Biological Soil Crusts in the Mojave Desert, USA: Micromorphology and Pedogenesis

TLDR

Biological soil crusts are bio‑sedimentary complexes that stabilize arid soils and influence water and nutrient dynamics, yet the interactions between their biota and sediments remain poorly understood. The study proposes a new conceptual model of hot‑desert BSC development that links micro‑ and macro‑scale processes across wet–dry cycles. The authors performed a detailed micromorphological survey of BSCs in Nevada using petrographic, light, SEM, and EDS analyses of over 1,800 observations, correlating micro‑features with macro‑scale crust morphology and soil geomorphology. The study found that BSC micro‑structures vary with crust morphology, trap surface water, and incorporate dust nutrients, thereby shaping landscape‑scale water dynamics, biogeochemical cycling, and overall arid pedogenesis.

Abstract

Biological soil crusts (BSCs) are bio-sedimentary complexes that play critical ecological roles in arid landscapes; however, the interactions between component biota and sediments are poorly understood. A detailed micromorphological investigation of BSC development and crust microstructure in the Muddy Mountains Wilderness Area, Nevada, examined features in thin section using petrographic microscopy, light microscopy, scanning electron microscopy, and energy dispersive x-ray spectroscopy. The >1800 microscopic observations were linked to crust macroscale features and soil geomorphology. Complex bio-sedimentary structures of BSCs reflect a dynamic genetic history and diverse formative processes, including: (i) stabilization and authigenic mineral precipitation; (ii) wetting–drying and expansion–contraction; (iii) dust capture; (iv) microscale mass wasting; and (v) vesicular (Av) horizon formation. A new conceptual model for hot deserts illustrates how these processes co-develop with BSC succession, during countless wet–dry cycles, to build up pinnacle microtopography while simultaneously forming Av horizons in the bio-rich and bio-poor zones. Complex surficial and internal bio-sedimentary structures, which vary as a function of crust morphology, trap surface water for uptake by crust organisms, while dust influx provides a source of nutrients. These phenomena influence landscape-scale water dynamics and biogeochemical cycling, increasing the availability of soil resources during times of biotic stress. Biological soil crusts uniquely facilitate the accumulation, morphology, and ecosystem function of dust and should, therefore, be considered critical agents in arid pedogenesis and landscape development.