Tiny organisms manage water in the world's driest places

Scientists at the University of New South Wales (UNSW) are describing how tiny organisms on the surface of soils called biocrusts help conserve water in the world’s driest regions. Biocrusts are diverse communities of mosses, cyanobacteria, lichens, algae, and microfungi that strongly interact with desert soils. 

Collectively, arid regions known as drylands make up the largest biome on the planet. Biological soil crusts, often referred to as the living skin of the Earth, help soil retain water and reduce erosion. Biocrusts also add nitrogen and carbon to the nutrient-poor desert soil, which helps support other plants.

“Biocrusts are critically important because they fix large amounts of nitrogen and carbon, stabilize surface soils, and provide a home for soil organisms,” said study lead author Professor David Eldridge. “But we still have a poor understanding of just how biocrusts influence hydrological cycles in global drylands.

“Accounting for biocrusts and their hydrological impacts can give us a more accurate picture of the impacts of climate change on dryland ecosystems and improve our capacity to manage those effects.”

For the investigation, the UNSW team collected data from more than 100 studies published over the past 30 years. The researchers used this information to assemble the largest global database containing scientific evidence on the role of biocrusts in water movement, storage, and erosion.

“Our emphasis was on dryland soils because biocrusts are often the dominant surface covering on these soils, particularly during dry times,” said Professor Eldridge.

A large increase in the number of publications on biocrusts over the past decade enabled the team to critically assess the links between water capture and storage and landscape stability in drylands.

“The global literature on biocrust effects on hydrology has often been conflicting, preventing us from making broadscale recommendations on how to manage them to manage water,” said study co-author Dr. Samantha Travers.

The researchers found that the presence of biocrusts on the soil surface reduced water erosion by an average of 68 percent worldwide.

“Cyanobacteria in the crusts secrete organic gels and polysaccharides that help to bind small soil particles into stable surfaces. Mosses in the crusts also trapped water and sediment on the soil surface, preventing the removal of soil particles,” explained Dr. Travers.

The study showed that biocrusts reduced the infiltration of water into the soil, but tended to increase water storage in the uppermost layers.

“This upper layer is where most of the nutrients and microbes are found – it is a critical zone for plant production and stability in dryland soils,” said Professor. Eldridge. “More water in the upper layers means greater productivity and stability.”

According to Professor Eldridge, the findings provide new insight into how biocrusts affect water relations in drylands. “However, the effects depend on factors such as the type of crust and whether it is intact or disturbed.”

Professor Eldridge has been studying the influence of biocrusts on Australia’s soils for more than 30 years. The research is focused on drylands because they occupy almost half of Earth’s land surface and support nearly 40 percent of the global population.

“Many people in drylands rely on pastoralism for their livelihoods, so the capture and use of water is critically important in these water-limited environments,” said Professor Eldridge. “Anything that alters the hydrological balance in drylands has the potential therefore to affect millions of people, hence the importance of these tiny surface communities.”

Professor Eldridge pointed out overgrazing by livestock is a major problem for sustainable management of drylands.

“Trampling by sheep and cattle breaks up the crust, destabilizing the soil surface and leading to increased water erosion – effects that are supported by our global analyses.”

“Preventing overgrazing by livestock is critical if we are to prevent the loss of biocrusts, but until recently, the magnitude of the effects have not been known.”

“The results of this work will be incorporated into global water balance and soil loss models so that managers and governments have a better understanding of the implications of losing biocrusts on the world’s dwindling water supplies,” said Professor Eldridge.

The study is published in the journal Global Change Biology.

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By Chrissy Sexton, Earth.com Staff Writer