Nutrient limitation of native and invasive N2-fixing plants in Willamette Valley prairies

Background/Question/Methods

Nutrient rich conditions often promote plant species invasion in ecosystems, which has fostered conservation approaches that seek to reduce nutrient availability as a way to favor native species.  Consequently, nutrient additions are rarely used to promote plant species conservation, but additions of non-nitrogen nutrients may provide a novel approach for promoting native nitrogen-fixing species in otherwise nitrogen-limited ecosystems dominated by non-native species.  Lupinus oreganus is a threatened nitrogen-fixing plant that is often outcompeted by non-native grasses in remnant prairies in western Oregon and southwest Washington.  We tested the effect of non-nitrogen fertilization with phosphorus (P), other macro and micronutrients (M), and a combination of these treatments on the growth, reproduction, tissue nitrogen (N) content, and stable isotope composition of Lupinus at three sites in the Willamette Valley, Oregon.  We also tested for treatment effects on plant community composition and the cover, tissue N content, and stable isotope composition of Viciaspp., which are non-native annual weeds in these communities.

Results/Conclusions

We observed different responses between the native, perennial Lupinus and non-native, annual Vicia in tissue N, N fixation, and utilization in response to fertilization.  At sites where P or M promoted Lupinus reproduction or growth, the same nutrient generally increased Lupinus tissue N.  For Lupinus, δ¹⁵N values remained near 0‰ regardless of treatment, suggesting an obligate reliance on fixed N for N nutrition.  Nutrients added in our treatments enhanced N fixation, leading to higher tissue N.  In contrast, Vicia responded to both M and P additions with increased tissue N, and accompanying shifts in δ¹⁵N towards atmospheric values.  The concomitant increase in tissue N with a shift towards atmospheric ¹⁵N suggests that Vicia is a facultative N fixer, and likely derives a portion of N from both soil pools and fixation, depending on availability of soil N and/or suitability of factors that allow N fixation.  Multivariate analyses found no fertilizer effects on plant community structure.  These preliminary results suggest that while fertilization with non-nitrogen fertilizers succeeded in increasing the growth and reproductive capacity of Lupinus with few non-target effects, the apparently greater benefit of invasive nitrogen fixers to treatments suggest this should only be used cautiously.