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) 中國長城有些部分是夯土結構，由天然材料與土壤夯實而成，這些部分一直被視為該建筑結構的薄弱環節。但一項新研究發現，這些土層的外面形成了一道天然防線以抗衡自然老化風險。 這項研究上月發表在《科學進展》雜志上，土壤生態學家馬修?鮑克是合著者之一，他說長城土壤表面覆蓋著一層“生物皮膚”，由微小、無根的植物和微生物組成，稱為生物土壤結皮。 鮑克是美國北亞利桑那大學副教授，他說：“（生物土壤結皮）在全世界干旱地區的土壤中很常見，但很少出現在人造建筑上。” 過去的研究發現，地衣和苔蘚生物簇會破壞文物的磚石結構，因為微生物群落影響美觀，產生酸和其他代謝物，改變微環境并導致侵蝕和巖石風化。基于這些研究，人們移除了長城頂上的植被。但新論文指出，生物土壤結皮對土壤結構會產生不同影響，藍藻和苔蘚群實際上讓長城更穩定，提高了建筑的抗侵蝕能力。 作者研究了1368年至1644年間建造的明長城遺址中超過483公里的范圍內八個夯土段采集的樣本，發現超過三分之二的區域被生物土壤結皮覆蓋。通過比較覆蓋了“生物皮膚”的樣本和未被覆蓋樣本的穩定性和強度，研究人員發現前者是后者強度的三倍。 該研究合著者、中國農業大學土壤學教授肖波說：“之前認為植被會破壞長城。我們的研究表明恰恰相反。生物土壤結皮在長城上很普遍，對保護長城非常有益。” 土壤結皮由藍藻、藻類、苔蘚、真菌和地衣等組成，棲息在干旱地區的表土上。這些由微小植物和微生物組成的群落估計覆蓋了地球表面12%， 需要幾十年甚至更長的時間形成。這些微型生態系統可以穩定土壤，增加保水性，調節氮和碳沉淀。 這些密集的生物集群在適當條件下可作為土壤孔隙的“防滲層”，還能吸收產生鹽害的營養物質。研究發現，生物集群的分泌物和這些結構交織在一起形成一個“粘性網絡”，從而增加土壤的強度和穩定性，抵御侵蝕。 研究人員發現，氣候條件、結構類型和生物簇類型都會影響生物土壤結皮的保護功能，它們抵御侵蝕的作用“遠大于”增加風化的風險。肖波說：“（生物土壤結皮）覆蓋在長城上，就像一塊毯子，把它和空氣、水、風隔絕開。” 報告作者還說，研究提供了依據，將進一步探索培養生物土壤結皮的可行性，以保護世界各地夯土結構的遺址。 (Translated by Debra) Words to Learn 相關詞匯 【夯實】hāngshí compress press or squeeze together 【土制的】tǔzhì de earthen made of earth or of baked clay | 