Rammed earth portions of the Great Wall of China — built by compressing natural materials with soils — have been regarded as a weak point in its structure. But these swaths of the iconic landmark developed a natural line of defense against the looming risk of deterioration, a new study has found.

These soil surfaces on the Great Wall are covered by a “living skin” of tiny, rootless plants and microorganisms known as biocrusts that are a source of the heritage site’s staying power, according to soil ecologist Matthew Bowker, a coauthor of the study published last month in the journal Science Advances.

“(Biocrusts) are common throughout the world on soils of dry regions, but we don’t typically look for them on human-built structures,” said Bowker, an associate professor at Northern Arizona University.

Past studies have found lichen and moss biocrusts to be a destructive threat to modern heritage stone structures due to the microbial communities’ long-term impacts on aesthetic value, production of acid and other metabolites, and alteration of microenvironments, which may cause erosion and rock weathering. Those findings have led to the removal of plants growing on the top of parts of the Great Wall. But the effects of biocrusts look different for earthen landmarks, and communities of cyanobacteria and moss actually increase the Great Wall’s stability and improve its resistance to erosion, according to the new paper.

Examining samples taken from over 483 kilometers across eight rammed earth sections of the site built during the Ming Dynasty between 1368 and 1644, the study authors found that more than two-thirds of the area is covered in biocrusts. When the researchers compared the stability and strength of samples layered in biocrust with samples sans “Earth’s living skin,” they discovered that samples with biocrusts were as much as three times stronger than those without.

“They thought this kind of vegetation was destroying the Great Wall. Our results show the contrary,” said study coauthor Bo Xiao, a professor of soil science at China Agricultural University. “Biocrusts are very widespread on the Great Wall and their existence is very beneficial to the protection of it.”

Made up of components such as cyanobacteria, algae, moss, fungi and lichen, biocrusts dwell on the topsoil of drylands. Covering an estimated 12% of the planet’s surface, the communities of tiny plants and microorganisms can take decades, or longer, to develop. Forming miniature ecosystems, biocrusts stabilize soil, increase water retention, and regulate nitrogen and carbon fixation.

They are able to do so partly thanks to a dense biomass, which acts as an “anti-infiltration layer” for soil pores under the right conditions, as well as a natural absorption of nutrients that promote salt damage. The secretions and structural layers of biocrusts also intertwine to form a “sticky network” of aggregating soil particles that promote strength and stability against corrosive forces threatening the Great Wall, according to the new study.

Climatic conditions, the type of structure and type of biocrust all play a role in a biocrust’s protective function, with its reduction of erodibility “much greater” than its risk of weathering, the researchers found.

“(Biocrusts) cover the Great Wall like a blanket that separates the Great Wall from air, from water, from wind,” Xiao said.

The study’s authors also say their work makes a case for exploring the possibility of cultivating biocrusts for preservation of other rammed earth heritage sites worldwide.

(SD-Agencies)

中国长城有些部分是夯土结构，由天然材料与土壤夯实而成，这些部分一直被视为该建筑结构的薄弱环节。但一项新研究发现，这些土层的外面形成了一道天然防线以抗衡自然老化风险。

这项研究上月发表在《科学进展》杂志上，土壤生态学家马修•鲍克是合著者之一，他说长城土壤表面覆盖着一层“生物皮肤”，由微小、无根的植物和微生物组成，称为生物土壤结皮。

鲍克是美国北亚利桑那大学副教授，他说：“（生物土壤结皮）在全世界干旱地区的土壤中很常见，但很少出现在人造建筑上。”

过去的研究发现，地衣和苔藓生物簇会破坏文物的砖石结构，因为微生物群落影响美观，产生酸和其他代谢物，改变微环境并导致侵蚀和岩石风化。基于这些研究，人们移除了长城顶上的植被。但新论文指出，生物土壤结皮对土壤结构会产生不同影响，蓝藻和苔藓群实际上让长城更稳定，提高了建筑的抗侵蚀能力。

作者研究了1368年至1644年间建造的明长城遗址中超过483公里的范围内八个夯土段采集的样本，发现超过三分之二的区域被生物土壤结皮覆盖。通过比较覆盖了“生物皮肤”的样本和未被覆盖样本的稳定性和强度，研究人员发现前者是后者强度的三倍。

该研究合著者、中国农业大学土壤学教授肖波说：“之前认为植被会破坏长城。我们的研究表明恰恰相反。生物土壤结皮在长城上很普遍，对保护长城非常有益。”

土壤结皮由蓝藻、藻类、苔藓、真菌和地衣等组成，栖息在干旱地区的表土上。这些由微小植物和微生物组成的群落估计覆盖了地球表面12%，

需要几十年甚至更长的时间形成。这些微型生态系统可以稳定土壤，增加保水性，调节氮和碳沉淀。

这些密集的生物集群在适当条件下可作为土壤孔隙的“防渗层”，还能吸收产生盐害的营养物质。研究发现，生物集群的分泌物和这些结构交织在一起形成一个“粘性网络”，从而增加土壤的强度和稳定性，抵御侵蚀。

研究人员发现，气候条件、结构类型和生物簇类型都会影响生物土壤结皮的保护功能，它们抵御侵蚀的作用“远大于”增加风化的风险。肖波说：“（生物土壤结皮）覆盖在长城上，就像一块毯子，把它和空气、水、风隔绝开。”

报告作者还说，研究提供了依据，将进一步探索培养生物土壤结皮的可行性，以保护世界各地夯土结构的遗址。

(Translated by Debra)

Words to Learn

相关词汇

【夯实】hāngshí

compress

press or squeeze together

【土制的】tǔzhì de

earthen

made of earth or of baked clay