The role of organic materials and calcium carbonates mediated by biocrusts in altering soil aggregate size fractions in drylands

Biological soil crusts (biocrusts) play a critical role in influencing soil biogeochemical properties and soil aggregate formation. While the effect of biocrusts on soil aggregate formation is well-established, the precise impact of biocrusts on specific aggregate size distribution (ASD) fractions, particularly in drylands, still requires conclusive determination. This study focused on the impact of biocrusts on organic material (OM) and calcium carbonate (CC) content in topsoil aggregates within the Sejzi Plain, Iran. Samples were collected from areas with and without biocrust coverage using a stratified random sampling method, and various soil properties were analyzed and compared. The study quantified OM and CC levels, assessed particle size distribution (PSD) through laser diffraction analysis, and compared aggregate size distribution (ASD). Scanning electron microscopy (SEM) observations revealed the presence of extracellular polymer substances (EPSs) in soils covered by biocrusts. High-performance liquid chromatography (HPLC) identified monosaccharides like glucose, arabinose, mannose, and xylose as components of the EPS polysaccharidic fraction. The ratio of hexose to pentose, calculated to be 2.41 in biocrust-covered soils, suggested the role of biocrusts in secreting EPSs into the soil. The results showed that the removal of CC and OM led to an increase in particle size in very fine sands, fine silts, and very fine silts, while aggregate size decreased in coarse sands in biocrust-covered soils. Soils lacking biocrusts exhibited minimal changes except in very coarse sands. In summary, the CC and OM produced by biocrusts enhance soil particle adhesion in fine silt and very fine sand fractions, contributing to the formation of coarse sand and very coarse sand aggregates.