The secret 'living skin' that protects the Great Wall

The rammed earth sections of the Great Wall of China were built by compacting natural materials with soil .

This is considered a weak point in the structure of the Great Wall.

But a new study finds that this iconic region has developed a natural defense against the looming threat of deterioration.

Improves erosion resistance

This "living skin" consists of small, rootless plants and microorganisms.

According to ecologist Matthew Bowker - co-author of the study published in the journal Science Advances, the land surfaces on the Great Wall are covered with a 'living skin' of small plants. This small, rootless and microscopic organism is called a biocrust.

They are the source of enduring strength of the Great Wall. Mr. Bowker - Associate Professor at Northern Arizona University, said: 'Biocrusts are common around the world on soils in arid regions, but we don't usually find them on human-built structures. build'.

Previous studies have found that lichen and moss biocrusts pose a destructive threat to modern heritage stone structures due to the long-term impact of the microbial community on their aesthetic value. cosmetics, production of acids and other metabolites as well as changes in the microenvironment, which can cause erosion and weathering.

Those discoveries led to the removal of plants growing on top of the Great Wall. However, according to new research, the impact of biocrust varies on land landmarks. Meanwhile, communities of cyanobacteria and moss actually increase the stability of the Great Wall . At the same time, improving the erosion resistance of this area.

The researchers examined samples taken from more than 300 miles (483 km) across eight rammed earth sections of the Ming Dynasty construction site from 1368 to 1644. The team found that, more than 2 /3 area covered in biocrust.

The researchers compared the stability and durability of samples layered in a biocrust with those without 'Earth's living skin'. They found that samples with a biocrust were three times more durable than those without a biocrust.

'Many opinions say that this type of vegetation is destroying the Great Wall. However, our results show the opposite. Biocrusts are very common on the Great Wall. Meanwhile, their existence is very beneficial to protecting this area ,' said study co-author Bo Xiao - Professor of soil science at China Agricultural University.

Biocrusts are very common on the Great Wall.

'Like a blanket'

Made up of components such as cyanobacteria, algae, moss, fungi and lichens, biocrusts live on topsoil of dry land. Covering an estimated 12% of the planet's surface, tiny communities of plants and microorganisms can take decades or longer to develop. Forming miniature ecosystems, biological crusts stabilize soil, increase water holding capacity and regulate nitrogen and carbon fixation.

They are able to do this in part thanks to their dense biomass, which acts as a 'waterproof layer' for soil pores under the right conditions, as well as the natural absorption of nutrients that promote promote salt destruction.

According to new research, the secretions and structural layers of the biocrust are also intertwined. From there, to form a 'sticky network' that gathers soil particles to increase strength and stability against the corrosive effects that threaten the Great Wall.

The researchers found that climatic conditions, structural type and biocrust type all play a role in protective function. They have a 'far greater' role in reducing the potential for erosion than the risk of weather.

Compared to bare rammed earth, sections covered with cyanobacteria, moss and lichen biocrusts of the Great Wall showed a 48% reduction in porosity, water retention, erodibility and salinity. At the same time, this coating increases compression strength, penetration resistance, and stability by up to 321%.

Among them, moss bark is said to be the most stable . 'The biocrust covering the Great Wall is like a blanket that separates the area from air, water and wind ,' researcher Xiao said.

By blocking water and preventing salt buildup, biocrust resists chemical weathering, he notes. From there, substances are created that act as a 'glue' to bind soil particles together against dispersion, making the soil's properties stronger.

Most of the communities that make up a biocrust start from a single growing organism. This organism also makes the environment in which it grows suitable for other organisms.

These creatures remain vulnerable to the effects of climate change, said Emmanuel Salifu, an assistant professor at Arizona State University who researches nature-based solutions for sustainable engineering. . However, they are constantly evolving to adapt to changes in the environment.

That inherent adaptability makes biocrusts a good candidate for nature-based interventions, said Salifu, who was not involved in the new study.

Thereby, aiming to solve the problem of structural preservation in our warming world. Even when temperatures are warmer, they are still suitable to operate in those conditions. They will be more likely to survive if we engineer the growth of this shell on a large scale.

Salifu sees this new research as evidence of the potential benefits of engineering biocrusts to preserve Earth's heritage, although it is still a new field.

Research demonstrates that natural communities of plants and microorganisms have the potential to improve the structural integrity, longevity and durability of earthen structures such as the Great Wall of China.

The study authors also said that their work provides a basis for exploring the possibility of cultivating biocrusts to preserve other rammed earth heritage around the world.

In addition to being a tourist destination that attracts millions of visitors each year, the Great Wall also has great cultural significance. That's why the biocrust that preserves it is so important.

According to Professor Xiao, the Great Wall is the cultural center of Chinese civilization. Therefore, it is extremely necessary to do our best to protect it for the next generations.