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

Abstract

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.