Phosphorus acquisition by Bromus madritensis ssp rubens from soil interspaces shared with Mojave Desert shrubs

1. Bromus madritensis ssp. rubens (L.) Husnot (Red Brome) is an invasive an nual grass that is associated with increases in fire frequency and decrease s in perennial plant diversity. The success of Red Brome in the Mojave Dese rt has been attributed to its competitive ability, but competition between Red Brome and native shrubs for below-ground resources has not been investi gated previously. In this study we present a modification of previous dual- isotope methods that assesses competitive interactions and responses among plants under field conditions. We then use this method to (i) determine if direct competition for phosphorus (P) occurs between Red Brome and native s hrubs, and (ii) evaluate the effectiveness of phosphate acquisition by Red Brome from soil interspaces shared with different Mojave Desert shrub speci es. 2. Clipping Red Brome to remove approximate to 85% of its foliage on the da y prior to labelling soil interspaces with P isotopes did not have the desi red effect of inhibiting phosphorus uptake by Red Brome. Therefore we were unable to verify that direct competition for P occurred between Red Brome a nd native shrubs. 3. Nonetheless, by sampling plant tissue from unclipped strips of Red Brome that were centred between two shrubs, we were able to evaluate the effecti veness of phosphate acquisition by Red Brome from interspaces shared with d ifferent shrubs. There were no differences in the amount of phosphorus acqu ired by Red Brome from soil interspaces shared with the evergreen shrub Lar rea tridentata (D.C.) Cov. versus soil interspaces shared with the drought deciduous shrub Lycium pallidum Miers. However, Red Brome acquired more tha n six times as much P from soil interspaces shared with Larrea than from so il interspaces shared with the drought deciduous shrub Ambrosia dumosa Payn e. Less P uptake by Red Brome from soil interspaces shared with Ambrosia su ggests greater effectiveness of Ambrosia compared with Larrea to deplete so il P.