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Effects of Land Surface Characteristics on Pedogenesis, Biological Soil Crust Community Diversity, and Ecosystem Functions in a Mojave Desert Piedmont Landscape

  • Author(s): Pietrasiak, Nicole
  • Advisor(s): Graham, Robert C
  • et al.

Abiotic and biotic land surface properties are often highly heterogeneous but can assemble in a repetitive manner forming landform mosaics at a mesoscale (ten to hundreds of meters). Nonetheless, we still do not fully understand their interactions or the mechanisms involved that change these properties during landscape evolution. The goal of this work was to relate land surface properties to functional group diversity of biological soil crusts and vascular plants, ecosystem functions and pedogenesis within a Mojave Desert landscape. Seven mosaic types were visually identified that occurred on three geomorphic-aged surfaces: young bars and swales, intermediate-aged flattened bars, flattened swales and bioturbation units, and old desert pavement and shrub zones. Sixty-three randomly selected landform mosaics served as study plots. In each plot a suite of morphometrical, physical and biological variables were determined. Multivariate analysis revealed that landform mosaics are statistically distinct according to specific sets of abiotic and biotic land surface properties. Within each landform type I also detected significant differences in functional group diversity of plants and biological soil crusts. The aggregate stability, nitrogen and carbon fixation of crusts were measured. An area based quality index (ABQI) was developed to evaluate these microbial communities based on ecological functions. The ABQI was computed for each landform mosaic and statistically compared. The ABQI was highest for bars and lowest for desert pavements.

Two landform evolutionary trajectories were identified: an abiogenic and a biogenic tract. In the abiogenic track, vegetation contracted and the surface increased in physical components. In the biogenic track vegetation diversity and abundance increased, and crust cover and diversity was high. Both of these trajectories were linked to different pedogeneses. Abiogenic soils promoted vesicular and calcic horizon development, as well as sodicity and alkalinity. Organic carbon and total nitrogen decreased. Biogenic soils were well mixed. No vesicular horizon could develop and calcic horizons were weakly expressed. Furthermore, these soils increased in organic carbon and total nitrogen.

Overall, this work showed that desert landscapes are highly diverse in landform mosaics defined by abiotic and biotic land surface properties at the mesoscale. Moreover, strong linkages and feedbacks occur between physical, biotic and pedologic landscape components that over time result in quite contrasting surface features.

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