These wetlands are vitally important for carbon storage, holding roughly 30% of global soil carbon, despite covering only around 3% of Earth’s surface. It is their saturated surface conditions that make peatlands such effective carbon stores. When peat mosses die, they sink into this wet environment, and the low-oxygen, often acidic conditions prevent microbes that decompose plant litter elsewhere from working effectively. Instead of breaking down, the dead mosses slowly build up, trapping the carbon dioxide that they sucked out of the atmosphere during photosynthesis underground.
But a new study suggests that in Europe peatland water tables are falling. In the journal Nature Geoscience, researchers from almost 30 institutions report that 60% of the 31 peatlands they studied were drier between 1800 and 2000 than at any point in the past 600 years. And 40% were drier than they have been for 1,000 years, whereas 24% were drier than they have been for 2,000 years.
From Sink to Source?
This finding is concerning because if peatlands dry out, microbial activity could increase and shift them from carbon sinks to carbon sources, with global consequences for climate change.
“If we allow peatlands to dry out as they appear to be doing, then the carbon stocks that they have built up over the last 10,000 to 12,000 years might be at risk,” explained Paul Morris, an ecohydrologist at the University of Leeds in the United Kingdom.
To reconstruct the historic water levels of Europe’s peatlands, Morris and his colleagues turned to testate amoebas. These single-celled, soil-dwelling creatures have very specific tolerances for how wet their environment can be. Comparing fossil amoebas from cores taken from peatlands with the environmental conditions that the various species inhabit today allowed the researchers to determine the water tables of the past.
Morris said that across Europe there are three distinct regions where, historically, the peatlands have behaved differently: Britain and Ireland, Scandinavia and the Baltic states, and continental Europe. But recently, that has changed. “In the last 200 to 300 years the consistent change that we see between all of these three regions is that everything is getting dry,” Morris said. “Almost all of our sites are getting drier. It’s the only time when the record starts to agree across the whole continent.”
The findings suggest that the wetness of many European peatlands is moving away from natural baselines, according to the study authors, with the change linked to climate change and human activities, such as cutting, drainage, burning, and grazing. However, Morris said, “We think this is driven principally by warming.”
Zicheng Yu, a paleoecologist at Lehigh University in Bethlehem, Pa., believes the study is impressive and important as “it shows consistent continental-wide change in hydrological conditions on peatlands.”
But Yu questions the idea that as water tables drop, these peatlands will release more carbon because even though the idea is sound in theory, there are currently no long-term data from these sites showing this happens. Instead, drying could simply lead to a change in the plant community to species that prefer a lower water table but “may still sequestrate carbon at the same rate,” he explained.
Indeed, another recent study found that peat moss species have different tolerances for warming and drying and that species composition could change as conditions shift, helping some peatlands hold on to their carbon.
A useful next step would be assessing how carbon sequestration changes in these peatlands as they dry, Yu said.
—Michael Allen, Science Writer