“Living Skin” Named Defense of the Great Wall of China

These soil surfaces on the Great Wall of China are covered with a “living skin” of tiny, rootless plants and microorganisms known as biocrusts, which are the source of the cultural heritage site’s resilience, according to soil ecologist Matthew Bowker, co-author of the study published in the journal Science Advances.

“Biocrusts are found throughout the world in soils in arid regions, but we don’t typically look for them on human-built structures,” says Bowker, an assistant professor at Northern Arizona University.

Past research has shown that bioremediation of lichens and mosses poses a destructive 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 the microenvironment, which can cause erosion and weathering of rocks. These findings, CNN notes, led to the removal of plants growing on the upper parts of the Great Wall of China. But the effects of bioremediation on landmarks appear different, and communities of cyanobacteria and moss actually improve the Great Wall’s resilience and make it more resistant to erosion, a new paper finds.

Examining samples taken from more than 483 kilometers of eight rammed earth plots at a site built during the Ming Dynasty between 1368 and 1644, the study authors found that more than two-thirds of the area was covered in biocrusts. When the researchers compared the stability and strength of samples covered with a layer of biocrustation with samples without the “living skin of the Earth,” they found that samples with biocrustation were three times stronger than those without it.

“They thought this type of vegetation was destroying the Great Wall of China. Our results show the opposite,” said study co-author Bo Xiao, a professor of soil science at China Agricultural University. “Biocorrections are very widespread on the Great Wall of China, and their existence is very useful for its protection.”

Biocrusts, composed of components such as cyanobacteria, algae, moss, fungi and lichens, live in the topsoil of drylands. Occupying an estimated 12% of the planet’s surface, communities of tiny plants and microorganisms can take decades or longer to evolve. Forming miniature ecosystems, biocrusts stabilize soil, increase water retention, and regulate nitrogen and carbon fixation.

They’re able to do this in part because of dense biomass, which under certain conditions acts as an “anti-infiltration layer” for soil pores, as well as the natural absorption of nutrients that contribute to salt damage, CNN tells CNN. According to the new study, exudates and structural layers of bioremediation also intertwine to form a “sticky network” of aggregating soil particles that contribute to the strength and resistance to corrosive attacks that threaten the Great Wall of China.

Forming miniature ecosystems, biocrusts stabilize soil, increase water retention, and regulate nitrogen and carbon fixation.

The researchers found that climate conditions, type of structure and type of biocorrosion all play a role in the protective function of biocorrosion, with the reduction in its deterioration being “much greater” than the risk of weathering.

Compared to bare rammed earth, sections of the Great Wall covered with biocorrosion by cyanobacteria, mosses and lichens showed reductions in porosity, water holding capacity, erodibility and salinity by up to 48%, while increasing compressive strength, penetration resistance, shear strength and aggregate stability by up to 321%. Of the entire group, the moss biocorpus turned out to be the most stable.

“The bio-hull covers the Great Wall of China like a blanket that separates the Great Wall of China from the air, from the water, from the wind,” says Professor Xiao.

He noted that the bio-hull, which retains water and prevents salt build-up, resists chemical weathering by producing substances that act as a “glue” to bind soil particles together against dispersion, enhancing soil properties.

Most communities that make up a biobush start as a single organism that grows and makes the environment in which it grows suitable for others, CNN reports. Although they are still vulnerable to the effects of climate change, these constantly evolving organisms are expected to engage internal mechanisms to adapt to future extreme events, said Emmanuel Salifu, an assistant professor at Arizona State University.

This innate adaptability makes biocrusts an excellent candidate for conservation efforts to address the challenge of maintaining structure in our warming world, Salifou says.

“Even if we have higher temperatures, they are already adapted to work in these conditions,” says the expert. “We expect they can survive better if we engineer their growth at scale.”

Wind erosion, rain erosion, salinization and freeze-thaw cycles have caused cracking and disintegration across thousands of miles of structures connecting the Great Wall of China, which is subject to severe deterioration and vulnerable to collapse. Rising temperatures and increased precipitation can also lead to a decrease in the wall’s bio-coverage.

The study’s authors say their work provides evidence to explore the possibility of cultivating bio-surfacing to help preserve other World Heritage sites.

The construction industry as a whole remains divided over the historical conservation potential of biocorrection, Salifou said.

“The generally accepted idea is that biological growth is harmful to structures. It affects the aesthetics, it causes degradation, it affects the overall structural integrity,” he said. However, there is a lack of specific research to support these findings, Salifu added, noting that “the jury is still out on this.”

Salifu sees the new research as evidence of the potential benefits of creating biocorrections for the conservation of earthen heritage sites, although the field is still in its infancy. The study establishes that natural communities of plants and microorganisms “have the ability to improve the structural integrity, longevity of earthen structures such as the Great Wall of China,” Salifu said.

Besides its status as a tourist destination attracting millions of visitors every year, the Great Wall of China has great cultural significance, which is why bioremediation to preserve it is so important, Xiao said.

“The Great Wall of China 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.”