The ‘living skin’ of the Great Wall
- Biocrusts, a combination of tiny plants and microorganisms, form a ‘living skin’ that covers the soil surfaces of the Great Wall. These biocrusts are not just a superficial layer; they play a crucial role in enhancing the wall’s resistance to natural degradation.
- Researchers have discovered that biocrusts, a layer of living organisms including cyanobacteria, mosses, and lichens, cover 67% of the surveyed sections of the Great Wall. These biocrusts significantly enhance the wall’s mechanical stability and reduce its erodibility. Compared to bare rammed earth, biocrust-covered sections showed a decrease in porosity, water-holding capacity, erodibility, and salinity by 2 to 48%, while increasing compressive strength, penetration resistance, shear strength, and aggregate stability by 37 to 321%.
The Great Wall’s vulnerability
Spanning 8851.8 km, mostly through dryland environments, the Great Wall is built largely with rammed earth, making it susceptible to wind erosion, rainfall scouring, and other environmental challenges.With only 5.8% of its total length well preserved, the role of biocrusts in protecting this heritage site is crucial. These biocrusts not only cover a significant portion of the wall but also vary in their protective function based on biocrust features, climatic conditions, and structure types.
Biocrusts vs modern heritage structures
Past studies have indicated that biocrusts, particularly lichen and moss, can be detrimental to modern heritage stone structures, leading to their removal from parts of the Great Wall. However, this new study reveals a different story for earthen landmarks. Contrary to being a threat, communities of cyanobacteria and moss actually bolster the Great Wall’s stability and improve its erosion resistance.
A comprehensive study
The research team examined samples from over 300 miles of the Great Wall, built during the Ming Dynasty. They found that more than two-thirds of this area is covered in biocrusts. When comparing the stability and strength of biocrust-layered samples to those without, the team discovered that the biocrust-covered samples were up to three times stronger.
The composition and role of biocrusts
Biocrusts are made up of cyanobacteria, algae, moss, fungi, and lichen. They cover about 12% of the planet’s surface and are crucial in stabilizing soil, increasing water retention, and regulating nitrogen and carbon fixation. Their dense biomass acts as an anti-infiltration layer, blocking soil pores and absorbing nutrients that mitigate salt damage. These secretions and structural layers form a network that aggregates soil particles, enhancing the Great Wall’s strength and stability against erosion.
The protective function of biocrusts
Climatic conditions, structure types, and biocrust types all influence a biocrust’s protective function. Compared to bare rammed earth, biocrust-covered sections of the Great Wall showed reduced porosity, water-holding capacity, erodibility, and salinity, while increasing compressive strength, penetration resistance, shear strength, and aggregate stability.