Understanding competition for soil nitrate between common shrub-steppe, potential reclamation species, and common invasive species is necessary to identify mechanisms associated with ecosystem invasion and can assist with developing weed management scenarios. We designed a field experiment to evaluate the differential competitive effects of the invasive annual grass downy brome, the invasive biennial forb dyer's woad, and the reclamation shrub prostrate kochia, on nitrate acquisition of the perennial grass crested wheatgrass, the native forb western yarrow, and the native shrub big sagebrush. Individual plants were grown in two-plant neighborhoods, and a K¹⁵NO₃ tracer was injected into the soil between plants and recovered from leaf material after 5 to 11 d. We also evaluated neighbor effects on shoot and root growth, leaf carbon : nitrogen ratio, and leaf nitrogen concentrations to better understand how these traits are associated with differences in nitrate acquisition and nitrogen allocation among the three growth forms. Nitrate acquisitions by crested wheatgrass and western yarrow were significantly lower when competing with downy brome than with dyer's woad and prostrate kochia; however, competitors had similar, negative effects on nitrate acquisition by big sagebrush. Nitrate acquisition ratios between competing neighbors revealed that: (1) the grasses always acquired more nitrate than neighbors of a different growth form, (2) western yarrow was equally competitive with dyer's woad and prostrate kochia, and (3) all neighbors acquired more nitrate than big sagebrush. More successful competition for nitrate in the grasses was associated with greater specific root length. Compared to species of the same respective growth form, the two invasive weeds (downy brome and dyer's woad) and prostrate kochia always had significantly lower leaf carbon : nitrogen ratio, and greater leaf nitrogen concentration, which have been broadly correlated with leaf lifespan and nutrient use efficiency, and indicate differing strategies to persist in the semiarid shrub-steppe ecosystems.

Nomenclature: Big sagebrush, Artemisia tridentat (Beetle & A. Young) Welsh; crested wheatgrass, Agropyron cristatum (L.) Gaertner; Downy brome, Bromus tectorum L. BROTE; dyer's woad, Isatis tinctoria L. ISATI; prostrate kochia, Kochia prostrata (L.) Schrad; western yarrow, Achillea lanulosa Nutt.