8 January 2024

Biocrusts protect The Great Wall of China from erosion


Typical of arid or semi-arid expanses, soil bio-crusts are an important and undervalued natural resource. An international study has highlighted their ability to counter erosion

by Matteo Cavallito


Biocrusts formed along the Great Wall of China allegedly helped protect it from weather erosion. This is supported by a study published in the journal Science Advances. Built over several centuries, beginning in 221 B.C., the structure is made of different materials, largely stone and rammed earth, which are particularly susceptible to degradation. So how was it possible for the barrier to survive for so long?

To answer this question, some researchers from the Chinese Academy of Sciences, the Instituto de Recursos Naturales y Agrobiología in Seville and Northern Arizona University, have analyzed the crusts, supposing that they have played a crucial role to date. And the data, they explain, seem to confirm the assumption.

The study

Typical of arid or semi-arid expanses, biocrusts or “biological soil crusts” are a natural resource as valuable as they are undervalued. This is due to the amazing ecosystem they host: a diverse world that includes fungi, lichens and especially microbes. For many years, says a statement released by the researchers, scientists speculated that these particular organisms could accelerate the erosion process. But latest investigation disproved this belief.

By analyzing in the laboratory some samples collected from different places in the wall, the scholars measured aspects such as mechanical strength and stability. They also directly tested parts of the wall, comparing those covered by the crusts and those directly exposed to the elements. The biocrusts proved to be stronger than the rammed earth material they were growing on, proving capable of preserving the structure.

Stability can increase by more than 300 percent

“We conducted an extensive biocrust survey across the Great Wall and found that biocrusts cover 67% of the studied sections. Biocrusts enhance the mechanical stability and reduce the erodibility of the Great Wall,” the study says. “Compared with bare rammed earth, the biocrust-covered sections exhibited reduced 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 scientists also “found that the protective function of biocrusts mainly depended on biocrust features, climatic conditions, and structure types.” The study, in brief, “highlights the fundamental importance of biocrusts as a nature-based intervention to the conservation of the Great Wall, protecting this monumental heritage from erosion.”

The importance of biocrusts

Also underscoring the importance of biological soil crusts last year was another study by the Chinese Academy of Sciences that sequenced long-term restoration processes in the southeastern edge of the Tengger Desert, highlighting the influence of these environments on changes occurring in subsurface bacterial communities. “Biocrusts cover approximately 30% of the global dryland surface area, constituting a crucial atmosphere–soil interface,” says the study published in the journal Plant and Soil.

“Bacteria living at this interface participate in almost all biogeochemical cycling processes that may profoundly alter soil and ecosystem multifunctionality and speed up ecosystem restoration.”

Of concern, however, is the deterioration of these peculiar surface layers on a global scale. In fact, early last year, a research by the U.S. Geological Survey highlighted how climate change was causing the lichens’ presence to decline dramatically, thus threatening the survival of the crust. In the absence of global climate mitigation strategies, the authors concluded, the world risks losing 25 percent to 40 percent of its crusts by 2070.