Aboveground Food Webs Research Articles

Plasticine models confirm high predator pressure on surfaced earthworms in different ecosystems

Aboveground communities may strongly depend on the detrital subsidy, i.e. the release of energy and nutrients from the soil food webs. Earthworms predated by aboveground animals are seemingly among the most important links connecting belowground and aboveground food webs. Here, we conducted an experiment with plasticine models in alpine tundra, temperate, and monsoon tropical forests to estimate predator pressure on surfaced earthworms compared to aboveground prey (caterpillars). Earthworm models on the ground were attacked several times more often than caterpillar models on the plants. The attack rate was highest in the tropical forest; the most common types of attacks on earthworms were arthropod bites in both forests and mammal bites in tundra.

A highly resolved network reveals the role of terrestrial herbivory in structuring aboveground food webs.

Comparative studies suggest remarkable similarities among food webs across habitats, including systematic changes in their structure with diversity and complexity (scale-dependence). However, historic aboveground terrestrial food webs (ATFWs) have coarsely grouped plants and insects such that these webs are generally small, and herbivory is disproportionately under-represented compared to vertebrate predator-prey interactions. Furthermore, terrestrial herbivory is thought to be structured by unique processes compared to size-structured feeding in other systems. Here, we present the richest ATFW to date, including approximately 580 000 feeding links among approximately 3800 taxonomic species, sourced from approximately 27 000 expert-vetted interaction records annotated as feeding upon one of six different resource types: leaves, flowers, seeds, wood, prey and carrion. By comparison to historical ATFWs and null ecological hypotheses, we show that our temperate forest web displays a potentially unique structure characterized by two properties: (i) a large fraction of carnivory interactions dominated by a small number of hyper-generalist, opportunistic bird and bat predators; and (ii) a smaller fraction of herbivory interactions dominated by a hyper-rich community of insects with variably sized but highly specific diets. We attribute our findings to the large-scale, even resolution of vertebrate, insect and plant guilds in our food web.This article is part of the theme issue 'Connected interactions: enriching food web research by spatial and social interactions'.

The hidden oases: unveiling trophic dynamics in Namib's fog plant ecosystem

The Namib Desert is a hyperarid coastal desert where fog is a major moisture source. We hypothesized that the fog-harvesting grass Stipagrostis sabulicola establishes an important ecological niche, termed the "Fog-Plant-Oases" (FPOs), and serves as the primary carbon source for the invertebrate community. To determine this, we measured the natural variations of the stable carbon and nitrogen isotopes (δ13C and δ15N) of invertebrates as well as that of plant biomass and belowground detritus and estimated the contributions of the fog plants in their diets. Our findings revealed a complex trophic structure and demonstrated that S. sabulicola fuels carbon flow from lower to higher trophic levels in the aboveground food web. The distinct δ13C values of bacterial- and fungal-feeding nematodes indicated however the separation of the aboveground niche, which is primarily sustained by S. sabulicola, from the belowground niche, where wind-blown sediments may serve as the main energy source for the soil biota. Our findings further accentuate the critical role of S. sabulicola FPOs in establishing complex trophic dynamics and a distinctive food web within the hyperarid Namib dunes.

Mast seeding in European beech (Fagus sylvatica L.) is associated with reduced fungal sporocarp production and community diversity.

Mast seeding is a well-documented phenomenon across diverse forest ecosystems. While its effect on aboveground food webs has been thoroughly studied, how it impacts the soil fungi that drive soil carbon and nutrient cycling has not yet been explored. To evaluate the relationship between mast seeding and fungal resource availability, we paired a Swiss 29-year fungal sporocarp census with contemporaneous seed production for European beech (Fagus sylvatica L.). On average, mast seeding was associated with a 55% reduction in sporocarp production and a compositional community shift towards drought-tolerant taxa across both ectomycorrhizal and saprotrophic guilds. Among ectomycorrhizal fungi, traits associated with carbon cost did not explain species' sensitivity to seed production. Together, our results support a novel hypothesis that mast seeding limits annual resource availability and reproductive investment in soil fungi, creating an ecosystem 'rhythm' to forest processes that is synchronized above- and belowground.

Fear of predators alters herbivore regulation of soil microbial community function.

Fear of predation can affect important ecosystem processes by altering the prey traits expression that, in turn, regulates the quantity and quality of nutritional inputs to soil. Here, we aimed to assist in bridging a knowledge gap in this cascading chain of events by exploring how risk of spider predation may affect grasshopper prey performances, and the activity of various microbial extracellular enzymes in the soil. Using a mesocosms field-experiment, we found that grasshoppers threatened by spider predation ate less, grew slower, and had a higher body carbon to nitrogen ratio. Herbivory increased activity of all microbial extracellular enzymes examined, likely due to higher availability of root exudates. Predation risk had no effect on C-acquiring enzymes but decreased activity of P-acquiring enzymes. We found contrasting results regarding the effect of predation on the activity of N-acetyl-glucosaminidase and leucine arylamidase N-acquiring enzymes, suggesting that predation risk may alter the composition of N-inputs to soil. Our work highlighted the importance of soil microbial enzymatic activity as a way to predict how changes in the aboveground food-web dynamics may alter key ecosystem processes like nutritional-cycling.

The role of Nyctelia circumundata (Coleoptera: Tenebrionidae) on litter fragmentation processes and soil fertility in northeastern arid patagonia

Decomposition of organic matter and soil properties linked to carbon (C) and nitrogen (N) cycling are considered key processes in soil fertility. In arid ecosystems, most plant litter remains intact for long periods and is gradually consumed by macrodetritivore arthropods. Tenebrionid beetles (Coleoptera: Tenebrionidae) are the most abundant macrodetritivores in many arid environments worldwide, and it is widely suggested that these beetles are responsible for most of the nutrient cycling from accumulated litter in warm and dry seasons in these ecosystems. In this study, the role of Nyctelia circumundata Lesne (Coleoptera: Tenebrionidae) on soil fertility and litter fragmentation during the warm-dry season was experimentally evaluated in northeastern Patagonia, using microcosm experiments in the laboratory.Our results showed that the presence of litter alone is not enough to incorporate organic matter (OM), as well as nitrogen (N) or carbon (C) into the soil. Adult individuals of N. circumundata consumed a significant fraction of the litter offered in the experimental pots (∼10 %) and this activity was also associated to increased soil N, C, and OM contents, and the C/N ratio. These facts highlighted that litter fragmentation, consumption, and its subsequent conversion into feces by N. circumundata positively affects soil fertility in northeastern Patagonia. Considering that tenebrionid beetles are a major group of primary consumers in deserts, these beetles play an important role in aboveground food web and a key energy conduit from plants (as litter) to higher trophic levels in this region. Finally, taking into account the ecological relevance of tenebrionid beetles for soil nutrient cycling (both at local and ecosytem scales) demonstrated in this work, we recommend incorporating these beetle assemblages into future conservation strategies for arid Patagonia.

Microarthropod communities and their ecosystem services restore when permanent grassland with mowing or low-intensity grazing is installed

The current focus on intensification and maximizing productivity in agriculture can endanger soil biota and the ecosystem services they provide in such a way that it acts counterproductive and increases the dependence on external inputs. In this study, we aimed to identify the factors that are most limiting for the restoration of soil biota and their ecosystem services on sandy soils. To this end, we assessed microarthropod communities, their relationship with the aboveground food web and their effect on organic matter decomposition, in two land-use types: grasslands with agricultural land use and grasslands with nature land use. The latter are grasslands converted from agricultural land use, for the development of the Dutch National Ecological Network. For these land-use types, we took into account two main factors of disturbance: the number of years since the last tillage (i.e., plowing event), and the current grassland management (mowing or grazing). We found that the diversity of microarthropods was higher in nature grasslands than in agricultural grasslands. The abundance of microarthropods increased with time since last tillage for grasslands that were mown, but not for grasslands that were grazed. An agricultural grassland without tillage since 39 years had a microarthropod abundance similar to reference natural grasslands reported in previous research. The number of predatory beetles increased with a higher microarthropod abundance in mown grasslands, but not so in grazed grasslands. The number of fungivorous and herbofungivorous grazer microarthropods positively influenced the decomposition of soil organic matter as measured with the Tea Bag Index. Furthermore, we found a negative effect of Difenyl and total fungicide concentrations in the soil on (herbo)fungivorous grazers. Contrary to our expectations, we found more pesticide residues in nature grasslands than in agricultural grasslands. In conclusion, to restore the soil microarthropods and the ecosystem services they contribute to, the best practice is to strive for permanent grassland (without tillage) with mowing or low-intensity grazing (without compaction of the topsoil).

Soil biological responses to, and feedbacks on, trophic rewilding.

Trophic rewilding-the (re)introduction of missing large herbivores and/or their predators-is increasingly proposed to restore biodiversity and biotic interactions, but its effects on soils have been largely neglected. The high diversity of soil organisms and the ecological functions they perform mean that the full impact of rewilding on ecosystems cannot be assessed considering only above-ground food webs. Here we outline current understanding on how animal species of rewilding interest affect soil structure, processes and communities, and how in turn soil biota may affect species above ground. We highlight considerable uncertainty in soil responses to and feedbacks on above-ground consumers, with potentially large implications for rewilding interactions with global change. For example, the impact of large herbivores on soil decomposers and plant-soil interactions could lead to reduced carbon sequestration, whereas herbivore interactions with keystone biota such as mycorrhizal fungi, dung beetles and bioturbators could promote native plants and ecosystem heterogeneity. Moreover, (re)inoculation of keystone soil biota could be considered as a strategy to meet some of the objectives of trophic rewilding. Overall, we call for the rewilding research community to engage more with soil ecology experts and consider above-ground-below-ground linkages as integral to assess potential benefits as well as pitfalls.This article is part of the theme issue 'Trophic rewilding: consequences for ecosystems under global change'.

Predator community composition is linked to soil carbon retention across a human land use gradient.

Soil carbon (C) storage is a major component of the carbon cycle. Consensus holds that soil C uptake and storage is regulated by plant-microbe-soil interactions. However, the contribution of animals in aboveground food webs to this process has been overlooked. Using insights from prior long-term experimentation in an old-field ecosystem and mathematical modeling, we predicted that the amount of soil C retention within a field should increase with the proportion of active hunting predators comprising the aboveground community of active hunting and sit-and-wait predators. This comes about because predators with different hunting modes have different cascading effects on plants. Our test of the prediction revealed that the composition of the arthropod predator community and associated cascading effects on the plant community explained 41% of variation in soil C retention among 15 old fields across a human land use gradient. We also evaluated the potential for several other candidate factors to explain variation in soil C retention among fields, independent of among-field variation in the predator community. These included live plant biomass, insect herbivore community composition, soil arthropod decomposer community composition, degree of land use development around the fields, field age, and soil texture. None of these candidate variables significantly explained soil C retention among the fields. The study offers a generalizable understanding of the pathways through which arthropod predator community composition can contribute to old-field ecosystem carbon storage. This insight helps support ongoing efforts to understand and manage the effects of anthropogenic land use change on soil C storage.

Fear and below-ground food-webs

Predator induced trait mediated indirect interactions (TMIIs) are likely the dominant facet of trophic interactions in aboveground food-webs. New research is beginning to show that TMIIs are also important in revealing how soil food-webs (SFWs) regulate biogeochemical processes. We suggest that TMIIs can modify SFW functions by (a) regulating the quantity and nutritional quality of plants and animal production known to drive the SFW, (b) inducing defense phenotypes in soil-organisms, and (c) changing soil environmental conditions. Currently, very few studies have explored the role TMIIs play in shaping SFW functions, especially the cascading effects on SFW dynamics. Much theoretical and empirical research is needed before we can successfully incorporate the non-consumptive effects of predation into SFW models.

Addition of crop residues affects a detritus-based food chain depending on litter type and farming system

The addition of crop residues is a common farming practice to increase the organic carbon content of agricultural soils with particular importance in organically managed crops. Residues can be added either from the crop plant itself or from other plants and the type of litter may differentially affect decomposer populations. Effects of litter addition may cascade up to affect generalist predators via trophic cascades or modifications of structural microhabitat properties with unknown consequences for the role of these predators in below- and aboveground food webs. Wheat and maize litter were added to organically and conventionally managed wheat fields and effects on generalist predator and Collembola numbers, litter decomposition and carbon utilization as estimated by stable isotope analysis were studied. Significantly more predators were sampled in plots with maize litter compared to plots with wheat litter and under organic farming. Collembola numbers were not significantly affected by litter type or farming system. Litter mass loss was highest in plots that received wheat litter under organic management and was negatively related to predator, but not Collembola numbers. Individuals of Lepidocyrtus spp. (Collembola) incorporated high percentages of maize-borne carbon compared to predator species. Two linyphiid spider species were more closely linked to the maize-consuming prey in organically managed fields and one carabid species showed this pattern in conventionally managed fields. High litter decomposition levels and generalist predator numbers were only observed in wheat litter plots in organically managed fields. The addition of crop residues from the growing crop under organic management may therefore be a promising farming practice to simultaneously promote decomposition services and numbers of natural enemies. Future studies need to address this practice and potential effects on the litter–decomposer–predator food chain across sites that cover a range of different soil types and climatic conditions.

An Updated Multilocus Phylogeny of the Lumbricidae (Annelida: Clitellata: Oligochaeta) Earthworms

Lumbricidae earthworms dominate agricultural lands and often natural terrestrial ecosystems in temperate regions in Europe. They impact soil properties and nutrient cycling, shaping plant community composition and aboveground food webs. The simplicity of the earthworm body plan has hampered morphology-based classifications and taxonomy; hence current research on Lumbricidae systematic relies mostly on molecular data from multiple or single locus [e.g., cytochrome oxidase subunit I (COI) barcodes] to infer evolutionary relationships, validate taxonomic groups and/or identify species. Here we use multiple nuclear and mitochondrial gene regions (including COI) to generate updated maximum likelihood and Bayesian phylogenies of the family Lumbricidae. We then compare these trees to new COI trees to assess the performance of COI at inferring lumbricid inter-generic relationships.

Underground evolution: New roots for the old tree of lumbricid earthworms

Earthworms belonging to the family Lumbricidae are extremely abundant in terrestrial temperate regions. They affect soil properties and nutrient cycling, thus shaping plant community composition and aboveground food webs. Some lumbricids are also model organisms in ecology and toxicology. Despite the intense research efforts dedicated to lumbricids over the last 130years, the evolutionary relationships and taxonomic classification of these organisms are still subject to great debate. Resolution of their systematics is hampered by the structural simplicity of the earthworm body plan and the existence of cryptic species. We sampled 160 earthworm specimens belonging to 84 lumbricid species (28 genera) and 22 Lumbricoidea outgroups, sequenced two nuclear genes, four mitochondrial genes and seven mitochondrial tRNAs and examined 22 morphological characters. We then applied a combination of phylogenetic methods to generate the first robust genus-level phylogeny of the Lumbricidae. Our results show that the current Lumbricidae classification and the underlying hypotheses of character evolution must be revised. Our chronogram suggests that lumbricids emerged in the Lower Cretaceous in the holarctic region and that their diversification has been driven by tectonic processes (e.g. Laurasia split) and geographical isolation. Our chronogram and character reconstruction analysis reveal that spermathecae number does not follow a gradual pattern of reduction and that parthenogenesis arose from sexual relatives multiple times in the group; the same analysis also indicates that both epigeic and anecic earthworms evolved from endogeic ancestors. These findings emphasize the strong and multiple changes to which morphological and ecological characters are subjected, challenging the hypothesis of character stasis in Lumbricidae.

Impact of the Red Imported Fire Ant (Hymenoptera: Formicidae) on Epigeic Arthropods of Cotton Agroecosystems

Abstract The red imported fire ant, Solenopsis invicta Buren (Hymenoptera: Formicidae), has been studied extensively in its role in aboveground food webs of agroecosystems of the southern United States. There is also a limited body of evidence suggesting that S. invicta may significantly influence the soil fauna. This study examined the influence of fire ants on the arthropod communities at the soil surface of cotton, Gossypium hirsutum L., fields at two field sites operated by the University of Georgia in Athens and Tifton, GA (1 yr at each location). Fire ant abundance was suppressed in large plots with ant-specific hydramethylnon-based bait, and arthropods were collected from treatment and control plots by using pitfall traps during multiple week-long sample periods to measure the abundance of epigeic arthropods. Sampling was conducted from June through September 2006 in Athens and from July through September 2007 in Tifton. Although fire ant suppression decreased the abundance of erythraeid mites and nitidulid beetles at both sites, the majority of effects were site-specific. Other taxa positively associated with fire ants included oribatid mites and gnaphosid and linyphiid spiders. In contrast, the abundance of springtails, earwigs, endomychid beetles, and thrips increased with fire ant suppression. This study demonstrates that S. invicta can significantly influence both pest and beneficial epigeic arthropods and that although fire ants exert clear effects on specific taxa, their effects are not uniform within a given trophic group as members from the same guild were differentially impacted.

The influence of seabird nutrient enrichment and grazing on the structure and function of island soil food webs

Marine inputs from seabirds (in the form of guano) to terrestrial coastal communities play an important role in supporting aboveground food webs. However, little is known about the importance of seabird-derived nutrient inputs for belowground food webs and their function relative to other factors that regulate belowground communities. Here, we tested the relative importance of nutrient enrichment from seabirds and grazing, a known driver of belowground properties, in determining the structure and function of the soil food web in an island system. This was tested by measuring the size and composition of the microbial community, the abundance of nematode feeding groups and rates of decomposition and net nitrogen (N) mineralisation in soil samples collected from grazed and ungrazed plots at coastal and inland locations, representing sites of high and low seabird influence respectively, on the Isle of May in the Firth of Forth, east Scotland. We found that proximity to seabird breeding colonies, and associated greater input of seabird-derived N, stimulated the size of the soil microbial biomass and the abundance of bacteria relative to fungi in the soil microbial community relative to inland areas that received significantly less N. Despite this, proximity to seabird colonies had no detectable effect on rates of decomposition or N-mineralisation. The short-term removal of mammalian grazers, in the form of rabbits, had only limited effects on the structure of the soil food web, mainly affecting the abundance of bactivorous nematodes which were greater in grazed than ungrazed situations. However, cessation of grazing did impact significantly on rates of N-mineralisation and decomposition, which were higher and lower in grazed than ungrazed situations respectively. In conclusion, our study provides evidence that allochthonous nutrient inputs from seabirds have significant impacts on the composition of the soil microbial community, and that these effects outweigh short-term effects of grazers as a driver of soil food web structure in the island system studied. Overall, our results indicate the important roles that natural sources of N and grazing play as drivers of soil food webs and their function.

Harvested perennial grasslands provide ecological benchmarks for agricultural sustainability

Perennial vegetation can provide multiple ecosystem services essential for sustainable production more effectively than production systems based on annual crops. However, the ability of annually harvested, unfertilized perennial systems to sustain long-term yields while also maintaining ecosystem services has not been widely studied. Here we compare the impacts of harvested perennial grass and annual crop fields on ecosystem functioning in KS, USA. Despite the lack of mineral fertilizer applications, the aboveground harvests of perennial fields yielded similar levels of N compared to those of conventional high-input wheat ( Triticum aestivum) fields and at only 8% of the in-field energy costs. Their 75-yr cumulative N yield per ha was approximately 23% greater than that from the region's wheat fields. In terms of aboveground food webs, perennial fields harboured greater numbers and/or diversity of insect pollinators, herbivores and detritivores. Belowground, perennial grass fields maintained 43 Mg ha −1 more soil carbon and 4 Mg ha −1 more soil nitrogen than annual crop fields in the surface 1 m. Soil food webs in perennial fields, as indicated by nematode communities, exhibited greater food web complexity and stability than did those in annual crop fields. In surrounding watersheds, increased annual cropland was correlated with higher riverine nitrate-nitrogen levels. Given their benefits, harvested perennial grasslands provide valuable ecological benchmarks for agricultural sustainability.

Inter- and intraspecific interactions in lumbricid earthworms: Their role for earthworm performance and ecosystem functioning

Summary Earthworm species differentially affect organic matter and energy transformations, and interrelationships among soil food web components. Hence, knowledge of earthworm species associations and their interactions is important not only for an understanding of the functioning of soil systems and predicting the consequences of invasions of peregrine earthworms for native earthworm fauna, soil food web structure and functioning, but also for the structure and functioning of the aboveground food web. Over 60 studies investigating a total of 19 species and about 20 response variables were studied to summarize the scattered information on inter- and intraspecific interactions in lumbricids. Information on interactions between species is restricted to species pairs, while there is no data on species interactions within more complex associations. For further analysis, interactions were classified into positive and negative (i.e., those enhancing or decreasing the fitness of a given species). Based on this approach, “interactive portraits” (combinations of responses and effects) for six widespread species and for major ecological groups of earthworms were composed. Species and group characteristics were confirmed by meta-analysis. In multispecies communities, species strongly react to species composition and community structure with the mechanisms of species interactions likely being context-specific. Interactions between both species and ecological groups strongly affect soil biota and soil processes, with synergistic and neutralizing effects being most important. At the species level, lumbricids are among the few groups of soil invertebrates where density-dependent regulation of populations appears to be prevalent. These conclusions have important implications for the role of earthworms acting as ecosystem engineers in terrestrial systems.

ABOVE- AND BELOWGROUND INTERACTIONS ARE MEDIATED BY NUTRIENT AVAILABILITY.

By influencing nutrient mineralization in the soil, decomposers may affect the performance of plants and their associated herbivores. The strength of above-belowground linkages may therefore depend on the availability of nutrients in ecosystems. We investigated the dependency of decomposer- and leaf-herbivore-mediated changes in plant performance on soil nutrient availability in microcosm systems. In separate treatments, Poa annua was used as host plant for the herbivore and was grown in combination with different herb species: Plantago lanceolata, Trifolium repens, and Sanguisorba minor. At three different levels of nutrient availability, the impact of Collembola (Folsomia candida) and aphids (Rhopalosiphum padi), as well as their interacting effects on plant performance, were investigated. We further assessed the effects of Collembola on the performance of aphids and vice versa, as well as the effects on element contents in P. annua and soil nutrients. Collembola increased total aboveground biomass and biomass of single plant species (except T. repens) only at low and moderate nutrient availability, whereas the negative effects of aphids on P. annua and total aboveground biomass were highest at high nutrient availability. Collembola decreased relative root allocation. The negative effect of aphids on plant biomass was more pronounced in the presence of Collembola. Nitrogen concentration in shoots of P. annua was increased by Collembola. Aphids decreased the nitrogen concentration in grass shoots only when Collembola were present. These interacting effects can be related to indirect effects of decomposers on herbivore performance. At low and moderate nutrient availability, aphid numbers strongly increased due to decomposers, whereas at high nutrient availability this effect was much weaker. In turn, aphid herbivory increased the number of Collembola only at high nutrient availability. We conclude that the role of both below- and aboveground food webs for the dynamics of vegetation depend on each other and the availability of nutrients in ecosystems. This suggests that nutrient enrichment of ecosystems may have important consequences for above-belowground linkages and their effects on natural communities.

Subsidy from the detrital food web, but not microhabitat complexity, affects the role of generalist predators in an aboveground herbivore food web

The activity and density of generalist predators, such as carabid beetles, rove beetles and spiders, may increase in response to: (1) increased availability of prey from the belowground subsystem and/or (2) enhanced complexity of aboveground vegetation. Organic farming practices support decomposer populations and enhance habitat complexity due to an increased weed density. A response by generalist predators to such below‐ or aboveground changes could affect predation rates on herbivores in the aboveground food web. We tested this hypothesis in a replicated field experiment conducted in a winter wheat field, where increased predator activity could lead to improved control of herbivorous pests. In a crossed design, we increased and lowered densities of decomposer prey, and manipulated vegetation complexity using artificial plants in order to examine the effect of structural complexity in isolation from effects of plant‐attracted additional prey. Isotomid Collembola exhibited lowest activity‐densities (AD) in plots treated with soil insecticide and had gradually increasing AD in untreated plots and plots receiving detrital subsidies. Carabid beetles and cursorial spiders did not respond to increased availability of isotomid prey, and they unexpectedly displayed higher AD in the structurally less‐complex plots. Aphid density mirrored the positive response of isotomids to detrital subsidies, suggesting that aphids benefited from reduced predation due to predators switching to abundant prey in the decomposer subsystem. The absence of a numerical response by surface‐active predators apparently strengthened this indirect effect of isotomids on aphids. Our results suggest that indirect predator‐mediated prey‐prey interactions can reduce beneficial effects of detrital subsidies on pest suppression. We further demonstrated that generalist predators may not per se benefit from structural complexity. Both results document the challenges associated with management practices that support generalist predators, as these measures may not necessarily improve herbivore suppression.

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

Summary 1Stable 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 15N in soil organisms with diet humification has also been reported, but the underlying mechanism has not been fully examined. 2To examine the effect of diet humification on 15N, 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 CO2 by photosynthesis; it was estimated by comparing the radiocarbon (14C) content of these organisms to atmospheric 14CO2 records. 3Stable 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. 4A 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. 5Given 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 15N with the humification of below-ground diets should be considered when interpreting stable N isotope ratios of terrestrial food webs.