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The compressed earth parts of the Great Wall, which was built by compressing natural materials with soil, were considered the weak point of its structure. But a new study has found that these landmark sites have developed a natural line of defense against the risk of impending degradation.
These soil surfaces at the Great Wall are covered with a “living skin” of tiny, rootless plants and microorganisms known as biocrusts, which is the source of the heritage site’s enduring strength, according to soil ecologist Matthew Bowker, one of the authors of the study published in December. 8th in the journal Science Advances.
“(Biocrusts) are common in soils of dry regions around the world, but we don’t typically look for them in human-made structures,” Bowker, an associate professor at Northern Arizona University, said in an email.
Past studies have revealed that lichen and moss biocrusts are a devastating threat to modern heritage stone structures due to the long-term effects of microbial communities on aesthetic value, production of acids and other metabolites, and alteration of microenvironments that can lead to erosion and rock formation. weather conditions. These findings led to the removal of plants growing on parts of the Great Wall. But the effects of biocrusts appear to differ for key points in the soil, according to the new paper, and communities of cyanobacteria and algae actually increase the stability of the Great Wall and increase its resistance to erosion.
Studying samples taken from more than 300 miles (483 kilometers) of eight rammed earth sections of the site built between 1368 and 1644 during the Ming Dynasty, the study authors found that more than two-thirds of the site was covered in biocrusts. When researchers compared the stability and strength of samples embedded in biocrust with samples outside the “living skin of the Earth,” they discovered that samples with biocrust were 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 science at China Agricultural University. “Biocrusts are very common in the Great Wall of China, and their presence is very beneficial to its preservation.”
‘like a blanket’
Biocrusts, composed of components such as cyanobacteria, algae, mosses, fungi and lichens, live in the topsoil of arid areas. Small communities of plants and microorganisms that cover an estimated 12% of the planet’s surface can take decades, or even longer, to develop. 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 the dense biomass that acts as an “anti-seepage layer” for soil pores under the right conditions, as well as the natural absorption of nutrients that enhances salt damage. According to the new study, the secretions and structural layers of biocrusts also intertwine, forming a “sticky network” where soil particles collect, increasing durability and stability against the erosive forces that threaten the Great Wall of China.
The researchers found that climatic conditions, type and type of biocrust structure all play a role in the protective function of the biocrust, and that erodibility is reduced “much more” than the risk of weathering.
Compared to bare rammed earth, sections of the Great Wall covered in cyanobacteria, moss and lichen biocrust exhibited up to 48% reduction in porosity, water holding capacity, erodibility and salinity, while increasing compressive strength, penetration resistance, shear strength and aggregate stability by up to 321% . Algae biocrusts were found to be the most stable among the group.
“(Biocrusts) cover the Great Wall of China like a blanket that separates the Great Wall from air, water and wind,” Xiao said.
Working to keep water out and prevent salt buildup, biocrusts resist chemical weathering, produce substances that act as a “glue” for soil particles to bind together against disintegration, and make soil properties stronger, he noted.
The role of biocrusts in an uncertain future
Most biocrust-forming communities start from a single organism that grows and adapts the environments in which it grows to others. While they are still vulnerable to the effects of climate change, these ever-evolving organisms are expected to use internal mechanisms to adapt to future extremes, said Emmanuel Salifu, an assistant professor at Arizona State University who studies nature-based solutions for sustainable engineering.
This natural adaptability makes biological shells a major contender for nature-based interventions for structural protection in our warming world, said Salifu, who was not involved in the new study.
“Even if our temperatures are higher, they are already fit to perform in those conditions,” he said. “We assume their survival will be better if we engineer their growth at scale.”
Wind erosion, rainfall eroding, salinization and freeze-thaw cycles have led to cracks and disintegration in thousands of kilometers of structures connecting the Great Wall; these structures are at serious risk of deterioration and are vulnerable to collapse. Increasing temperatures and increased precipitation may also cause the biological crust cover of the wall to decrease.
Still, the construction industry is divided on the historic preservation potential of bioshells, according to Salifu.
“The conventional wisdom is that biological growth is not good for structures. It affects aesthetics, it causes deterioration, it affects overall structural integrity,” he said. However, there is a lack of concrete research to support these conclusions, Salifu said, adding that “the jury is still out on this issue.”
Salifu sees the new study as evidence of the potential advantages of biocrust engineering for the conservation of soil heritage sites; but this is still a developing field. Salifu said the research revealed that natural plant and microorganism communities “have the capacity to improve the structural integrity, longevity and durability of earthen structures such as the Great Wall of China.”
He noted that the paper “goes a long way in moving the clock forward by bringing the industry closer to where we can start thinking about (bioshell engineering).”
The study’s authors also say their work sets an example for exploring the possibility of growing biocrusts to protect other rammed earth heritage sites around the world.
Beyond being a tourism destination that attracts millions of visitors every year, the Great Wall has great cultural significance, which is why the biological shells that protect it are so important, Xiao said.
“The Great Wall of China is the cultural center of Chinese civilization,” he told CNN. “We must do our best to preserve it for our future generations. For our children, for our grandchildren,” he said.
Ayurella Horn-Muller He covered climate change for Axios. His first book, “Devoured: The Extraordinary Story of Kudzu, the Vine That Devoured the South,” will be released in the spring.
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