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The rammed earth portions of the Great Wall of China, built by compressing natural materials with earth, have been considered a weak point in its structure. But these swathes of the iconic landmark have developed a natural line of defense against the imminent risk of deterioration, according to a new study.
These soil surfaces on the Great Wall are covered by a “living skin” of small rootless plants and microorganisms known as biocrusts that are a source of the heritage site’s staying power, according to soil ecologist Matthew Bowker, co-author of the published study. in December. 8 in the journal Science Advances.
“(Biocrusts) are common around the world in soils from dry regions, but we don’t typically look for them in human-built structures,” Bowker, an associate professor at Northern Arizona University, said in an email.
Previous studies have found that lichen and moss biocrusts are a destructive threat to modern heritage stone structures due to the long-term impacts of microbial communities on aesthetic value, production of acid and other metabolites, and alteration of microenvironments, which can cause erosion and deterioration of the rock. weathering. Those findings have led to the removal of plants growing on top of parts of the Great Wall. But the effects of biocrusts appear different for terrestrial landmarks, and cyanobacteria and moss communities actually increase the stability of the Great Wall and improve its resistance to erosion, according to the new paper.
Examining samples taken from more than 483 kilometers (300 miles) in 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 in biocrust. When the researchers compared the stability and strength of samples placed in biocrust layers with samples without “living Earth skin,” they found that samples with biocrust were up to three times stronger than those without.
“They thought this type of vegetation was destroying the Great Wall. Our results show the opposite,” said study co-author Bo Xiao, a professor of soil sciences at China Agricultural University. “Biocrusts are widespread on the Great Wall and their existence is very beneficial for its protection.”
‘Like a blanket’
Biocrusts, made up of components such as cyanobacteria, algae, moss, fungi and lichens, inhabit the upper soil layer of drylands. Covering approximately 12% of the planet’s surface, communities of small plants and microorganisms can take decades or longer to develop. By forming miniature ecosystems, biocrusts stabilize soil, increase water retention, and regulate nitrogen and carbon fixation.
They are able to do this in part thanks to a dense biomass, which acts as an “anti-infiltration layer” for soil pores under the right conditions, as well as a natural absorption of nutrients that promote salt damage. According to the new study, the secretions and structural layers of the biocrusts also interlock to form a “sticky network” of aggregated soil particles that promote resistance and stability against the corrosive forces that threaten the Great Wall.
Climatic conditions, structure type and biocrust type all play a role in the biocrust’s protective function, with its reduction in erodibility being “much greater” than its erosion risk, the researchers found.
Compared to bare rammed earth, sections of the Great Wall covered in biocrust of cyanobacteria, moss and lichen exhibited reduced porosity, water-holding capacity, erodibility and salinity by up to 48%, while increasing compressive strength, penetration resistance, shear strength and aggregate resistance. stability up to 321%. Of the group, moss biocrusts were found to be the most stable.
“(The biocrusts) cover the Great Wall like a blanket separating it from air, water and wind,” Xiao said.
Working to keep water out and prevent salt buildup, biocrusts resist chemical erosion, he noted, producing substances that act as a “glue” for soil particles to bind together against dispersal, strengthening soil properties. .
The role of biocrusts in an uncertain future
Most communities that make up a biocrust start from a single organism that grows and makes the environments in which it grows suitable for others. Although still vulnerable to the impacts of climate change, these ever-evolving organisms are expected to deploy internal mechanisms to adapt to future extremes, said Emmanuel Salifu, an assistant professor at Arizona State University who studies nature-based solutions for engineering. sustainable.
That inherent adaptability makes biocrusts great 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 set up to function in those conditions,” he said. “Our hypothesis is that they will be able to survive better if we engineer their growth to scale.”
Wind erosion, rainfall, salinization and freeze-thaw cycles have caused cracks and disintegration in the thousands of kilometers of structures that link the Great Wall, which is at risk of serious deterioration and is vulnerable to collapse. Rising temperatures and increased precipitation could also lead to a reduction in the wall’s biocrust cover.
Still, the broader construction industry remains divided over the historic conservation potential of biocrusts, according to Salifu.
“The conventional wisdom is that biological growth is not good for structures. It affects aesthetics, it leads to degradation, it affects overall structural integrity,” he said. However, there is a lack of concrete research to support these conclusions, Salifu added, noting that “the jury is still out on this.”
Salifu sees the new study as evidence of the potential advantages of engineering biocrusts for the conservation of land heritage sites, although it is still an emerging field. The research establishes that 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 paper “goes a long way toward pushing the clock even further to get the industry closer to the point where we might start thinking (about biocrust engineering),” he said.
The study’s authors also say their work justifies exploring the possibility of growing biocrusts for the preservation of other rammed earth heritage sites around the world.
Beyond its status as a tourist destination that attracts millions of visitors each year, the Great Wall has great cultural relevance, which is why the biocrusts that preserve it are so important, Xiao said.
“The Great Wall is the cultural center of Chinese civilization,” he told CNN. “We must do everything we can to protect it for our next generations. For our children, for our grandchildren.”
Ayurella Horn-Müller has covered climate change for Axios. His first book, “Devoured: The Extraordinary Story of Kudzu, the Vine That Ate the South,” will be out in the spring.
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