Gradual enrichment of ¹⁵N with humification of diets in a below‐ground food web: relationship between ¹⁵N and diet age determined using ¹⁴C

Abstract

Summary 1.Stable nitrogen (N) isotope has been widely used to disentangle food webs and to infer trophic positions of organisms based on an assumption that the stepwise enrichment occurs along trophic levels. The enrichment of 15 N in soil organisms with diet humification has also been reported, but the underlying mechanism has not been fully examined. 2. To examine the effect of diet humification on 15 N, we estimated the stable N isotope ratios and diet ages of earthworms and termites. These organisms feed on organic matter with various degrees of humification, ranging from undecomposed plant materials to humified organic matter (soil organic matter), in a gallery forest and a savanna in the Ivory Coast. We defined diet age as the time elapsed since carbon (C) in the diet of earthworms and termites was fixed from atmospheric CO 2 by photosynthesis; it was estimated by comparing the radiocarbon ( 14 C) content of these organisms to atmospheric 14 CO 2 records. 3. Stable N isotope ratios increased along the humification gradient of diets, and values for earthworms and termites varied from 1·8‰ to 9·9‰ and from -1·5‰ to 15·9‰, respectively. Epigeic (litter-feeding) earthworms had younger diet ages (2-4 years), whereas endogeic (soil-feeding) earthworms generally exhibited older diet ages (5-9 years). Grass-feeding termites had young diet ages (2 years), and wood/soil-feeding termites had the oldest diet ages ( c. 50 years). Soil-feeding termites were similar in diet age (7-12 years) to wood feeders (8-11 years), with the exception of one species (18-21 years) that consumes large-diameter wood. 4. A significant positive relationship was found between diet ages and stable N isotope ratios of the two groups in the savanna. This relationship held in the gallery forest when termites feeding on woody tissues were not considered. These results show that the stable N isotope ratios of organisms can increase with diet age, unless C in the diet has been stored as organic matter, such as woody tissue, that is able to age without being subject to humification processes. 5. Given that above-ground food webs are often sustained directly by material and energy flow from below-ground food webs, in addition to trophic interactions, gradual enrichment of 15 N with the humification of below-ground diets should be considered when interpreting stable N isotope ratios of terrestrial food webs.

A strontium isotope analysis on the relationship between ritual tooth ablation and migration among the Jomon people in Japan

“…Extraction of bone collagen and purification of graphite were carried out at the RIHN following the methods of Kusaka et al (2008) and Hyodo et al (2008), respectively. The results of carbon isotope measurements of the bone collagen have already been described by Kusaka et al (2008).…”

Feeding habits of Hymenoptera and Isoptera in a tropical rain forest as revealed by nitrogen and carbon isotope ratios

“…In particular, the soil-feeding termite, D. nemorosus exhibited values of d 15 N and d 13 C that were indistinguishable from those of the army ant L. diminuta and the fly-hunting wasp Bembix sp., despite clear differences in the feeding habits of these species. This result indicates that values of d 15 N and d 13 C can increase not only from trophic enrichment, but also from the humification of feeding substrates in terrestrial food webs (Hyodo et al, 2008). Therefore, this study highlights that stable isotope techniques should be coupled with knowledge of the natural history and physiology of focal organisms to achieve a more precise interpretation of the isotopic signatures.…”

Earthworms facilitate carbon sequestration through unequal amplification of carbon stabilization compared with mineralization

“…Thus, the effects of earthworms on soil C loss, humification intensity (which was indicated by increasing 15 N natural abundance; ref. 23) and C stabilization (aggregates formation) were investigated. In addition, three different soil treatments, that is, Amynthas-worked soil (Asoil), Lumbricus-worked soil (Lsoil) and control soil incubated without earthworms (control soil) were obtained.…”