Sem-derived microstructure of cyanobacterial and moss biocrusts and their connections to mechanical stability of two contrasting dryland soils

As an important biological component of the soil surface in drylands, biocrusts play an indispensable role in maintaining soil ecosystem multifunctionality. However, the microstructure of these biocrusts remains unclear. For this end, both cyanobacterial and moss crusts inhabiting sandy and loessial soils were collected on the Chinese Loess Plateau, and their profiles were thoroughly studied using scanning electron microscope (SEM). A full SEM image of each biocrust sample were quantitatively analyzed and their mechanical stability was measured. The SEM images showed that moss crusts had a more complex soil structure compared to cyano crusts. Furthermore, the filament density, ratio indices of soil particles to filaments (N_P/L_F and L_F/A_P), and the pore area ratio (PAR) of the moss crusts were higher than that of the cyano crusts on both soils. The penetration resistance (PR) and shear force of the moss crusts were higher than those of the cyano crusts on both soils, but the same type of biocrusts was less stable on the sandy soil than that on loessial soil. In addition, our results showed that the microstructure of biocrusts showed an obvious correlation with their PR, with an apparent threshold value for the effect of soil particles to filaments ratio indices of biocrusts on their mechanical stability. On the sandy soil, the PR of the biocrusts reached the maximum when N_P/L_F was 10.0–13.0 mm⁻¹, and L_F/A_P was 12.0–14.0 mm m⁻² (cyano crusts) and 21.0–29.0 mm m⁻² (moss crusts). While on the loessial soil, the PR of the biocrusts reached its maximum when N_P/L_F and L_F/A_P were 37.0–40.0 mm⁻¹ and 11.0–12.5 mm m⁻², respectively. Our study elucidated the microstructural characteristics of the biocrust profiles on the different soils and their thresholds affecting mechanical stability, which may help in the site-specific application of biocrusts in soil remediation and ecological restoration.