Nitrogen isotope patterns of biological soil crusts and two representative Mojave Desert shrubs, Larrea tridentata and Ambrosia dumosa

Biological soil crusts (BSC) are composed of cyanobacteria, lichens, and mosses which provide stability and nutrients to the soils of arid ecosystems. The role of biological soil crusts as a potential nitrogen source for shrubs in the nitrogen-limited eastern Mojave Desert, USA was investigated over one year using a stable isotope approach. A common lichen in BSCs of the Mojave Desert, Collema spp., fixes atmospheric nitrogen into a biologically active compound that, during the wet winter season, may be deposited into the surrounding soil. Local plants may utilize this biologically fixed nitrogen as a nutrient source. Stable nitrogen isotope ratios (¹⁵N/¹⁴N, δ¹⁵N) were used to measure changes in nitrogen dynamics in Mojave Desert soils and two common shrubs, Larrea tridentata and Ambrosia dumosa. Nitrogen isotopes were measured monthly for one year; results indicate that nitrogen isotope values fluctuate in both soils and plants similarly over time. L. tridentata had a δ¹⁵N value significantly higher than A. dumosa throughout the study, which is indicative of the different rooting depths of these species. Collema spp. showed a δ¹⁵N close to that of atmospheric nitrogen. All soils and plants showed similar δ¹⁵N patterns over time, suggesting that nitrogen pulses in the system are observable throughout the soil profile. These results suggest that nitrogen use patterns may be predictable on a local scale in the Mojave Desert, and that shifts in climate may lead to changes in nutrient dynamics in both soils and shrubs.