Actinorhizal plant species capable of fixing atmospheric dinitrogen (N2) into plant available forms of nitrogen (N) are common and diverse in the dry montane ecosystems of the western United States. While several actinorhizal species are known to contribute N to more humid ecosystems via N2 fixation, the ecological role and impact on the N cycle of these putatively N2 fixing species is less understood in this region. In these ecosystems with generally low productivity, even a small change in N may impact plant vitality and ecosystem carbon (C) cycling. Here, we used foliar 15N natural abundance to measure differences in N sources for three widespread actinorhizal shrubs on two different mountain ranges: curlleaf mountain mahogany (Cercocarpus ledifolius), antelope bitterbrush (Purshia tridentata), and snowbrush ceanothus (Ceanothus velutinus). We asked how these species influenced N cycling and the N status of non-fixing plants in the surrounding areas by comparing foliar δ15N and %N and soil δ15N, %N, %C, and C:N ratios. We also used assays of potential nitrification rates, denitrifying enzyme activity, and a stable isotope mass balance model to assess whether different N loss pathways could provide an alternative explanation for isotopic differences between actinorhizal and non-actinorhizal vegetation types.
Results/Conclusions
Several lines of evidence suggest that actinorhizal plants fixed N2 and influenced the N status of the surrounding ecosystem. Foliage of actinorhizal plants had significantly lower foliar δ15N than nearby non-actinorhizal shrubs and trees. Non-actinorhizal shrubs and forbs growing in actinorhizal plots had significantly depleted foliar δ15N (1-4‰) and higher %N (~0.5%) than the same shrub species growing in plots without actinorhizal shrubs. Soils had depleted δ15N and higher %N in actinorhizal plots relative to non-actinorhizal plots. There were no significant differences in potential rates of soil nitrification and denitrification between sites. The mass balance model suggested that fractionating N losses would have to be 15-32% higher in the reference plot than actinorhizal plots for N losses, rather than N2 fixation, to explain the foliar δ15N differences we observed. However, we observed a stronger signal of N2 fixation in C. ledifolius and P. tridentata between the two mountain ranges, suggesting that small shifts in topography and climate may influence N2 fixation by actinorhizal plants in the region. The presence of actinorhizal plants changes the availability of plant N in of these dry ecosystems with likely consequences for plant productivity and community composition.