Foliar amino acid δ15N patterns reflect aridity and plant structure with implications for consumer diet reconstruction.

Compound specific amino acid (AA) nitrogen isotope (δ15N) analysis complements direct observations by revealing food web structure and ecological interactions, yet interpretation of consumer δ15N values depends on understanding natural variability in autotroph AA δ15N values. To test hypotheses about how foliar AA δ15N values vary with lignin content, phenolic content, and aridity, AA δ15N values were gathered from foliage collected from 86 plants growing across an aridity gradient in SW Madagascar. There exist consistent differences between sympatric grasses and woody taxa, including grass tyrosine and glycine relatively enriched in 15N by > 5‰. Consequently, consumer AA δ15N values carry information about the dietary fraction of woody plants vs. grasses, complementing inferences from carbon isotope (δ13C) analyses of C3/C4 diet proportions. Meanwhile, mean annual precipitation is negatively associated with most foliar AA δ15N values, averaging a 1.2‰ increase per 100 mm/yr decrease, though the association is not necessarily linear. Notably, lysine and tyrosine δ15N values increase with aridity, making them candidate proxies for habitat aridity in heterotroph tissues. Additionally, foliar δ15N lysine-glutamate offsets are more conserved than phenylalanine-glutamate offsets (CV = 15% vs. 40%), likely due to the involvement of phenylalanine in the phenylpropanoid pathway. Marked taxon-specific variation in foliar phenylalanine and threonine δ15N values likely affect δ15N values of consumer AAs. Improved characterizations of natural variation in plant AA metabolism and foliar AA δ15N values are essential for refining inferences of consumer ecology.