Blue Nitrogen: The Nutrient Story Coastal Wetlands Have Been Hiding

Every time this podcast talks about coastal wetlands, the conversation goes straight to carbon. Marshes, mangroves, and seagrasses pull carbon dioxide out of the atmosphere and lock it in soil for centuries, and that story has earned coastal ecosystems a real seat at the climate table. Dr. Adam Langley, a global change ecologist and professor of biology at Villanova University and a research associate at the Smithsonian Environmental Research Center, thinks there's a second story hiding in that same soil that could matter just as much, or more: nitrogen. Langley and 37 co-authors just published the first global estimate of how much nitrogen the world's marshes and mangroves actually bury, and the number is staggering.

Pulling from over 8,000 soil measurements across 255 tidal wetland sites worldwide, Langley's team calculated that coastal wetlands sequester roughly 3.2 teragrams of nitrogen every year, a figure Langley makes tangible by comparing it, loosely, to the mass of 700,000 African elephants or to the fertilizer applied to every cornfield in the United States. That's equal to somewhere between 13 and 15 percent of all the nitrogen buried in the world's oceans, a share nobody had managed to calculate on a global scale before this study. Nitrogen doesn't get the same climate spotlight as carbon, but its damage is intensely local: too much of it running off farmland and into estuaries fuels the algal blooms, oxygen dead zones, and red tides that can sicken and kill fish, manatees, and people. Coastal wetlands act, in Langley's words, like nature's kidneys, filtering that nitrogen out of the water and locking it away in oxygen-starved soil where it stays inert, as long as the wetland itself survives.

And that's the catch. These wetlands have kept pace with sea level rise for 5,000 to 10,000 years by building soil vertically, but Langley's research suggests they can only keep up with about 8 millimeters of sea level rise a year, a threshold the global rate, now roughly double what it was 75 years ago, is edging closer to. Andrew and Langley dig into what happens on the other side of that threshold: wetland loss, boat-wake erosion along Florida's Intracoastal Waterway, and mangroves creeping poleward into places like Georgia for the first time on record, possibly helping some marshes hold their ground in the process. They also talk dollars: blue carbon's global value has been estimated at $191 billion, and Langley's team calculates blue nitrogen could be worth even more, a case he hopes gets policymakers to start managing nutrients with the same urgency as carbon.

Takeaways:

• Coastal wetlands (marshes and mangroves) bury roughly 3.2 teragrams of nitrogen every year, drawn from a global database of over 8,000 soil measurements across 255 sites, the first estimate of its kind.
• That nitrogen burial equals 13 to 15 percent of the world's marine nitrogen burial.
• Human activity has more than doubled natural nitrogen inputs to ecosystems, arguably a bigger disruption to the nitrogen cycle than to the carbon cycle, even though nitrogen's damage shows up locally rather than globally.
• Blue carbon's global value has been estimated at $191 billion a year; blue nitrogen's could be even higher, an estimated $70 to $339 billion a year.
• Wetlands can generally keep pace with roughly 8 millimeters of sea level rise per year by building soil, but the global rate has already roughly doubled since the mid-20th century.
• Mangroves are expanding poleward, including a newly documented population in Georgia, which may help some marshes build soil and resist erosion better, though Adam expects overall coastal wetland area to keep shrinking as seas rise faster.