Compartmentalization of the soil animal food web as indicated by dual analysis of stable isotope ratios (N/N and C/C)

The soil animal food web has become a focus of recent ecological research but trophic relationships still remain enigmatic for many taxa. Analysis of stable isotope ratios of N and C provides a powerful tool for disentangling food web structure. In this study, animals, roots, soil and litter material from a temperate deciduous forest were analysed. The combined measurement of δ¹⁵N and δ¹³C provided insights into the compartmentalization of the soil animal food web. Leaf litter feeders were separated from animals relying mainly on recent belowground carbon resources and from animals feeding on older carbon. The trophic pathway of leaf litter-feeding species appears to be a dead end, presumably because leaf litter feeders (mainly diplopods and oribatid mites) are unavailable to predators due to large size and/or strong sclerotization. Endogeic earthworms that rely on older carbon also appear to exist in predator-free space. The data suggest that the largest trophic compartment constitutes of ectomycorrhizal feeders and their predators. Additionally, there is a smaller trophic compartment consisting of predators likely feeding on enchytraeids and potentially nematodes.     

Dual stable isotope analysis (δC and δN) of soil invertebrates and their food sources

More research is required to validate and refine natural abundance stable isotope ratio techniques as a tool for the investigation of the feeding ecology of soil animals and trophic relations in soil food webs. Isotope ratios of C (δ¹³C) and N (δ¹⁵N) were measured in herbivorous and detritivorous invertebrate groups, namely lumbricid earthworms (7 species), enchytraeid worms (3 species), slugs (3 taxa), and their potential food sources in an arable system. Intrapopulation δ¹⁵N variation in the slug Deroceras reticulatum (n=52) was large (range 4.2‰), possibly reflecting spatial variability in the food sources. Significant correlations between C:N ratios and isotope ratios in earthworms suggest that factors other than feeding may influence isotopic patterns. One enchytraeid species, Enchytraeus buchholzi, was enriched in ¹³C and strongly depleted in ¹⁵N compared to all other groups. Invertebrates formed a continuum when considered in relation to C and N separately, but fell into two distinct groups on the basis of combined C and N isotope ratios. The less enriched group represents herbivorous and litter-feeding species, while the more enriched group represents soil feeders. It is concluded that δ¹³C measurements could provide a means of assigning separate baseline δ¹⁵N values to primary and secondary decomposers, which in turn could improve the inference of higher trophic levels, omnivory and intraguild predation.     

Stable isotope N and C labelling of different functional groups of earthworms and their casts: A tool for studying trophic links

Earthworms (Oligochaeta: Lumbricidae) have substantial effects on the structure and fertility of soils with consequences for the diversity of plant communities and associated ecosystem functions. However, we still lack a clear understanding of the functional role earthworms play in terrestrial ecosystems, partly because easy-to-use methods to quantify their activities are missing. In this study, we tested whether earthworms and their casts can be dual-labelled with ¹⁵N and ¹³C stable isotopes by cultivating them in soil substrate amended with ¹⁵N ammonium nitrate and ¹³C-glucose. Additionally, we also wanted to know whether (i) earthworms from different functional groups (soil-feeders vs. litter-feeders) and their casts would differ in their incorporation of stable isotopes, (ii) if enrichment levels are higher if the same amount of isotopes is applied in one dose or in staggered doses, and (iii) if isotopic enrichment in casts changes when they are stored in a conditioning cabinet or in a pot filled with soil placed in a greenhouse. Our findings show the feasibility of dual-labelling tissues and casts of both litter-feeding (Lumbricus terrestris) and soil-feeding (Aporrectodea caliginosa) earthworms using the same method. The advantage of this method is that earthworms and their casts can be labelled under realistic conditions by cultivating them for only four days in soil that received a one-time addition of commercially available stable isotopes instead of offering labelled plant material. In earthworms, the isotopic enrichment remained at a stable level for at least 21 days; labelled casts could be stored for at least 105 days without significantly decreasing their isotopic signals. This simple and efficient method opens new avenues for studying the role of these important ecosystem engineers in nutrient cycling and their functional relationships with other organisms.     

Seasonal and age-related changes in the stable isotope composition (15N/14N and 13C/12C) of millipedes and collembolans in a temperate forest soil

Seasonal changes in environmental conditions and biotic interactions are often ignored when using stable isotope analysis for reconstructing the trophic structure of soil communities in temperate ecosystems. In this study, we estimated seasonal and age-related changes in δ¹³C and δ¹⁵N values in three epigeic species of collembolans (Pogonognathellus longicornis, Orchesella flavescens and Isotoma viridis) and two litter-dwelling species of millipedes (Polydesmus denticulatus and Leptoiulus proximus) in deciduous and coniferous forest stands in central Russia. Age-related changes in δ¹³C or δ¹⁵N values were either absent or negligible (within 1‰) in L. proximus, but adult and subadult specimens of P. denticulatus were enriched in ¹⁵N compared to early larval stages. Since the adults of P. denticulatus were generally more enriched in ¹⁵N than adults of L. proximus, they presumably occupy more distinct trophic niches than juveniles do. Age-related changes in isotopic composition were small or absent in collembolans studied. Neither δ¹³C nor δ¹⁵N values of millipedes changed significantly during the vegetation season. In contrast, consistent seasonal changes in δ¹³C and δ¹⁵N values were found in collembolans. Increased δ¹³C values coincided with the period of minimum soil moisture and correlated with a decreased C/N ratio in collembolan tissues. These changes can largely be attributed to the depletion of lipid-rich storage tissues. Seasonal changes in δ¹⁵N values were similar among collembolan species, yet slightly varied between habitats. A general trend of increasing δ¹⁵N values from June to September–October may indicate either a reduced importance of non-vascular plants (algae and lichen) in collembolan diet or variation in the isotopic composition of these plants. Overall, our data show that seasonal variations should be taken into account when estimating the isotopic composition of epigeic collembolans in forest soils.     

Trophic structure of a macroarthropod litter food web in managed coniferous forest stands: a stable isotope analysis with δN and δC

We studied the composition of a litter detrital community in a temperate coniferous forest using stable isotopes of nitrogen and carbon. Samples of mineral soil, bulk litter material, macroarthropods and understory plants were collected from ten experimental forest stands. Half of the stands were previously thinned 17–42 years ago, the other half served as controls. Values of δ¹⁵N and δ¹³C were based on the analysis of almost 500 individuals of at least 22 species in 11 arthropod families. The isotopic analysis showed a significant increase in δ¹⁵N and δ¹³C values with soil depth. Isotopic signatures of macroarthropods ranged from −26.51‰ to −20.52‰ for δ¹³C and −2.85‰ to 5.10‰ for δ¹⁵N. All consumers showed levels of ¹³C enrichment substantially higher than those of primary producers and litter. Predators were generally significantly more ¹⁵N enriched than detritivores and herbivores, but their δ¹³C levels were similar to those of primary consumers. Our data indicate that this community consists of at least 2–3 trophic levels with a considerable amount of variation in the ¹⁵N enrichment among detritivores and predators. We suggest that the spread of δ¹⁵N values of predators likely reflects the diversity of potential prey among detritivores and a varying degree of intraguild predation among different species. Our findings generally agree closely with the results of similar studies from other forest litter communities. Thinning did not appear to influence the overall isotopic composition of the detrital food web. Extensive omnivory and intraguild predation among litter consumers may buffer long-term effects of thinning on the trophic structure of these species-rich communities.