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Competition for soil nitrate and invasive weed resistance of three shrub-steppe growth forms

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 may assist with developing weed management scenarios. We designed a field experiment to evaluate the differential competitive effects of the invasive annual grass downy brome (Bromus tectorum L.), the invasive biennial forb dyer’s woad (Isatis tinctoria L.), and the reclamation shrub prostrate kochia (Kochia prostrata [L.] Schrad.) on nitrate acquisition of the perennial grass crested wheatgrass (Agropyron cristatum [L.] Gaertner x A. desertorum [Fisch. ex Link] Schultes), the native forb western yarrow (Achillea lanulosa Nutt.), and the native shrub big sagebrush (Artemisia tridentata var.wyomingensis [Beetle & A. Young] Welsh) at two sites in northern Utah. Individual plants were grown in two-plant neighborhoods and a KNO3 tracer was injected into the soil between plants and recovered from leaf material within 15 days. We also evaluated neighbor effects on shoot and root growth, leaf carbon: nitrogen ratio, and leaf N concentrations as well as a post hoc comparison of the six species to better understand how these traits are associated with differences in nitrate acquisition and nitrogen allocation among the three growth forms. Nitrate acquisition of crested wheatgrass and western yarrow 21 were lower when competing with downy brome than with dyer’s woad and prostrate kochia (P < 0.01); however, the three growth forms had similar competitive effects on nitrate acquisition of 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 an invasive forb and prostrate kochia, and 3) all neighbors acquired more nitrate than big sagebrush. Greater competition for nitrate in the grasses was associated with greater specific root length. The two invasive weeds and prostrate kochia always had lower leaf C: N ratio (P < 0.01), and greater leaf N concentration (P < 0.01), which have been broadly correlated with leaf longevity, nutrient use efficiency, and indicate differing strategies to persist in the semiarid shrub-steppe ecosystems.