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. 2023 Dec 8;9(49):eadk5892.
doi: 10.1126/sciadv.adk5892. Epub 2023 Dec 8.

Biocrusts protect the Great Wall of China from erosion

Yousong Cao  1   2   3 , Matthew A Bowker  4   5 , Manuel Delgado-Baquerizo  6 , Bo Xiao  1   2   3   7
Affiliations

Biocrusts protect the Great Wall of China from erosion

Yousong Cao et al. Sci Adv. .

Abstract

The Great Wall of China, one of the most emblematic and historical structures built by humankind throughout all of history, is suffering from rain and wind erosion and is largely colonized by biocrusts. However, how biocrusts influence the conservation and longevity of this structure is virtually unknown. Here, 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. 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%. We further found that the protective function of biocrusts mainly depended on biocrust features, climatic conditions, and structure types. Our work highlights the fundamental importance of biocrusts as a nature-based intervention to the conservation of the Great Wall, protecting this monumental heritage from erosion.

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Figures

Fig. 1.
Fig. 1.. Porosity and water-holding capacity of biocrusts and bare rammed earth on the Great Wall.
(A) Box chart of capillary porosity of bare (bare rammed earth), cyan (cyanobacterial biocrusts), and moss (moss biocrusts) in different climates (arid, arid climate; S-arid, semiarid climate) and structure types (wall; fort, fortress). The red stars indicate the mean value. (B) Box chart of total porosity. (C) Box chart of Sc (saturated water content) and Fc (field capacity). (D) Total porosity in different defense districts (NX, Ningxia; YS, Yansui; SX, Shanxi; DT, Datong). The * designates a significant difference between different defense districts at the 0.05 level of probability.
Fig. 2.
Fig. 2.. Mechanical stability of biocrusts and bare rammed earth on the Great Wall.
(A) ɛ (strain) and Ϭ (stress) curve of bare (bare rammed earth), cyan (cyanobacterial biocrusts), and moss (moss biocrusts). (B) Box chart of compressive strength. The red stars indicate the mean value. (C) Box chart of penetration resistance. (D) Box chart of shear strength in different climate regions. (E) Box chart of compressive strength in different structure types (wall; fort, fortress).
Fig. 3.
Fig. 3.. Erodibility indicators of biocrusts and bare rammed earth on the Great Wall.
(A) Box chart of R (macroaggregate content) of bare (bare rammed earth), Cyan (cyanobacterial biocrusts), and Moss (moss biocrusts). The red stars indicate the mean value. (B) Box chart of soluble salt content. (C) Box chart of Ec (electrical conductivity). (D) Box chart of GMD. (E) Box chart of MWD. (F) Box chart of K (erodibility coefficient) in different climates (arid climate and semiarid climate) and structure types (wall and fortress). (G) K in different defense districts (Ningxia, Yansui, Shanxi, and Datong).
Fig. 4.
Fig. 4.. Diagram showing the overall protective functions of biocrusts against erosion in the Great Wall as well as the influencing pathways.
In comparison to bare rammed earth, biocrusts exert protective effects mainly by reducing erosive force, strengthening soil mechanical stability, and reducing soil erodibility. The developmental level and protective function of biocrusts on fortresses surpassed that on walls. EPS, extracellular polymeric substances. C means other organic matter such as exudate, secretion, mucilage, and sloughed residue.
Fig. 5.
Fig. 5.. Location of study sites.
(A) Location of the Great Wall. (B) Zoomed in map of the sampling sites: (C) Hengchengcun, (D) Xingwuying, (E) Yangjiquan, (F) Liuyangbao, (G) Changchengzhen, (H) Laoyingzhen, (I) Erfenguan, and (J) Bianqianghao. (K) Cyanobacterial biocrusts. (L) Moss biocrusts.
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