Preferences for 15N-ammonium, 15N-nitrate,and 15N-glycine differ among dominant exotic and subordinate native grasses from a California oak woodland

Following an invasion of exotic annual grasses into California oak woodlands, grass species dominance shifted from native perennials to exotic annuals. In combination with other ecosystem and species characteristics, species-specific N preferences may influence species coexistence and dominance. If species N preferences follow dominance patterns in California oak woodlands, then the more dominant exotic grasses should prefer the most abundant inorganic soil N form (NH₄⁺), while the subordinate native grasses should prefer the less available inorganic (NO₃^?) or organic (glycine) soil N forms. To investigate this prediction, we applied ¹⁵N-labeled NH₄⁺, NO₃^?, and glycine to soil and measured % ¹⁵N recovery by two dominant annual grasses (Bromus diandrus and Bromus hordeaceus) and two subordinate perennial grasses (Elymus glaucus and Nassella pulchra). As expected, shoots of B. diandrus recovered more ¹⁵N-NH₄⁺ (74%) than either ¹⁵N-NO₃^? (51%) or ¹⁵N-glycine (39%). B. diandrus also captured at least 3.2 times more ¹⁵N-NH₄⁺ than subordinate grasses. Dominant B. hordeaceus, however, demonstrated no N form preferences. As hypothesized, shoots of subordinate E. glaucus and N. pulchra recovered 2.1–2.3 times more ¹⁵N-NO₃^? than ¹⁵N-NH₄⁺ and increased %N by 4.8–5.7% in response to the application of ¹⁵N-NO₃^?. Both subordinate grasses did not prefer ¹⁵N-glycine over ¹⁵N-NH₄⁺, suggesting that the importance of this N form in structuring species coexistence in California oak woodlands is minimal. These results support our theory that species N preferences follow dominance patterns in California oak woodlands. To further understand the role of these species-specific N preferences in structuring dominance, the importance of N form versus such characteristics as rooting distribution and species phenologies needs to be explored in the presence of interspecific competition.