Recent evidences have suggested that compound-specific stable isotope analysis (CSIA) of amino acids has been employed as a new powerful method with that enables the estimation of trophic level of organisms in not only aquatic but also terrestrial food webs. This CSIA approach is based on contrasting the 15N-enrichment with each trophic level between two common amino acids: glutamic acid shows significant enrichment of +8.0‰ with each trophic level, whereas phenylalanine shows little enrichment of +0.4‰. These 15N-enrichments are well consistent in both aquatic and terrestrial organisms. The trophic level of organisms can be estimated within a small error as 1σ=0.12 for aquatic and 0.17 for terrestrial food webs, employing the eq.: [Trophic level]=(δ15Nglutamic acid-δ15Nphenylalanine+β)/7.6+1, where β represents the isotopic difference between these two amino acids in primary producers (-3.4‰ for aquatic cyanobacteria and algae, +8.4‰ for terrestrial C3 higher plants, and -0.4‰ for terrestrial C4 higher plants). Here, we briefly review this new method (i.e. CSIA of amino acids) and its application to natural organisms in terrestrial environments.
