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

Compound specific amino acid (AA) nitrogen isotope ( TM15 N) analysis complements direct observations by revealing food web structure and ecological interactions, yet interpretation of consumer δ 15 N values depends on understanding natural variability in autotroph AA δ 15 N values. To test hypotheses about how foliar AA δ 15 N values vary with lignin content, phenolic content, and aridity, AA δ 15 N 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 15 N by > 5‰. Consequently, consumer AA δ 15 N values carry information about the dietary fraction of woody plants vs. grasses, complementing inferences from carbon isotope (δ 13 C) analyses of C3/C4 diet proportions. Meanwhile, mean annual precipitation is negatively associated with most foliar AA δ 15 N values, averaging a 1.2‰ increase per 100 mm/yr decrease, though the association is not necessarily linear. Notably, lysine and tyrosine δ 15 N values increase with aridity, making them candidate proxies for habitat aridity in heterotroph tissues. Additionally, foliar δ 15 N 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 δ 15 N values likely affect δ 15 N values of consumer AAs. Improved characterizations of natural variation in plant AA metabolism and foliar AA δ 15 N values are essential for refining inferences of consumer ecology.