that grow between the high-tide and low-tide marks in tropical and subtropical coastal regions. Their tall, tangled roots trap sediments rich in microbes that break down nitrogen in the water into nitrogen gas (N2) and nitrous oxide (N2O), effectively removing this nutrient from the ecosystem.”

     The scientists admit that they are just beginning to understand how the microbes enable denitrification.

"We're still really in the infancy of trying to understand what is driving this nitrogen removal," Benoit Thibodeau, an assistant professor in the Department of Earth and Environmental Science at The Chinese University of Hong Kong, said in a joint interview with his co-author Ziyan Wang, a doctoral student in environmental science at the same university. "You're taking reactive nitrogen … and you're removing it to the atmosphere as N2, which is nonreactive and has a residence time of thousands of years."

     Nitrogen pollution from fertilizer and manure runoff, and sewage, collects in coastal areas where it depletes the oxygen in the water, a process known as eutrophication, and feeds algal blooms.

     The researchers estimated global nitrogen-removal rates in mangrove forests, noting different rates of nitrogen removal in different forests. They then estimated the amount of nitrogen that mangrove forests could soak up if temperature, salinity, and nitrogen levels were optimal. They utilized previous studies along with their own measurements to arrive at the estimates.

“Microbes in mangrove forests remove nitrogen via two main pathways: denitrification and anaerobic ammonium oxidation (anammox). Denitrification transforms nitrate in the water into nitrogen gas and nitrous oxide, which is a greenhouse gas. Anammox, on the other hand, converts nitrite and ammonium into nitrogen gas, which makes up 78% of the atmosphere and is not a greenhouse gas. These pathways work best with relatively high nitrogen concentrations, but there is a threshold past which removal slows, according to the study.”

“These pathways also occur in seagrass meadows and other coastal environments, but mangrove forests are especially good at removing nitrogen because their sediments are oxygen-poor, which promotes the right kind of microbial activity, Wang said.

     The researchers utilized a market valuation akin to carbon credits for the nitrogen-removal benefits to estimate the value of mangrove forests. They are known as blue nitrogen credits.

“Based on what municipalities in countries like Australia and the U.S. pay to get rid of nitrogen in their water systems, Thibodeau and Wang settled on a price of just over $10,000 for every metric ton of nitrogen removed anywhere in the world.”

     The market valuing nitrogen removal is much less mature than carbon markets, which generally have agreed-upon regional prices.

“At the current rate of nitrogen removal, mangroves' cleanup service is worth $8.7 billion per year globally. If removal rose to 5.5 million tons per year, it would be worth around $57 billion annually, according to the study.”

     Below are some of the variables used in estimating nitrogen removal potential in mangrove forests.

     Below, they note that their estimates suggest that nitrogen removal offers much better overall environmental or ecosystem benefits than carbon sequestration, with an economic value potentially 12 times higher.

“The researchers also calculated the economic value of carbon sequestration in mangrove forests and found it was 12 times smaller than that of nitrogen removal. Notably, carbon sequestration is also less stable than nitrogen removal is, because mangroves store carbon in sediments that can be disturbed. On the flip side, mangrove forests convert nitrogen in the water mostly into nitrogen gas, which stays in the atmosphere, Thibodeau said. Nevertheless, mangrove forests "have a very high rate of storage of carbon compared to other ecosystems," he added.

“Mangroves are mostly threatened by sea-level rise and land clearance for infrastructure, Thibodeau said. The results highlight that "we're not only losing space or nature, but we're also losing a very important financial value."

     The researchers note that conventional approaches to wastewater treatment make nitrogen pollution mitigation prohibitively expensive. They also say that water infrastructure funding in the U.S. is inadequate by about $81 billion.

     The authors explain denitrification via anaerobic reduction as the main reaction below, as well as the secondary reaction that converts ammonium and nitrite to nitrogen gas.

“Denitrification, the anaerobic reduction of NO3− to nitrous oxide (N2O) and N2 is generally the primary process responsible for N removal in mangroves (e.g., Reis et al., 2017). This process is thought to be fueled by the amount of available NO3− and organic carbon (OC) (Kraft et al., 2014; Rivera-Monroy & Twilley, 1996). Anammox can also contribute to N removal by converting NH4+ and nitrite (NO2−) directly into N2, partly decoupled from the availability of OC (Fernandes et al., 2012; Meyer et al., 2005). However, despite the increasing recognition of mangroves for their capacity to buffer coastal waters from N pollution (Adame et al., 2019; Lee et al., 2009; Wu et al., 2008), a robust global assessment is lacking.”

     They note that accurate estimation of the global nitrogen removal potential of different mangrove forests is essential to the valuation of nitrogen removal credits in an offset market.

     The graph below shows that carbon sequestration, nitrogen removal, and potential future nitrogen removal are all better where mangrove forests occur in estuarine and deltaic environments compared to open-coast and lagoon environments.

 

References:

 

Mangroves clean up $8.7 billion of nitrogen pollution every year, study finds. Sascha Pare. Live Science. May 8, 2026. Mangroves clean up $8.7 billion of nitrogen pollution every year, study finds | Live Science

Blue Nitrogen: Global Rates and Economic Importance. Ziyan Wang and Benoit Thibodeau. Earth’s Future. Volume 14, Issue 5. May 2026. Blue Nitrogen: Global Rates and Economic Importance - Wang - 2026 - Earth's Future - Wiley Online Library