James Cook University (JCU) researchers say it's time to look deeper – literally – as groundwater may determine whether coastal wetlands lock away carbon or leak it back into the atmosphere.
As global efforts intensify around restoring coastal wetlands to curb climate change, a new JCU-led study (published in Reviews of Geophysics) is the first to link wetland restoration and carbon cycling with groundwater dynamics, showing that subsurface flows can tip the balance from storing carbon to emitting it.
"Coastal wetlands are vital for global carbon storage, but their drainage for agriculture and development has led to the release of much of that stored carbon," said lead researcher, JCU's Dr Mahmood Sadat-Noori.
"Over the last 150 years, the world has lost coastal wetlands equivalent to an area the size of India.
"Globally, the loss of this carbon-rich habitat is estimated to release the equivalent of emissions from around 118 coal-fired power plants each year."
Increasing global interest in wetland restoration is driven by their ability to store carbon, support biodiversity, and provide vital ecosystem services.
This focus aligns with the United Nations Decade on Ecosystem Restoration (2021–2030), which promotes restoring degraded ecosystems to combat both climate change and biodiversity loss.
However, Dr Sadat-Noori and colleagues highlight a key missing piece in wetland restoration: groundwater.
While most restoration projects focus on surface water and vegetation, the researchers show that ignoring groundwater may leave a major gap in understanding how wetlands store or release carbon.
"When we restore wetlands, we typically measure how much carbon is stored in sediments and how much carbon goes into the atmosphere – basically we study the vertical movement of carbon in a wetland," he said.
"But we're finding that carbon also moves horizontally through the groundwater and that can offset part of the carbon stored in wetland sediment.
"Ignoring groundwater means our carbon accounting could be incomplete."
Groundwater interacts with surface water and sediments, influencing salinity, acidity, and microbial activity, all of which determine how much carbon remains stored, is transported, or is released back into the atmosphere.
"When groundwater levels drop, more sediment is exposed to oxygen, which promotes the breakdown of organic matter and releases carbon dioxide," explained Dr Sadat-Noori.
"But high groundwater levels keep sediments waterlogged and limit oxygen exposure, helping to preserve stored carbon.
"In coastal settings, saline groundwater may also suppress methane production, which further reduces greenhouse gas emissions.
"Overall, restoring tides and groundwater connectivity can help coastal wetlands shift from being carbon sources to long-term carbon sinks."
The researchers call for a more holistic approach to wetland restoration—one that measures and manages both surface water and groundwater processes.
Dr Sadat-Noori said that integrating groundwater dynamics into restoration is essential to ensure that restored wetlands deliver the full carbon and biodiversity benefits they promise.
"If we have a better quantitative understanding of groundwater influence on wetland carbon, then we can design projects that optimise the amount of carbon stored after restoration," he said.
