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

Carbon and nitrogen stable isotope ratios (delta C-13 and delta N-15) 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 delta C-13 values are similar (< 1aEuro degrees difference) to those of their diet, while consumer delta N-15 values are about 3aEuro degrees 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 delta C-13 and delta N-15 in belowground systems, e.g., enrichment of C-13 and N-15 in soil organic matter (likely related to soil microbial metabolism). These enrichments should be associated with the high C-13 (similar to 3aEuro degrees) enrichment in belowground consumers relative to litter and soil organic matter and with the large variation in delta N-15 (similar to 6aEuro degrees) 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 delta C-13 and delta N-15 analyses of terrestrial food webs. Meanwhile, by measuring the delta C-13 of key predators, the linkage between above- and belowground systems could be estimated based on observed differences in delta C-13 of primary producers, detritivores and predators. Furthermore, radiocarbon (C-14) measurements will allow the direct estimation of the dependence of predators on the belowground systems.