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The rammed earth sections of the Great Wall of China – built by compressing natural materials with soils – were seen as a weak point in its structure. But these interpretations of the iconic landmark have developed a natural line of defense against the looming risk of decay, a new study has found.
These soil surfaces on the Great Wall are covered with a “living skin” of tiny rootless plants and microorganisms called bioscrews which are the source of the heritage site’s staying power, according to soil ecologist Matthew Bowker, co-author of the study published in December. 8 in the journal Science Advances.
“(Biobarriers) are common worldwide on soils of arid regions, but we don’t usually find them on human-built structures,” Bowker, an associate professor at Northern Arizona University, said in an email.
Past studies have found lichens and mosses to be a destructive threat to modern heritage stone structures due to long-term impacts of microbial communities on aesthetic value, production of acids and other metabolites, and alteration of microenvironments, which may cause corrosion and rock. weather. As a result of those results plants growing on top of parts of the Great Wall were removed. But the effects of biocrushing on clay landmarks look different, with communities of cyanobacteria and moss increasing the Great Wall’s stability and improving its resistance to erosion, according to the new paper.
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 with bio-belts. When the researchers compared the stability and strength of samples layered in bioscrews with samples sans “Earth’s living skin,” they found that samples with bioscrews 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 coauthor Bo Xiao, professor of soil science at China Agricultural University. “Biocrustaceans are very widespread on the Great Wall and having them there is very beneficial in protecting it.”
‘like a blanket’
Composed of components such as cyanobacteria, algae, moss, fungi and lichens, biocrustaceans inhabit the topsoil of dry lands. Covering about 12% of the planet’s surface, communities of tiny plants and microorganisms can take decades, or longer, to develop. By forming mini-ecosystems, bioscrubs stabilize soil, increase water retention, and regulate nitrogen and carbon fixation.
They are able to do so in part thanks to dense biomass, which acts as an “anti-infiltration layer” for soil pores under the right conditions, as well as the natural absorption of nutrients that promote salt damage. According to the new study, the secretions and structural layers of the bioscour combine to aggregate an “adhesive network” of soil particles that promotes strength and stability against erosive forces that threaten the Great Wall.
Climatic conditions, the type of structure and the type of bioremediation play a role in the protective effect of biodegradation, and the reduction in erosion is “far greater” than the risk of weathering, according to the researchers.
Compared to bare rammed earth, the sections of the Great Wall that were covered with cyanobacteria, moss and lichen biodegradation showed reduced porosity, water retention capacity, corrosion and salinity up to 48%, and an increase in compressive strength, penetration resistance, shear strength and aggregate. stability up to 321%. Of the cluster, the moss bioscrews were found to be the most stable.
“(Biobarriers) cover the Great Wall like a blanket that separates the Great Wall from the air, from the water, from the wind,” Xiao said.
As they work to keep water out and prevent salt build-up, the bioscrews resist chemical weathering, he noted, producing substances that act as “glue” to bind soil particles together against dispersal, making soil properties stronger.
The role of biofencing in an uncertain future
Most of the communities that comprise a biocrust grow from a single organism that makes the environments in which it grows suitable for others. Although they remain vulnerable to the impacts of climate change, these ever-evolving organisms are expected to deploy internal mechanisms to adapt to future extreme conditions, said Emmanuel Salifu, an assistant professor at Arizona State University who study of nature-based solutions for sustainable engineering.
That inherent adaptability makes biocrustaceans strong contenders for nature-based interventions to address structural preservation 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 able to survive better if we engineer their growth at scale.”
As a result of wind erosion, rain scouring, salinization and freeze-thaw cycles, the thousands of miles of structures that connect the Great Wall have cracked and disintegrated, which are in serious danger of decay and collapse. Rising temperatures and increased rainfall can lead to a reduction in the wall’s bioscrew cover.
Still, the broader construction industry remains divided on the historic preservation potential of bioscourers, according to Salifu.
“The conventional thinking is that biological growth is not great for structures. It affects the aesthetics, it leads to degradation, it affects the overall structural integrity,” he said. However, concrete research supporting those conclusions is lacking, Salifu said, noting that “the jury is still out on that.”
Salifu sees the new study as evidence of the potential benefits of biofence engineering for the preservation of earthen heritage sites — although this is still an emerging field. The research establishes that natural communities of plants and microorganisms have the potential to “enhance the structural integrity, longevity and durability of earthen structures such as the Great Wall of China,” Salifu said.
The paper goes a long way to beating the clock to get the industry closer to where we could start thinking about (engineering biorestrictions),” he said.
The study’s authors also say that their work makes a case for exploring the possibility of exploiting biocrustaceans to preserve other ram clay heritage sites around the world.
Beyond its status as a tourist destination that draws millions of visitors each year, the Great Wall has great cultural relevance, which is why the biodiversity it preserves is so significant, Xiao said.
“The Great Wall is the cultural center of Chinese civilization,” he told CNN. “We should do our best to protect it for future generations. For our children, our grandchildren.”
Ayurella Corn-Muller Climate change is covered for Axios. Her first book, “Devoured: The Extraordinary Story of Kudzu, the Vine that Devoured the South,” is due out in the spring.
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