Ecologists are only beginning to understand how spatial scale can influence patterns in distributions of soil organisms. In arid ecosystems, the distribution of aboveground organisms has been characterized as heterogeneous and patchy. Does the abundance and diversity of belowground organisms mirror aboveground patterns? Few studies have empirically examined the distributions of soil microbes in arid environments. Furthermore, the appropriate measurement scales to capture microbial variation are poorly understood. Critical components of the soil microbial community in arid ecosystems are arbuscular mycorrhizal fungi (AMF), obligate root symbionts that confer increased resource uptake in exchange for host plant photosynthates. In June 2004, we used hierarchical sampling to measure AMF abundance and diversity within semi-arid Artemisia shrublands in southern Utah (USA) at four spatial scales: shrub/canopy microsites (1m), sites (1ha), regions (5,000ha), and entire study area (600,000ha). We assessed AMF abundance using four different measures: hyphal density, glomalin-related soil protein (GRSP), spore density and relative viable propagule abundance as measured by a greenhouse bioassay.
Results/Conclusions
Hyphal density, GRSP, and viable propagules varied at the larger regional scale, such that certain regions had higher levels of these AMF measures than others. Differences in AMF abundance across regions are attributed to variation in soil organic matter (r=0.73) and phosphorus (r=0.48). At the smaller microsite scale, hyphal density and viable propagules did not vary, such that similar amounts were found between shrub canopies and interspaces. However, GRSP and spore density differed at the microsite scale and were both more concentrated underneath shrub canopies. Overall, AMF spore species richness was high across all sites, with a total of 47 observed spore species and an average of 10 species per sample. This level of diversity is higher than that of certain mesic sites which have been studied over several seasons and years. Spore richness and evenness were similar across both microsite and regional scales. Variation in AMF abundance increased with scale for certain measures, but not for others. This study indicates that soil microbes, even obligate symbionts, are not restricted to life in the rhizosphere and can extend into unvegetated interspaces. Furthermore, AMF abundance can be variable or constant depending on the spatial scale and fungal measure in question. This is important because most studies sample at a single scale and measure one AMF indicator. An improved understanding of AMF spatial variation is required before small-scale measurements are used to predict biome levels of AMF abundance and diversity.
