"Spilling The Tea" On Wetlands: Scientists Find Link Between Temperature And Carbon Storage Rates

A huge global study has used teabags to measure carbon storage in wetlands.

Scientists, including those from NIWA, buried 19,000 bags of tea in 180 wetlands across 28 countries.

This included three official sites in New Zealand: Pahurehure Inlet (Manukau Harbour), Bayswater (Auckland) and Whangateau Harbour.

RMIT University’s Dr Stacey Trevathan-Tackett lead the study as part of an Australian Research Council DECRA Fellowship while at Deakin University.

Over three years, Dr Trevathan-Tackett and other researchers measured how much tea had degraded, which gave an indication of the wetland’s ability to hold carbon in the soil.

The project, which is the first long-term study of its kind, found that warmer temperatures reduce the ability of wetlands to store carbon, meaning more is released into the atmosphere.

Dr Carolyn Lundquist is a NIWA Principal Scientist for Marine Ecology and Associate Professor at the University of Auckland. She co-authored the study along with fellow marine ecologist Richard Bulmer.

"Teabags may not be the first thing that comes to mind when you think of a scientific instrument, but they’re a simple, cheap and standardized way to identify how different factors influence carbon breakdown rates in wetlands," said Dr Lundquist.

Tea contains organic matter, which is made up of carbon. Like all organic matter, it decomposes and releases that carbon into the environment. By burying teabags in wetland soil and measuring the tea’s remaining organic mass over certain time periods, scientists can assess how well each wetland is holding onto its carbon stores.

"Wetlands are a key carbon sink, meaning they hold onto carbon and stop it entering the atmosphere, which helps lower emissions and mitigates climate change. However, some inevitably escapes into the air. It’s therefore vital to know which wetlands are leaking the most carbon," said Dr Lundquist.

Two types of teabags - green and rooibos - were used to build a more complete picture of the wetlands’ carbon storage capacity, as they can represent different kinds of organic matter found in soil.

At each site, between 40 and 80 tea bags were buried roughly 15 cm underground and collected at various time intervals to measure their degradation. By using local weather station data and comparing different climates, scientists could then see if there were any links between temperature and carbon storage rates.

Dr Stacey Trevathan-Tackett said they compared different climates and local weather station data to see if there were any links between temperature and carbon storage rates.

"Generally, warmer temperatures led to increased decay of organic matter, which translates to reduced carbon preservation in soil. The two tea types also acted differently with increasing temperature," said Dr Trevathan-Tackett.

The organic matter in green tea decomposes easily and in rooibos it is slower. The study found that rooibos tea always had more decay under higher temperatures, regardless of other factors such as soil type. In contrast, green tea bags decayed at different rates under higher temperatures depending on the type of wetland they were buried in, being faster in freshwater wetlands but slower in mangrove and seagrass wetlands.

"Overall, freshwater wetlands and tidal marshes had the highest tea mass remaining, indicating a greater potential for carbon storage in these ecosystems," said Dr Trevathan-Tackett.

Dr Lundquist says this work will help us to tackle global carbon emissions.

"The study’s findings are helping 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," said Dr Lundquist.

Notes:

‘Climate effects on below ground tea litter decomposition depend on ecosystem and organic matter types in global wetlands’ is published in Environmental Science and Technology (DOI: 10.1021/acs.est.4c02116).

The global study involved 110 co-authors on the paper, along with many others who helped such as undergraduate students and citizen scientists. Core team members included Dr Martino Malerba and Professor Peter Macreadie from Deakin University and RMIT, Dr Sebastian Kepfer-Rojas from the University of Copenhagen in Denmark and Dr Ika Djukic from The Swiss Federal Institute for Forest, Snow and Landscape Research WSL.

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