Long-term nitrogen isotope dynamics in Encelia farinosa reflect plant demographics and climate.

While plant δ N values have been applied to understand nitrogen (N) dynamics, uncertainties regarding intraspecific and temporal variability currently limit their application. We used a 28 yr record of δ N values from two Mojave Desert populations of Encelia farinosa to clarify sources of population-level variability. We leveraged > 3500 foliar δ N observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to δ N values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and δ N, intra-plant fractionations, and relationships between soil and plant δ N values. We observed extensive within-population variability in foliar δ N values and found plant age and foliar %N to be the strongest predictors of individual δ N values. There were consistent differences between root, stem, and leaf δ N values (spanning c. 3‰), but plant and bulk soil δ N values were unrelated. Plant-level variables played a strong role in influencing foliar δ N values, and interannual relationships between climate and δ N values were counter to previously recognized spatial patterns. This long-term record provides insights regarding the interpretation of δ N values that were not available from previous large-scale syntheses, broadly enabling more effective application of foliar δ N values.