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The rammed earth portions of the Great Wall of China – built by compressing natural materials with earth – have been seen as a weak point in its structure. But these sections of this iconic monument 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 with a “living skin” of tiny rootless plants and microorganisms known as biocrusts which are a source of sustainability of the heritage site, according to a soil ecologist. Matthew Bowker, co-author of the study published December 8 in the journal Science Advances.
“(Biocrusts) are common worldwide on soils in dry regions, but we generally do not look for them on human-built structures,” Bowker, an associate professor at Northern Arizona University, said in a letter. electronic.
Previous studies have shown that lichen and moss biocrusts pose a destructive threat to modern heritage stone structures due to long-term impacts of microbial communities on aesthetic value, acid production and other metabolites and alteration of microenvironments, which can cause erosion and rocks. erosion. These discoveries led to the removal of plants growing atop parts of the Great Wall. But the effects of biocrusts appear different for earthen monuments, and communities of cyanobacteria and moss actually increase the stability of the Great Wall and improve its resistance to erosion, according to the new paper.
Courtesy of Bo Xiao
“Biocrusts are very widespread on the Great Wall,” explains Bo Xiao, co-author of the study, “and their existence is very beneficial for its protection.”
Examining samples taken from more than 300 miles (483 kilometers) across eight sections of adobe from 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 biocrusts. When the researchers compared the stability and strength of biocrust-covered samples with samples without the “living skin of the Earth,” they found that samples containing biocrusts were up to three times stronger than those without biocrusts.
“They thought this kind of vegetation was destroying the Great Wall. Our results show the opposite,” said Bo Xiao, co-author of the study and professor of soil science at China Agricultural University. “Biocrusts are widespread on the Great Wall and their existence is very beneficial for its protection.”
Made up of components such as cyanobacteria, algae, moss, fungi and lichens, biocrusts inhabit the topsoil of arid areas. Covering about 12% of the planet’s surface, communities of tiny plants and microorganisms can take decades or more to develop. Forming miniature ecosystems, biocrusts stabilize the soil, increase water retention and regulate nitrogen and carbon fixation.
They achieve this in part through dense biomass, which acts as an “anti-seepage layer” for soil pores under the right conditions, as well as natural uptake of nutrients that promote salt damage. The secretions and structural layers of biocrusts also intertwine to form a “sticky network” of aggregated soil particles that promote resistance and stability against corrosive forces threatening the Great Wall, according to the new study.
Courtesy of Bo Xiao
Forming miniature ecosystems, biocrusts stabilize the soil, increase water retention and regulate nitrogen and carbon fixation.
Climatic conditions, structure type and biocrust type all play a role in a biocrust’s protective function, with the reduction in erodibility “much greater” than its risk of weathering, the researchers found.
Compared to bare rammed earth, sections of the Great Wall covered with biocrust of cyanobacteria, moss, and lichens had reduced porosity, water-holding capacity, erodibility, and salinity by up to 48%, while increasing compressive strength, penetration resistance, shear strength and aggregates. stability up to 321%. Among the group, moss biocrusts were found to be the most stable.
“(The biocrusts) cover the Great Wall like a blanket that separates it from air, water and wind,” Xiao said.
Working to keep water out and prevent salt buildup, biocrusts resist chemical weathering, he noted, producing substances that act like a “glue” allowing soil particles to bond together to avoid dispersion, thus strengthening the properties of the soil.
Most of the communities that make up a biocrust arise from a single organism that grows and makes the environments in which it grows suitable for others. Although they are still vulnerable to the impacts of climate change, these 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 solutions based on nature for sustainable engineering.
This 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 though we have warmer temperatures, they are already adapted to perform in those conditions,” he said. “We hypothesize that they will be better able to survive if we organize their growth on a large scale. »
Wind erosion, precipitation scour, salinization, and freeze-thaw cycles have led to the cracking and disintegration of thousands of miles of structures that connect the Great Wall, which is at risk of serious deterioration and collapse. Rising temperatures and increased precipitation could also lead to a reduction in the biological cover of the wall.
Courtesy of Bo Xiao
The study authors say their work warrants exploring the possibility of growing biocrusts to help preserve other dirt heritage sites.
Yet, according to Salifu, the broader construction industry remains divided over the historic potential for biocrust conservation.
“The conventional wisdom is that biological growth is not good for structures. This affects aesthetics, leads to degradation and affects overall structural integrity,” he said. However, there is a lack of concrete research to support these conclusions, Salifu added, noting that “opinion is not yet clear on this point.”
Salifu sees the new study as evidence of the potential benefits of biocrust engineering for the conservation of earthen 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 earthen structures like the Great Wall of China,” Salifu said.
The paper “goes a long way toward moving the needle of time by moving the industry closer to the point where we could start thinking about (biocrust engineering),” he noted.
The study authors also say their work makes the case for exploring the possibility of cultivating biocrusts for the preservation of other clay 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 significance, which is why the biocrusts that preserve it are so important, Xiao explained.
“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 Horn-Müller covered climate change for Axios. His first book, “Devoured: The Extraordinary Story of Kudzu, the Vine That Ate the South,” is expected to be published in the spring.
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