Linking aboveground and belowground food webs through carbon and nitrogen stable isotope analyses

Abstract Carbon and nitrogen stable isotope ratios (δ 13 C and δ 15 N) have been used for more than two decades in analyses of food web structure. The utility of isotope ratio measurements is based on the observation that consumer δ 13 C values are similar (<1‰ difference) to those of their diet, while consumer δ 15 N values are about 3‰ higher than those of their diet. The technique has been applied most often to aquatic and aboveground terrestrial food webs. However, few isotope studies have examined terrestrial food web structure that includes both above‐ and belowground (detrital) components. Here, we review factors that may influence isotopic signatures of terrestrial consumers in above‐ and belowground systems. In particular, we emphasize variations in δ 13 C and δ 15 N in belowground systems, e.g., enrichment of 13 C and 15 N in soil organic matter (likely related to soil microbial metabolism). These enrichments should be associated with the high 13 C (~3‰) enrichment in belowground consumers relative to litter and soil organic matter and with the large variation in δ 15 N (~6‰) of the consumers. Because such enrichment and variation are much greater than the trophic enrichment generally used to estimate consumer trophic positions, and because many general predators are considered dependent on energy and material flows from belowground, the isotopic variation in belowground systems should be taken into account in δ 13 C and δ 15 N analyses of terrestrial food webs. Meanwhile, by measuring the δ 13 C of key predators, the linkage between above‐ and belowground systems could be estimated based on observed differences in δ 13 C of primary producers, detritivores and predators. Furthermore, radiocarbon ( 14 C) measurements will allow the direct estimation of the dependence of predators on the belowground systems.