Exotic plant species can become ecosystem engineers in their new habitatsand dramatically alter above- and belowground properties and processes.Cheatgrass (Bromus tectorum) is one such exotic annual grass that canpotentially convert diverse, nitrogen (N)-limited sagebrush steppeecosystems to homogenous, N-mineralizing environments. While many studieshave characterized the pools and fluxes of inorganic N species (i.e.,ammonium and nitrate) in sagebrush and cheatgrass rhizosphere soils, littleresearch has focused on the relative contributions of specific microbialgroups to N cycling in these soils. Previous work had determined that soilcommunity structure and composition from cheatgrass-invaded and uninvadedsagebrush/perennial bunchgrass plots in eastern Oregon and southwesternIdaho differed significantly, most notably in fungal abundance anddiversity. In order to assess the role of fungi in N cycling in sagebrushand cheatgrass rhizosphere soils, we added the fungal protein synthesisinhibitor cycloheximide (CHX) to a 15NH4+ pool dilution experiment. Wedetermined gross NH4+ production and consumption, net mineralization andnitrification, and microbial biomass C and N at 0-24 and 24-48 h.

Results/Conclusions

There were no differences between control sagebrush and cheatgrass soils inany of the measured gross or net N cycling rates. Gross NH4+ mineralizationand consumption significantly decreased in both soil types in the presenceof CHX. Cycloheximide-treated sagebrush soils exhibited marginallysignificant increases in net N mineralization and nitrification relative tocheatgrass soils during the 24-48 h incubation period. Our results suggestthat N cycling has not been measurably affected by cheatgrass invasion intosagebrush steppe ecosystems of the northern Great Basin. We provideevidence that soil fungi play a large role in N cycling in both sagebrushand cheatgrass soils and that, even when released from competition withfungi, bacterial communities do not necessarily respond and may beconsidered functionally redundant in this ecosystem. We further posit thatfungal heterotrophic nitrification may be a significant process in thesagebrush steppe.