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Rammed earth sections of the Great Wall of China – built by compressing natural materials with earth – are considered a weak point in its structure. But these parts of the iconic landmark developed a natural line of defense against the looming risk of deterioration, a new study shows.
These soil surfaces on the Great Wall are covered with a “living skin” of small, rootless plants and microorganisms known as biocrusts, which are a source of heritage endurance, according to soil ecologist Matthew Bowker, co-author of the in research published in December. 8 in the journal Science Advances.
“(Biocrusts) are common on dryland soils around the world, but we don’t typically look for them on human-built structures,” Bowker, an associate professor at Northern Arizona University, said in an email.
Previous studies have shown that lichen and moss biocrusts pose a destructive threat to modern heritage stone structures due to the long-term effects of the microbial communities on aesthetic value, production of acid and other metabolites, and alteration of microenvironments, which can cause erosion and rock cause. weathering. These findings have led to the removal of plants growing on parts of the Great Wall. But the effects of biocrusts look different for earthen landmarks, and communities of cyanobacteria and moss actually increase the Great Wall’s stability and improve its resistance to erosion, the new paper says.
Examining samples taken from more than 300 miles (483 kilometers) across eight rammed earth sections of the site built during the Ming Dynasty between 1368 and 1644, the study authors found that more than two-thirds of the area is covered by biocrusts. When the researchers compared the stability and strength of samples in biocrusts with samples without “living skin of the Earth,” they found that samples with biocrusts were as much as three times stronger than samples without.
“They thought this type of vegetation was destroying the Great Wall. Our results show the opposite,” said co-author Bo Xiao, professor of soil science at China Agricultural University. “Biocrusts are very widespread on the Great Wall and their existence is very beneficial for its protection.”
‘Like a blanket’
Biocrusts consist of components such as cyanobacteria, algae, moss, fungi and lichens and live on the topsoil of arid areas. The communities of small plants and microorganisms cover an estimated 12% of the Earth’s surface and can take decades or even longer to develop. Biocrusts form miniature ecosystems and stabilize soils, increase water retention and regulate nitrogen and carbon fixation.
They can do this partly thanks to a dense biomass, which acts as an ‘anti-infiltration layer’ for the soil pores under the right conditions, and through natural absorption of nutrients that promote salt damage. The secretions and structural layers of biocrusts are also interwoven, forming a “sticky network” of converging soil particles that promote strength and stability against corrosive forces that threaten the Great Wall, the new study says.
Climatic conditions, type of structure and type of biocrust all play a role in a biocrust’s protective function, with the reduction in erodibility “much greater” than the risk of weathering, the researchers found.
Compared to bare rammed earth, the cyanobacteria-, moss-, and lichen-covered parts of the Great Wall showed reduced porosity, water-holding capacity, erodibility, and salinity by as much as 48%, while compressive strength, penetration resistance, shear strength, and aggregate increased. stability up to 321%. Of the entire set, the moss biocrusts turned out to be the most stable.
“(Biocrusts) cover the Great Wall like a blanket that separates the Great Wall from air, water and wind,” Xiao said.
The biocrusts work to keep water out and prevent salt buildup and resist chemical weathering, he noted, producing substances that act as a “glue” for soil particles to bond together against spreading, improving the soil’s properties getting stronger.
The role of biocrusts in an uncertain future
Most of the communities that make up a biocrust start with a single organism that grows and makes the environment in which it grows suitable for others. While they are still vulnerable to the effects of climate change, these constantly evolving organisms are expected to deploy internal mechanisms to adapt to future extremes, says Emmanuel Salifu, an assistant professor at Arizona State University who studies nature-based studies solutions for sustainable technology.
That inherent adaptability makes biocrusts strong contenders for nature-based interventions to address structural conservation in our warming world, said Salifu, who was not involved in the new study.
“Even if we have warmer temperatures, they are already suited to perform in those conditions,” he said. “We hypothesize that they will be better able to survive if we achieve their growth at scale.”
Wind erosion, abrasive rainfall, salinization and freeze-thaw cycles have led to cracks and disintegration in the thousands of kilometers of structures connecting the Great Wall, which is at risk of severe deterioration and vulnerable to collapse. Rising temperatures and increasing rainfall may also result in a reduction in the biocrust cover of the wall.
Yet, according to Salifu, the broader construction industry remains divided over the historic conservation potential of biocrusts.
“The conventional idea is that organic growth is not good for structures. It affects aesthetics, leads to degradation and affects overall structural integrity,” he said. However, there is a lack of concrete research supporting these conclusions, Salifu added, noting that “the jury is still out on that.”
Salifu sees the new study as evidence of the potential benefits of developing biocrusts for the conservation of earthen heritage sites – although that is still an emerging field. The research shows that the natural communities of plants and microorganisms “have the ability to improve the structural integrity, longevity and durability of earth structures such as the Great Wall of China,” Salifu said.
The article “goes a long way in pushing the needle on the clock and bringing the industry closer to what we might be thinking about (developing biocrusts),” he noted.
The study’s authors also say their work makes a case for exploring the possibility of cultivating biocrusts for the conservation of other heritage sites around the world.
In addition to its status as a tourist destination that attracts millions of visitors every year, the Great Wall has great cultural relevance. That’s why the biocrusts that maintain the wall are so important, Xiao said.
“The Great Wall is the cultural center of Chinese civilization,” he told CNN. “We must do our best to protect it for our next generations. For our children, for our grandchildren.”
Ayurella Hoorn-Muller covers climate change for Axios. Her debut book, “Devoured: The Extraordinary Story of Kudzu, the Vine That Ate the South,” will be published in the spring.
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