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Biocrusts: Unseen protectors of the Great Wall of China

 

Erosion is a contemporary threat to the Great Wall of China, an engineering marvel and a representation of the tenacity of ancient people. But a surprising defender of this unique structure is emerging: a natural phenomena called biocrusts. An unexpected ally in the preservation of this landmark structure has come to light through a new study published in “Science Advances.”

The Great Wall’s ‘living skin’

A “living skin” of microorganisms and microscopic plants called biocrusts covers the Great Wall’s soil surfaces. These biocrusts are more than simply a surface layer; they are integral to the wall’s ability to withstand natural deterioration.
Researchers have found that 67% of the Great Wall’s examined portions are coated in biocrusts, a layer of living things that includes mosses, lichens, and cyanobacteria. The wall’s erodibility is decreased and its mechanical stability is greatly increased by these biocrusts. Sections covered with biocrust demonstrated increases in compressive strength, penetration resistance, shear strength, and aggregate stability of 37 to 321%, whereas porosity, water-holding capacity, erodibility, and salinity decreased by 2 to 48% when compared to bare rammed earth.

The Great Wall, which is 8851.8 kilometers long and primarily traverses dryland areas, was primarily constructed using rammed earth, which leaves it vulnerable to environmental problems including wind erosion and rainfall scouring.The significance of biocrusts in safeguarding this historical landmark cannot be overstated, as just 5.8% of its whole length remains intact. These biocrusts not only cover a large area of the wall, but their protective qualities also differ according to the characteristics of the biocrust, the climate, and the kind of building.

Biocrusts versus contemporary historic buildings

According to earlier research, biocrusts—especially lichen and moss—may be harmful to contemporary heritage stone constructions, which is why they were removed from several sections of the Great Wall. For earthen markers, though, the results of this new study paint a different picture. Communities of moss and cyanobacteria do not pose a hazard; rather, they enhance the stability and resilience to erosion of the Great Wall.

A thorough investigation

The Great Wall was constructed during the Ming Dynasty, and the research team looked at samples from more than 300 miles of it. They discovered that biocrusts cover more than two thirds of this area. The researchers found that the biocrust-covered samples were up to three times stronger when comparing the stability and strength of the biocrust-layered samples to those without.

The nature of biocrusts and their function

Algae, moss, lichen, cyanobacteria, and fungi comprise biocrusts. They make up around 12% of the Earth’s surface and have a major role in controlling the fixation of carbon and nitrogen, retaining more water in the soil, and stabilizing it. Their thick biomass blocks soil pores and absorbs nutrients to lessen the damage caused by salt, functioning as an anti-infiltration barrier. The network created by these secretions and structural layers collects soil particles, strengthening and stabilizing the Great Wall against erosion.

The role of biocrusts in protection

The protective role of a biocrust is influenced by structural kinds, biocrust types, and climate conditions. Sections of the Great Wall covered in biocrust had higher compressive strength, penetration resistance, shear strength, and aggregate stability, while lower porosity, water-holding capacity, erodibility, and salinity when compared to bare rammed earth.

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