Drylands, collectively, cover nearly half Earth's land surface. They are the world's largest biome, supporting almost 40 percent of the global human population.

And yet, the effects of biocrusts on the biome's water cycle are poorly understood, researchers say.

New research suggests these living crusts -- an amalgamation of mosses, lichens, cyanobacteria and other kinds of microscopic organisms, including bacteria and fungi -- have a significant influence on the ability of drylands to hold water.

"Biocrusts are critically important because they fix large amounts of nitrogen and carbon, stabilize surface soils, and provide a home for soil organisms," lead researcher David Eldridge, professor and an expert in dryland ecology at the University of New South Wales, said in a news release.

"But we still have a poor understanding of just how biocrusts influence hydrological cycles in global drylands," he said. "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."

In a new paper, published this week in the journal Global Change Biology, Eldridge and his colleagues reviewed the findings of dozens of studies on dryland biocrusts published over the last three decades.

"The global literature on biocrust effects on hydrology has often been conflicting, preventing us from making broad-scale recommendations on how to manage them to manage water," said study co-author Samantha Travers, a postdoctoral researcher in the Aric Ecology Lab at UNSW.

Despite these contradictions, researchers found that the presence of biocrusts was responsible for a large reduction in water erosion, 68 percent on average.

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

The new literature review showed biocrusts prevent water from penetrating deep into the ground. However, the presence of biocrusts enhanced water storage in the soil's upper layers -- the place where most nutrients and microbes are found. The data showed biocrusts are a boon for biological production and ecological stability on drylands.

Despite the consistent hydrological and ecological benefits provided by biocrusts, their effects were not uniform. Researchers found the type of crust and whether it was disturbed or not had a strong impact on the influence of biocrusts.

Many studies have previously shown that overgrazing can increase water erosion and diminish water retention. The latest research suggests overgrazing can also prevent biocrusts from working as a protective and stabilizing force.

"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," Eldridge said. "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."

Researchers said they hope their work will be incorporated into into global water balance and soil loss models, as well as be used by governments and policy makers working to safeguard the planet's dwindling water supplies.

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