An international research team has found large sections of the Great Wall of China held together by ‘biocrust’, thin layers of organic material that helped protect the architectural marvel from erosion.

According to a study recently published in the journal Science Advances, the Great Wall of China stretches an astonishing 8,851.8 kilometers in dryland environments, and has been recognized as a World Heritage Site due to its unparalleled construction period and geographical scale.

The Great Wall of China has existed for more than five centuries and is an indispensable expression of the Chinese nation and a priceless treasure of human civilization. One of the most common materials used to build large structures in the ancient world, it was built in many places and periods.

As an iconic structure of the rammed earth, the Great Wall is highly vulnerable to wind erosion, precipitation, salinity, and freeze-thaw cycles, leading to serious problems such as cracking, deformation, and eventual collapse.

Given the effects of global climate change, the Great Wall is at risk of severe erosion, which could affect the long-term life of its soil structure. To date, only 5.8% of its total length is still well preserved, while 52.4% has disappeared or is severely degraded. Therefore, security measures should be urgently implemented.

How did biocrusts… protect the wall?

Biocrusts – photosynthetic autotrophic communities consisting mainly of cyanobacteria, algae, lichens, other microorganisms and tightly bound soil particles – are known to cover large areas of the Great Wall of China.

Although biocrusts colonize only a few centimeters of the soil surface, they can act as environmental engineers supporting and regulating many key processes of soil and terrestrial ecosystems.

But biocrusts can also affect the mechanical stability and physical and chemical properties of the rammed earth used to build the Great Wall, which acts as a natural living cover to protect terrestrial materials in dry climates. However, how biocrusts contribute to the preservation of the Great Wall is virtually unknown.

“Ancient builders knew which materials could make structures more stable,” said study co-author Bo Xiao, a professor of soil science in the College of Land Science and Technology at China Agricultural University in Beijing.

“To increase mechanical strength, the wall’s rammed earth was constructed by the original builders with clay, sand and other adhesives such as lime,” Xiao told Live Science. “These materials provide fertile ground for organisms that form biological crusts.”

To test the strength and integrity of the Great Wall of China, researchers collected samples from eight different sections built during the Ming Dynasty from 1368 BC to 1644 BC, and found that 67% of the samples contained “biocrusts,” which Xiao called “engineers.” “Ecosystem”. Using small mechanical instruments on site and in the laboratory, the researchers measured the mechanical strength and soil stability of the samples and compared that data to sections of the wall containing bare earth.

They found that “biocrust” samples were sometimes three times stronger than solid Earth samples. According to the study, samples with algae were particularly fattening. Cyanobacteria and other life forms within the biocrust secrete substances, such as polymers, that “tightly bind” to the colliding earth particles, essentially helping to “enhance their structural stability” by forming cement, Xiao said.

These cement materials, biological threads and soil aggregates within the biocrust layer finally form a cohesive network with strong mechanical strength and stability against external erosion, he said.