Soil resource heterogeneity in the Mojave Desert

Heterogeneity of soil resources was investigated in the Mojave Desert to better understand spatial variability of soil attributes in a Larrea tridentata–Ambrosia dumosa community. Six soil samples were collected at each of eight microsites, with this sampling scheme replicated for three sites that were 0·7 km apart on a 10-km long bajada. The eight microsites were: five different shrub types (individual shrubs of L. tridentata, individual shrubs ofLycium pallidum , individual shrubs of A. dumosa, mixtures of shrub species with small mammal burrows, and mixtures of shrubs without small mammal burrows), a grass microsite (around individual plants of the tussock grass Pleuraphis rigida), and two microsites without perennial vegetation (one within small washes and the other within the interspace between perennial plants). Fifteen soil parameters were assessed, and all were found to differ among microsites. Nutrients were assessed as the pools extractable by vegetation. Mineral nutrients except for Ca, organic matter, and litter were higher in soils collected from shrub microsites compared to the others. Shrub microsites with small mammal burrows had significantly more N and P than those without burrows. Non-vegetated microsites in washes and plant interspaces contained very low N and P levels. Mg, Na, pH and cation exchange capacity were highest in soils under Lycium shrubs, while Ca levels were highest in wash soils. Arbuscular mycorrhizal spore densities were generally low (0–0·2 spores g−1 soil) except for Lycium soils, which contained 1·8 spores g−1 soil. Wash and interspace soils lacked spores but had mycorrhizal inoculum potential values similar to the other microsites, indicating that mycorrhizal propagules were present. Thus, the presence of perennial shrubs strongly influenced microsite soil characteristics and resulted in higher nutrient levels. Small mammal burrows further enhanced the mineral nutrient content of soils. In addition, all soil characteristics except for spore density differed among the three sites across the bajada, and interactions between microsite and site were common. Clearly, soil resources for plants are spatially heterogeneous in this desert ecosystem and differ greatly between adjacent micro- and macrosites in thisL. tridentata–A. dumosa community.