LANZHOU, June 29 (Xinhua) -- For millennia, China's Great Wall has stood as a human marvel, yet time, wind and rain have quietly eroded what was once built by hand. Now, Chinese scientists are looking to apply complex communities of microorganisms -- biocrusts as a "natural guardian."
Researchers from the Dunhuang Academy and the Northwest Institute of Eco-Environment and Resources (NIEER) under the Chinese Academy of Sciences have systematically integrated recent field surveys and experimental data, revealing that biocrusts can benefit rammed-earth sections of the Great Wall.
The benefits depend on a critical threshold determined by three factors: the prevailing climatic environment, the wall's material composition, and the successional stage of the biocrust community.
They propose a novel perspective -- biocrusts could offer a low-cost, eco-friendly protective layer, leveraging their net effect to safeguard the Great Wall by harnessing the wisdom and power of nature to withstand environmental damage.
This purely nature-based insight and solution that harnesses the power of biocrusts, dubbed the Great Wall's "living skin," echoes today's mainstream trend in global cultural heritage conservation.
MISSION FOR SCIENTIFIC PROTECTION
As a UNESCO World Heritage Site, the Great Wall was built over more than two millennia, from the Spring and Autumn Period (770 B.C.-476 B.C.) to the Ming Dynasty (1368-1644).
"During long-term field surveys, the study team discovered that rammed-earth wall sections along the Great Wall in Gansu, Ningxia and Shaanxi, northwest China, commonly suffer from surface weathering and spalling, with wall surfaces continuously 'shedding soil' and thinning over time," said Wu Fasi, a researcher at the Dunhuang Academy.
At the base of walls, rainwater erosion and salt activity have caused severe alkali efflorescence and undercutting, as if the structures were being "gnawed away." Some wall sections have developed extensive fissures due to wet-dry and freeze-thaw cycles, with localized collapse.
"These deteriorations not only damage the Great Wall's appearance, but also threaten its structural stability," Wu said.
Researchers shoulder an important mission: exploring a more appropriate, scientific solution to protect the Great Wall that avoids the problems associated with the currently widely used chemical reinforcement materials.
"Chemical reinforcement materials can improve wall strength and reduce weathering in the short term. However, they also suffer from poor compatibility with the rammed-earth substrate, and short effective lifespans," said Duan Yulong, an associate researcher at the NIEER.
Duan explained that after consolidation, some chemical materials may alter the wall's breathability, preventing internal moisture from escaping and thereby causing salt accumulation.
LEVERAGING WISDOM FROM NATURE
"Green, eco-friendly, and sustainable conservation philosophies are now gaining acceptance and being applied. Our research team turned to exploring more natural materials," said Jia Rongliang, a researcher at the NIEER.
During field surveys at Great Wall sites, researchers were surprised to discover that biocrusts are widely present as coverings on rammed-earth sections of the Great Wall.
Biocrusts cling tightly to the wall surface, naturally integrating with the rammed-earth matrix to form a thin "skin" that stabilizes soil, prevents wind erosion, reduces abrasion, and protects the wall.
Jia explained that biocrusts are surface composites cemented together by microorganisms such as cyanobacteria and lichens, along with lower plants such as mosses and soil particles, which are hailed as "ecosystem engineers." In arid and semi-arid sections of the Great Wall, they cover up to 67 percent of the rammed-earth surface.
In past conservation and restoration efforts, biocrusts were often cleared away as harmful organisms. "Their functions were unclear," Jia said.
He explained that biocrusts are self-sustaining and self-repairing ecosystems fully compatible with rammed-earth materials. "They are ideal green conservation bio-materials."
Biocrusts can play multiple roles in protecting the Great Wall. For example, they can help reduce raindrop splash erosion by providing the wall with a "protective umbrella." They can also limit rainwater infiltration by equipping the wall with a "breathable raincoat."
Biocrusts can also enhance mechanical stability by adding a "resilient exoskeleton" to the wall. "It is because of fungal hyphae that moss rhizoids can intertwine and strengthen soil particles, jointly forming a 3D network structure that significantly improves the compressive, shear, and erosion resistance of rammed earth," Wu said.
The study also noted that biocrusts have limitations -- in semi-humid and humid zones, excessive moisture may trigger vegetation growth and root penetration. They may also attract ants, millipedes and other small fauna, introducing extra mechanical disturbance and damage.
ADVANCING NATURE-BASED SOLUTIONS
"In recent years, nature-based solutions have become a trend in global cultural heritage conservation. Scientists and conservators at home and abroad are endeavoring to explore nature-based solutions to heritage site conservation," Wu said.
Researchers of the University of Oxford have employed the soft capping of ruins as a form of heritage conservation. In China, researchers from the Dunhuang Academy collaborated with NIEER and the Sichuan Provincial Cultural Relics and Archaeology Research Institute to explore ecological approaches to safeguarding earthen sites, such as the Qin Great Wall site in Gansu's Tongwei County, and the Sanxingdui Ruins in Sichuan's Guanghan City.
The study team is currently developing an artificial biocrust cultivation-inoculation technical system to accelerate the natural formation of biocrusts.
"Our new study offers a perspective for the green conservation of the Great Wall and other earthen sites," Jia said.
Compared with traditional methods for relic conservation, biocrusts offer irreplaceable advantages in ecological compatibility. It is because biocrusts are self-sustaining, self-repairing "living protective layers" requiring no repeated inputs. It is also a green conservation solution as biocrusts can deliver additional ecological benefits, including carbon sequestration, nitrogen fixation, and dust suppression.
Jia said the team has developed an integrated system with sand barrier, trees, shrubs, grass, and biocrusts, and applied it in the wind-sand control regions of the country's Three-North Shelterbelt Forest Program, benefiting both cultural heritage protection and ecological restoration.
Going forward, the study team will conduct cross-climatic-zone surveys of Great Wall biocrusts, optimize artificial cultivation and inoculation techniques for dominant biocrust species, and establish long-term monitoring systems.
"Biocrusts are a promising natural solution. We will move ahead for a better understanding of their nature and learning to precisely manage them, with the target of enhancing cultural heritage conservation from passive emergency repair toward proactive ecological coexistence," Wu said. Enditem
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