Peat bogs, swamps and mangrove forests are unlikely to feature on many people’s summer holiday itineraries. We tend to undervalue wetlands and take their capacity to clear pollution and store carbon for granted.
But globally, peat wetlands hold twice as much carbon as all of the world’s forests combined, even though they cover less than 3% of the Earth’s surface. Most of this carbon is stacked away deep underground and has long been considered as permanent storage. But as temperatures continue to rise, some wetlands start leaking carbon into the atmosphere.
More than 100 researchers from across the globe have joined forces to monitor changes in wetland carbon fluxes in a warming world, with the help of an unusual scientific instrument – teabags.
This may not be the first thing that comes to mind as a research method, says Carolyn Lundquist, principal scientist, marine ecology at Niwa, who was part of the study. “But teabags are a simple, cheap and standardised way to identify how different factors influence carbon breakdown rates in wetlands.”
Collectively, the global team buried 19,000 teabags in 180 wetlands across 28 countries. In New Zealand, this included mangrove forests in the Pahurehure Inlet in Manukau Harbour, at Bayswater in Auckland and at Whangateau Harbour (Omaha). At each site, 40-80 teabags were buried underground and collected at various intervals during the past three years to measure decomposition.
Tea leaves are organic, made up of carbon. When they decompose, they release that carbon into the environment. Burying teabags in wetland soils and measuring their remaining organic mass is a good proxy for how well each wetland is holding onto its carbon stores.
To add detail to their research, the team used two types of tea: green and rooibos. These tea leaves represent different kinds of organic matter found in soil. Green tea decomposed more easily and rooibos took longer to break down.
Once the teabag data set was complete, the team incorporated information from local weather stations for each site to account for differences in climate regions. Overall, the researchers found that, unsurprisingly, warmer temperatures led to increased decay of organic matter, which in turn meant reduced carbon storage in soil.
But the two teas behaved differently. For rooibos, the type of wetland in which the teabags were buried didn’t matter. Higher temperatures always led to more decay, which suggests that the kind of organic material we’d expect to last longer in the soil is in fact more vulnerable to warming.
In contrast, green teabags decayed at different rates, depending on the type of wetland. “If carbon decays more slowly, this means there is higher carbon storage potential in that location,” says Lundquist. “The winners here are freshwater wetlands and coastal saltmarshes, which show higher carbon storage than other habitats.”
The global study includes sites across a range of temperate to tropical locations, which allowed the team to assess how climate change might affect future carbon storage potential. Freshwater wetlands show a decline in carbon storage with increasing temperature, but for coastal wetlands (mangroves, saltmarshes and seagrass meadows), there is no clear decline. “This is good news for coastal restoration projects, to know they will retain their value for carbon storage into the future.”
New Zealand has lost about 90% of wetlands since European settlement. They were drained for cities and pasture. Many continue to be degraded, often because people are unaware of their importance, not just for carbon storage but as breeding grounds for native fish and birds.
Lundquist says the research helps piece together the puzzle of wetland carbon sequestration on a global scale. “Now we know which environments are storing more carbon, we can start putting things in place to protect them from degradation.”
