Nitrogen Cycling and Root-Zone Microbial Communities in Rangeland Restoration Plots.

In the Intermountain West (USA), cheatgrass (Bromus tectorum) has infested over 40 million ha of native sagebrush – bunchgrass communities and these communities are quite resistant to re-establishment of desirable native perennials. We hypothesize that belowground feedbacks from shifts in nutrient cycling and microbial communities are responsible for the observed persistence of cheatgrass and ask whether intrinsic characteristics of the soil or plant history are more influential on these shifts. We examined microbial and nutrient dynamics in soils collected from replicated restoration plots established 25 years ago in Western Colorado, USA. We assessed ammonia–oxidizing bacteria (AOB) and archaea (AOA), soil bacteria, soil fungi and arbuscular mycorrhizal fungi (AMF) in response to contrasting soil history and plant species dominance. The AO community was examined by real-time PCR and clone libraries of amoA. The ratio of AOA /AOB amoA copies was higher (17) in the undisturbed native soil than in either the fumigated cheatgrass dominated (2.1) or the fumigated climax plant community soil (1.0). Theses AOB and AOA numbers were related to nitrification potentials and net mineralization. Bacterial and fungal community structure was assessed by amplified ribosomal spacer analysis (ARISA). ARISA results suggested only minor shifts in the overall microbial community structure with the contrasting plant communities. The survey of AMF was accomplished by DNA extraction from plant roots (4 plant species) followed by PCR amplification targeting AMF, cloning, and RFLP of the 18S rRNA gene. Our results showed that plant species could affect shifts in the AMF community and that these differences were more pronounced in fumigated soils than in the native plant control soils. In fumigated soils, different plants had a different cohort of AMF; but in the control undisturbed soil, the composition of the AMF community was more similar on roots from different plants suggesting a soil history effect.