Long-Term Nitrogen Isotope Dynamics In Encelia Farinosa Reflect Plant Demographics And Climate

While plant delta N-15 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 delta N-15 values from two Mojave Desert populations of Encelia farinosa to clarify sources of population-level variability. We leveraged > 3500 foliar delta N-15 observations collected alongside structural, physiological, and climatic data to identify plant and environmental contributors to delta N-15 values. Additional sampling of soils, roots, stems, and leaves enabled assessment of the distribution of soil N content and delta N-15, intra-plant fractionations, and relationships between soil and plant delta N-15 values. We observed extensive within-population variability in foliar delta N-15 values and found plant age and foliar %N to be the strongest predictors of individual delta N-15 values. There were consistent differences between root, stem, and leaf delta N-15 values (spanning c. 3 parts per thousand), but plant and bulk soil delta N-15 values were unrelated. Plant-level variables played a strong role in influencing foliar delta N-15 values, and interannual relationships between climate and delta N-15 values were counter to previously recognized spatial patterns. This long-term record provides insights regarding the interpretation of delta N-15 values that were not available from previous large-scale syntheses, broadly enabling more effective application of foliar delta N-15 values.