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

Abstract

While plant δ15N 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 δ15N values from two Mojave Desert populations of Encelia farinosa to clarify sources of population-level variability. We leveraged > 3500 foliar δ15N observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to δ15N values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and δ15N, intra-plant fractionations, and relationships between soil and plant δ15N values. We observed extensive within-population variability in foliar δ15N values and found plant age and foliar %N to be the strongest predictors of individual δ15N values. There were consistent differences between root, stem, and leaf δ15N values (spanning c. 3‰), but plant and bulk soil δ15N values were unrelated. Plant-level variables played a strong role in influencing foliar δ15N values, and interannual relationships between climate and δ15N values were counter to previously recognized spatial patterns. This long-term record provides insights regarding the interpretation of δ15N values that were not available from previous large-scale syntheses, broadly enabling more effective application of foliar δ15N values.