Submitted to: Rhizosphere
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2016
Publication Date: 8/18/2016
Citation: Blank, R.R., Mackey, B.E., Morgan, T.A. 2016. Soils conditioned by native vegetation and by the exotic invasive annual grass Bromus tectorum: do a native perennial and two exotic grasses sense the substrates similarly. Rhizosphere. 2:38-47.
Interpretive Summary: Invasion by the exotic annual grass cheatgrass often increases availability of soil nitrogen (N) relative to soils occupied by native vegetation. We tested if the native perennial grass Snake River wheatgrass and the exotic perennial grass crested wheatgrass also benefit from elevated soil nutrient availability due to conditioning by cheatgrass. The experiment used soil from three cheatgrass-dominated areas, paired with adjacent areas dominated by native vegetation in the northern Great Basin. Soils planted to cheatgrass, Snake River wheatgrass, and crested wheatgrass and grown for 70 days (controls were left unplanted). Following harvest, above-ground and root biomass, tissue N and P, and several measures of soil availability of N and P were quantified. Above-ground biomass of the exotics cheatgrass and crested wheatgrass averaged over 2 times greater in cheatgrass conditioned soil relative their growth in native conditioned soil; biomass of the native Snake River wheatgrass was statistically similar when grown in either soil treatment. Overall, plants grown in soil conditioned by cheatgrass had greater tissue N and P than plants grown in soil conditioned by native vegetation. Overall, our data suggest that soil conditioned by invasion of cheatgrass is a superior growth medium providing greater availability of N and P for the two exotics tested than soil conditioned by native vegetation. We conjecture that the native Snake River wheatgrass did not benefit from cheatgrass conditioned soil because it does not have the inherent nutrient uptake kinetics and growth rate to efficiently utilize elevated N and P availability.
Technical Abstract: Invasion by the exotic annual grass Bromus tectorum often increases soil nutrient availability. It is unclear, however, if other grasses benefit from this higher nutrient status. Soil from three sites in the northern Great Basin U.S.A. conditioned by B. tectoruminvasion (BTCS=B. tectorum conditioned soil), paired with adjacent areas conditioned by native vegetation (NCS=native conditioned soil), were freshly-collected. Replicates were sown to B. tectorum, Elymus wawawaiensis (native perennial grass), and Agropyron cristatum (exotic perennial grass) and grown for 70 days. Above-ground biomass of the exotics B. tectorum and A. cristatum averaged over 2 times greater in BTCS relative to their growth in NCS. Above-ground biomass of the native E. wawawaiensis was statistically similar whether grown in BTCS or NCS. Overall, plants grown in BTCS had statistically similar tissue N and significantly greater tissue P concentrations compared to plants grown in NCS. The native perennial grass had significantly more negative '15N values when grown in BTCS relative to NCS suggesting it is accessing isotopically different N sources between the two substrates that B. tectorum or A. cristatum do not sense. Post-harvest soil mineral N was significantly greater for BTCS compared with NCS. Grown in BTCS, both exotic grasses significantly reduced soil mineral N relative to unplanted controls, but the native grass did not. The variable with the strongest explanatory power for above-ground plant mass was % utilization of the pre-plant soil pool of bicarbonate-extractable P. For the two exotics tested, BTCS is a superior growth medium than NCS. The native E. wawawaiensis may not benefit from growth in BTCS because it has slower nutrient uptake kinetics and growth to efficiently utilize greater N and P availability.