Biological Soil Crusts and the Fertile Island Effect: Soil-Geomorphic Insights From the Mojave Desert, USA.

Biological soil crusts (BSCs), composed of cyanobacteria, moss, and lichen, protect arid soil surfaces.  These microbial-sediment complexes influence dust capture, fix nutrients, and manage soil resources.  This study assessed the impact of soil-geomorphology and the distribution of BSCs on the development of fertile islands within the Mojave.  Extensive site characterization, soil surface sampling, and soil physicochemical analyses were conducted.  ANOVA, t-tests, and multivariate statistical analyses indicate that fertility relationships are strongly tied to soil-geomorphology and soil interspace cover.  Soil-geomorphology controls landscape stability, surface characteristics, and soil development that ultimately determine interspace cover.  Interspace cover controls the relative magnitude of the fertile island effect.  The most dramatic islands of fertility occur in areas of desert pavement cover, where moderate interspace dust accumulation and high runoff lead to the greatest disparity between shrub canopy “Fertility Islands” and interspace “Fertility Deserts”.  Moss-lichen crusts capture large volumes of dust and increase moisture-availability by creating vesicular horizons that trap water directly underneath the biological crusts.  Enhanced biological activity promotes nutrient availability.  These crusts provide interspace “Oases of Fertility” that surround shrub “Fertility Islands”.  Cyanobacteria-dominated sand sheets have low dust capture and water availability.  As a result, overall fertility is low, while the nutrient disparity between canopies and interspaces is moderate.  These canopies can therefore be considered “Islands of Fertility” and interspaces considered “Fertility Deserts”.  Biogeochemical patterns are primarily controlled by inferred dust-related nutrient inputs, water availability, and biological nutrient transformations.   Our hierarchical and scalar model of soil resource allocation provides greater insight into the complexities of nutrient distributions in the Mojave.  We propose the use of the soil-geomorphic template as a conceptual framework for understanding the expression of fertile islands in arid lands.  This new landscape perspective should be considered as scientists, policy makers, and land managers address future desertification dynamics.