Comparisons of Root-Zone Microbial Communities in Response to Vegetation Changes in a Rangeland Soil.

In the Intermountain West, cheatgrass (Bromus tectorum) has infested over 40 million hectares of native sagebrush – bunchgrass communities and these cheatgrass dominated communities appear quite resistant to re-invasion by native perennials. We hypothesize that positive feedbacks resulting from shifts in nutrient cycling and microbial community structure are responsible for the long-term persistence of cheatgrass dominated communities. We are examining the soil microbial and nutrient cycling characteristics in soil collected from replicated restoration plots established 24 years ago in Western Colorado. We compare soil bacterial, fungal and arbuscular mycorrhizal fungal (AMF) communities in response to contrasting soil history and plant species dominance. Bacterial and fungal community structure in these soils was assessed by automated ribosomal intergenic spacer analysis (ARISA), a DNA profiling approach. Preliminary results suggest only minor shifts in the overall microbial community structure in soils with contrasting plant communities. The survey of AMF was accomplished by DNA extraction from plant roots followed by PCR amplification, cloning, and sequencing of the variable region of the 18S rRNA gene targeting AMF. We examined DNA extracted from roots of four plant species (sagebrush, needle-thread grass, bluebunch wheatgrass, and cheatgrass). Our results showed that plant species could affect shifts in the AMF community and that these differences were more pronounced when soils had been fumigated 24 years prior than in the native plant control plots. In soils that had received fumigation treatment, different native plants had a different cohort of AMF; but in the control native treatment, the composition of the AMF community was more similar on roots of the different plants. These investigations examine whether the intrinsic characteristics of the soil or plant history are more influential on shifts in nutrient cycling processes.