Amino acid isotopic analysis in agricultural systems

A relatively new approach to stable isotopic analysis—referred to as compound-specific isotopic analysis (CSIA)—has emerged, centering on the measurement of ¹⁵N:¹⁴N ratios in amino acids (glutamic acid and phenylalanine). CSIA has recently been used to generate trophic position estimates among animal species; however, the validity of such estimates is dependent on the consistency of the trophic discrimination factor (inter-trophic enrichment/depletion/routing of isotopes). To address whether the trophic discrimination factor may scale with trophic level, we created model communities with discrete trophic groups: a basal resource, strict herbivores, strict predators, and predators of strict predators. We used CSIA to analyze the organisms representing these trophic levels, and then tested the method using free-roaming specimens from a terrestrial ecosystem. We provide the first evidence that the discrimination factor remains consistent across a wide range of trophic levels. Based on these findings and previous studies in aquatic and marine systems, this factor (+7.6 ‰) appears to be a portable, non-scaling enrichment factor. Employing this parameter within formulae for trophic position estimation, we derived highly accurate estimates of animal trophic positions. These estimates permitted the assembly of trophic hierarchies, revealing the highest trophic position (5.07) and thus longest food chain ever measured using CSIA. Our findings suggest that CSIA facilitates extremely accurate measurement of animal trophic positions, which will allow bio-control researchers to accurately characterize the lifetime trophic tendencies/spectra of bio-control agents.