Exploring the role of biological soil crusts in protecting rammed earth sites: A case study of the Shanhaiguan Great Wall in Bohai Bay

The aim of this study was to investigate the potential of biological soil crusts (BSCs), particularly filamentous cyanobacteria, in protecting rammed earth sites from erosion in the Bohai Bay region.

BSC samples were collected from various sites on and around the rammed earth Great Wall in Shanhaiguan. The diversity and characterization of BSCs of the Great Wall were analyzed using genetic testing techniques. Seven strains of filamentous cyanobacteria from BSCs were isolated, purified and identified. The protective effect of filamentous cyanobacteria inoculation on the soil of the Great Wall was assessed.

This study revealed greater bacterial diversity on natural surface crusts than on the wall itself, with lower sections of the wall exhibiting greater diversity than areas near the wind inlets at the top of the wall. Cyanobacteria constituted a dominant group in BSCs, accounting for 36.85–48.56% of the soil bacteria. Microcoleus and Leptolyngbya were identified as the dominant genera within Cyanobacteria. Filamentous cyanobacteria have sheaths, can resist drought and cold, and can produce polysaccharides. The filamentous cyanobacteria and rammed earth gravel interact with each other, and the polysaccharides provide bonding and covering effects, which jointly increase the stability of the soil particles. The soil erosion resistance in areas with filamentous cyanobacteria BSCs was approximately 40 times greater than that in areas without crust.

These findings suggest that filamentous cyanobacteria-based BSCs can effectively protect rammed earth sites from erosion. These results provide a scientific foundation for implementing BSCs in the preservation of rammed earth structures such as the Great Wall against soil erosion.