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For global wetlands, intensifying droughts pose a ‘diabolical’ threat

  • Covering almost one-tenth of the land area of Earth, places with wet soils provide an estimated $27 trillion in benefits to humanity per year, but, according to a newly published review paper, researchers still have much to learn about the effects of drought on wet soils.
  • The review, which draws upon more than 200 published studies, says “drought poses a significant threat to wet soils, a threat which can be difficult to determine before an event but which poses a catastrophic risk to some sites.”
  • Among the “most pressing” findings of the review are that published information on the effects of drought on wet soils is absent for most parts of the world outside Europe, North America and Australia, and that there’s very little research that can be applied to water management.
  • The review also discusses the ramifications of droughts to climate change: As wet soils dry, increased oxygen content in the soil speeds up decomposition, releasing greenhouse gases into the atmosphere.

People seldom think of wet soil until they step in it, but the Earth’s soggy places quietly do the work of legions: holding together coasts, giving shelter to young fish, preventing floods, filtering water, storing carbon and providing a legacy of fertile soil — quite a job for some mud.

Covering roughly one-tenth of the land area of Earth, places with wet soils — wetlands, fens, springs, swamps, peatlands, floodplains, moorlands — provide an estimated $27 trillion in benefits to humanity every year.

However, as climate change accelerates, the number, intensity, and speed of droughts in these places are expected to rise. And, according to a review paper published in Earth-Science Reviews, researchers still have much to learn about the effects of drought on wet soil.

“Fens, swamps, bogs, floodplains, springs and moorlands are some of the most productive places on the planet, and we’re losing them at an astonishing rate,” study co-author Erinne Stirling, from Zhejiang University, China, and the University of Adelaide, Australia, wrote in a blog post about the review.

The review, which draws upon more than 200 published studies, says, “drought poses a significant threat to wet soils, a threat which can be difficult to determine before an event but which poses a catastrophic risk to some sites.”

A great blue heron (Ardea herodias) eating a common snapping turtle (Chelydra serpentina) hatchling. These animals depend on wetlands for their survival. Photo by John Harrison via Wikimedia Commons. (CC BY-SA 3.0).

The researchers describe how droughts lead to the acidification, cracking and compaction of wetland soils. Sometimes these changes are irreversible, altering the soil forever, with consequences for water quality, ecosystems, and the living things that rely on them.

One example of such change is in the Murray-Darling Basin, a major agricultural center in Australia that has experienced a 12-year drought, devastating agricultural yields.

“We have seen many examples of how drought in the Murray-Darling Basin has caused major issues including acidification of soil and water due to acid sulfate soils exposure in wetlands, co-author Luke Mosley, a soil scientist and deputy director of the Acid Sulfate Soils Centre at the University of Adelaide, said in a statement.

“The Murray-Darling Basin is a food bowl,” Julian Cribb, a journalist and professor of science communications at the University of Technology Sydney, said in an interview with Circle of Blue. “There are many food bowls like it. The Indus-Ganges food bowl in India. The Yangtze-Yellow River food bowl in China. The Volta-Chad food bowl in Africa. In almost every one of these cases, the rivers are in trouble.

“The rivers are being emptied, the ground water is being drained, the soils are drying out, the lakes are filling in with sediment,” Cribb said, “…there is a diabolical problem emerging for the world.”

A white salt inflorescence along the edge of a salinized wetland in a drought-stressed Australian wetland. Photo by Luke Mosley.

Among the “most pressing” findings of their review, Stirling writes, is the lack of published information on the effects of drought on wet soils for most parts of the world outside Europe, North America and Australia, and that there’s very little research that can be applied to water management.

“Fresh water is a precious commodity that people have fought over for millennia,” Stirling said in an email to Mongabay. “There will always be more things that can use fresh water than there is water. Agriculture, cities and industry are forever fighting against the environment for ‘their’ share … How can we allocate enough water to the farmers while also stopping some irreparable environmental disaster occurring downstream?”

Because of this fight, water management is a “fact of life,” but making decisions about water is an extra challenge, Stirling says, because of the lack of research. Water managers have little more to work with than “common sense and history,” leaving them with tough choices to make.

“Decisions are made about water and wetlands all the time and the people making those decisions don’t know enough about the situation to make good ones,” Stirling said. “And worse, scientists don’t know either because so few researchers have had the time and money to take all those measurements around the world.”

Dissolved iron in a wetland pool in Australia acidified by drought. The small plant growing in the acid pool is likely Typha australis – an extremely hardy emergent water plant that forms monocultures and crowds out all other plants. Photo by Luke Mosley.

The review also sheds light on how the drying of wet soils may contribute to climate change. The specifics vary in each situation but consider an example of a drying wetland. As the water evaporates, more more air makes its way close to and then into the mud. This converts the bacteria in the soils from anaerobic (operating without oxygen) to aerobic, increasing the rate of decomposition of organic matter such as dead leaves.

“In simple terms, there is a strong positive feedback loop,” Jay Bell, a professor in the Department of Soil, Water and Climate at the University of Minnesota, who was not involved in the study, told Mongabay in an email. “As the organic matter is decomposed, it releases carbon dioxide, a greenhouse gas… So drought drives the production of greenhouse gases and the greenhouse gases will further drive climate change — in some regions.”

Climate change has widely different impacts in different regions of the world, and this example illustrates what happens only within a wetland. But as we know, these wet places do not exist in isolation but are part of much more complex systems, connected to entire watersheds. It’s hard to say what the effects of droughts are at the ecosystem scale.

“Wetlands are such a key part of so many ecosystems that their loss or drastic changes could have a broad impact beyond the edge of the wetland,” Bell said.

Marshlands and riverine habitats teem with life in Kaziranga, India. Photo by Shashank Gupta via Flickr (CC BY-SA 2.0).

Environments are resilient, Stirling says, but many have “tipping points” at which the changes brought on by drought become so dramatic the ecosystem cannot recover. And we don’t know what those tipping points are until we reach them.

“It’s obvious that completely draining a peatland will be a local environmental disaster. But what about 5% less water? 10%? [A] 5°C higher nighttime temperature?” Stirling said. “What is the threshold and what do we need to do to protect these places?

“We need good observations of these systems in real life,” she added, “and we need good experiments to find what just-before-disaster looks like. Because just-after is too late.”

Citation:

Stirling, E., Fitzpatrick, R. W., & Mosley, L. M. (2020). Drought effects on wet soils in inland wetlands and peatlands. Earth-Science Reviews, 210, 103387. doi:10.1016/j.earscirev.2020.103387

Banner image of a drought-stressed Australian wetland by Luke Mosley.

Liz Kimbrough is a staff writer for Mongabay. Find her on Twitter @lizkimbrough

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