Actinorhizal species influence plant and soil nitrogen status of semiarid shrub-dominated ecosystems in the western Great Basin, USA

Actinorhizal plants form symbiotic root associations with dinitrogen (N₂) fixing Frankia and are abundant in North American cold deserts. However, the extent to which actinorhizal species are actively fixing N₂ or altering ecosystem nitrogen (N) availability remains unclear. We used the ¹⁵N natural abundance technique to measure how three widespread actinorhizal species in the western Great Basin of western North America acquired N and influenced soil N cycling and the N status of the surrounding non-fixing plant community. We compared foliar and soil N concentrations and δ¹⁵N and soil biogeochemistry between reference plots and plots dominated by actinorhizal species. Actinorhizal species may be actively fixing N and influencing the N status of the surrounding ecosystem. Foliar δ¹⁵N of actinorhizal shrubs was significantly depleted compared to non-actinorhizal species. Non-actinorhizal plants in the presence of actinorhizal species showed depleted foliar δ¹⁵N and higher foliar N concentrations than in reference plots. Rates of N transformations in reference plots were similar to rates in actinorhizal plots; isotopic differences between plot types could not be explained by differences in N loss pathways. Actinorhizal species influence N cycling and availability in N-limited cold deserts, in a manner similar to leguminous plants in warm arid regions.