Study Finds A "Living Skin" Is Protecting The Great Wall Of China

The Great Wall of China is one of the most fascinating structures built by humans. Constructed over several centuries by China's emperors to protect their territory, it has stood the test of time and attracted millions of tourists who are in awe of the structure built around 3,000 years ago. It was constructed by compressing natural materials with soils and developed weak points in its structure. But they developed a natural line of defence against the constant risk of deterioration, a new study has found.

This defence mechanism is in the form of "living skin" made of tiny, rootless plants and microorganisms known as biocrusts, the study published in Science Advances on December 8, 2023, said.

"Biocrusts are common throughout the world on soils of dry regions, but we don't typically look for them on human-built structures," soil ecologist Matthew Bowker, a co-author of the study and an associate professor at Northern Arizona University, told CNN.

The biocrusts have covered the soil surfaces of the Great Wall. And it is not just a superficial layer, the microorganisms play a crucial role in enhancing the structure's resistance to natural degradation and rock weathering.

The researchers took samples from over 480 kilometres of the site and found that more than two-thirds of the area is covered in biocrusts.

Earlier studies had said biocrusts were destructive threat to the stone structures since the microbes produce acid, and alter microenvironments. But Mr Bowker and his team compared the stability and strength of samples layered in biocrust with samples without the "living skin" and discovered that samples with biocrusts were as much as three times stronger than those without.

"They thought this kind of vegetation was destroying the Great Wall. Our results show the contrary," said study co-author Bo Xiao, a professor of soil science at China Agricultural University. "Biocrusts are very widespread on the Great Wall and their existence is very beneficial to the protection of it."

The biocrusts are made up of cyanobacteria, algae, moss, fungi and lichen, and are found on the top layer of the soil. The layer can take decades, or even longer, to develop. The miniature ecosystem formed by these biocrusts helps stabilise soil, increase water retention, and regulate nitrogen and carbon fixation.