15 N uptake by mycorrhizal native and invasive plants from a N-eutrophied shrubland: a greenhouse experiment

Abstract

In the coastal sage scrub (CSS) community of southern California, Artemisia californica and other native shrubs are gradually being replaced by invasive annual grasses, especially Bromus madritensis ssp. rubens. This decline may be attributed, in part, to local atmospheric N deposition, which causes elevated soil NO3 −. Unaffected soils have low N with a relatively higher concentration of NH4 + than NO3 −. The objectives in this study were: (1) to compare the short-term uptake of 15NO3 − and 15NH4 + by B. madritensis and A. californica and (2) to discern whether 15NO3 − or 15NH4 + uptake is influenced by the mycorrhizal status of either plant species. Analysis of 15N concentrations indicated that both A. californica and B. madritensis took up more 15NO3 − than 15NH4 +, but overall, B. madritensis took up 6–15 times more of both forms of 15N than A. californica. Mycorrhizal A. californica had an increased 15NH4 + concentration in roots but not shoots. In B. madritensis, the only mycorrhizal response was a reallocation of 15NO3 −, with mycorrhizal plants retaining a higher proportion of 15NO3 − in roots. Overall, arbuscular mycorrhizae had a small effect on 15N uptake in this short-term study. However, the ability of B. madritensis to take up so much 15N may explain in part why it has been so successful in replacing A. californica in CSS.