Belowground Food Webs Research Articles

How extreme is an extreme climatic event to a subarctic peatland springtail community?

Extreme climate events are increasing in frequency and duration and may directly impact belowground foodwebs and the activities of component soil organisms. The soil invertebrate community, which includes keystone decomposers, might respond to these newly induced soil microclimate conditions by shifts in density, species composition, spatial patterning and/or functional traits.To test if and how short-term extreme climatic conditions alter the structure, the vertical stratification and the community weighted trait means of the springtail (Collembola) community in sub-arctic peatbogs, we experimentally subjected Sphagnum peat cores in a field setting to factorial treatments of elevated temperature and episodically increased moisture content.The large precipitation peaks did not affect the springtail community, but an average soil temperature increase of 4 °C halved its density in the shallower peat layers, mainly caused by the reduced dominance of Folsomia quadrioculata. A hypothesized net downward shift of the surface-dwelling springtail community, however, was not observed. We observed species-specific responses to warming but the overall community composition in subsequent organic layers was not significantly altered. Although the effects of an extreme warming event on density, species composition and vertical stratification pattern seemed subtle, functional trait analysis revealed directional community responses, i.e. an overall increase of soil-dwelling species due to warming, even though warming did not alter layer-specific community weighted trait means.We suggest that subtle changes in moisture conditions, due to increased evapotranspiration, have decreased typically surface-dwelling species relative to soil-dwelling species. The extent to which this directional change in the community is maintained after an extreme event, and its costs for the community's resilience to multiple sequential extreme events will consequently determine its longer-term effects on the community and on ecosystem functioning.

Fungal communities influence root exudation rates in pine seedlings

Root exudates are hypothesized to play a central role in belowground food webs, nutrient turnover, and soil C dynamics in forests, but little is known about the extent to which root-associated microbial communities influence exudation rates in trees. We used a novel experimental technique to inoculate loblolly pine (Pinus taeda L.) seedlings with indigenous forest fungi to examine how diverse fungal communities influence exudation. Surface-sterilized seeds were sown in intact, unsieved soil cores for 14weeks to promote root colonization by fungi. After 14weeks, we transferred seedlings and root-associated fungi into cuvettes and measured exudate accumulation in trap solutions. Both the abundance and identity of root-associated fungi influenced exudation. Exudation rates were greatest in root systems least colonized by ectomycorrhizal (ECM) fungi and most colonized by putative pathogenic and saprotrophic fungi. However, the ECM community composition was not a strong determinant of exudation rates. These results suggest that environmental conditions that influence the degree to which tree roots are colonized by pathogenic and saprotrophic vs. mutualistic fungi are likely to mediate fluxes of labile C in forest soils, with consequences for soil biogeochemistry and ecosystem processes.

Stable isotope evidence for marine-derived avian inputs of nitrogen into soil, vegetation, and earthworms on the isle of Rum, Scotland, UK

The largest breeding colony of Manx shearwaters (Puffinus puffinus) in the world is found on the Isle of Rum in the inner Hebrides of Scotland. We collected a feather, guano, soil, vegetation, and earthworms near shearwater burrows to determine whether inputs of nitrogen (N) from guano were incorporated into belowground foodwebs. For comparison, similar samples were collected from a nearby plot that was experimentally fertilised 40 years prior. The shearwater feather had the highest level of 15N enrichment (+18‰), followed by guano (+12‰). Soil (+7.5‰) and vegetation (+5.7‰) collected at the burrow entrance were enriched with 15N relative to those collected at 2 m or more away (+0.5 to +3.1‰ for soil, and −2.5 to −4.6‰ for vegetation). In contrast, soil inside the fertilised plot had δ15N ≈ 0, but was enriched with 15N away from the plot edge. Earthworms collected from shearwater greens had enriched 15N signatures relative to earthworms from the fertilised plot (+3.8 and −0.9‰, respectively). Our data suggest that available N is tightly cycled in vegetation and soil for decades, and that shearwater derived N is substantially assimilated by earthworms. Therefore, because earthworms do not occur outside areas of shearwater influence, the birds should be viewed as ecosystem engineers of soil invertebrate foodwebs on Rum.

Root herbivore effects on aboveground multitrophic interactions: patterns, processes and mechanisms.

In terrestrial food webs, the study of multitrophic interactions traditionally has focused on organisms that share a common domain, mainly above ground. In the last two decades, it has become clear that to further understand multitrophic interactions, the barrier between the belowground and aboveground domains has to be crossed. Belowground organisms that are intimately associated with the roots of terrestrial plants can influence the levels of primary and secondary chemistry and biomass of aboveground plant parts. These changes, in turn, influence the growth, development, and survival of aboveground insect herbivores. The discovery that soil organisms, which are usually out of sight and out of mind, can affect plant-herbivore interactions aboveground raised the question if and how higher trophic level organisms, such as carnivores, could be influenced. At present, the study of above-belowground interactions is evolving from interactions between organisms directly associated with the plant roots and shoots (e.g., root feeders - plant - foliar herbivores) to interactions involving members of higher trophic levels (e.g., parasitoids), as well as non-herbivorous organisms (e.g., decomposers, symbiotic plant mutualists, and pollinators). This multitrophic approach linking above- and belowground food webs aims at addressing interactions between plants, herbivores, and carnivores in a more realistic community setting. The ultimate goal is to understand the ecology and evolution of species in communities and, ultimately how community interactions contribute to the functioning of terrestrial ecosystems. Here, we summarize studies on the effects of root feeders on aboveground insect herbivores and parasitoids and discuss if there are common trends. We discuss the mechanisms that have been reported to mediate these effects, from changes in concentrations of plant nutritional quality and secondary chemistry to defense signaling. Finally, we discuss how the traditional framework of fixed paired combinations of root- and shoot-related organisms feeding on a common plant can be transformed into a more dynamic and realistic framework that incorporates community variation in species, densities, space and time, in order to gain further insight in this exciting and rapidly developing field.

Root carbon flow from an invasive plant to belowground foodwebs

Aims Soil foodwebs are based on plant production. This production enters belowground foodwebs via numerous pathways, with root pathways likely dominating supply. Indeed, root exudation may fuel 30–50 % of belowground activity with photosynthate fixed only hours earlier. Yet we have limited knowledge of root fluxes of recent-photosynthate from invasive plants to belowground foodwebs.

The fate of glucose, a low molecular weight compound of root exudates, in the belowground foodweb of forests and pastures

Increasing evidence suggests that much of belowground, heterotrophic activity in terrestrial ecosystems is fueled by inputs of low molecular weight carbon compounds (LMWCCs). Root exudation (rhizodeposition) is a primary source of these inputs and will likely increase with rising atmospheric CO2. Yet the fates of these compounds belowground, as well as the environmental factors that influence them, are relatively unexplored. Using stable isotopes we track the fate of one dominant LMWCC, glucose, in three pasture and three forest sites located in South Carolina, USA. We resolve glucose-derived C in CO2, dissolved and soil organic C (DOC, SOC), microbial biomass, and microarthropods (Collembola, oribatid and mesostigmatid mites). After 72h, the greatest proportions of glucose-C are in microbial biomass and SOC, followed by CO2, DOC, and microarthropods. Within this short time frame, glucose-C propagates through the foodweb to the highest trophic level, predatory mesostigmatid mites. The biomass of these predators is the only variable that explains the relative partitioning across sites of glucose-C, with higher biomass associated with reduced partitioning of glucose-C to respiration and hence greater retention belowground. Our results suggest that LMWCCs entering belowground systems may propagate through soil foodwebs rapidly, and that their partitioning belowground may potentially be determined by higher trophic levels.

Carbon flow into microbial and fungal biomass as a basis for the belowground food web of agroecosystems

Abstract The origin and quantity of plant inputs to soil are primary factors controlling the size and structure of the soil microbial community. The present study aimed to elucidate and quantify the carbon (C) flow from both root and shoot litter residues into soil organic, extractable, microbial and fungal C pools. Using the shift in C stable isotope values associated with replacing C3 by C4 plants we followed root- vs. shoot litter-derived C resources into different soil C pools. We established the following treatments: Corn Maize (CM), Fodder Maize (FM), Wheat + maize Litter (WL) and Wheat (W) as reference. The Corn Maize treatment provided root- as well as shoot litter-derived C (without corn cobs) whereas Fodder Maize (FM) provided only root-derived C (aboveground shoot material was removed). Maize shoot litter was applied on the Wheat + maize Litter (WL) plots to trace the incorporation of C4 litter C into soil microorganisms. Soil samples were taken three times per year (summer, autumn, winter) over two growing seasons. Maize-derived C signal was detectable after three to six months in the following pools: soil organic C (C org ), extractable organic C (EOC), microbial biomass (C mic ) and fungal biomass (ergosterol). In spite of the lower amounts of root- than of shoot litter-derived C inputs, similar amounts were incorporated into each of the C pools in the FM and WL treatments, indicating greater importance of the root- than shoot litter-derived resources for the soil microorganisms as a basis for the belowground food web. In the CM plots twice as much maize-derived C was incorporated into the pools. After two years, maize-derived C in the CM treatment contributed 14.1, 24.7, 46.6 and 76.2% to C org , EOC, C mic and ergosterol pools, respectively. Fungi incorporated maize-derived C to a greater extent than did total soil microbial biomass.

Impact of the plant community composition on labile soil organic carbon, soil microbial activity and community structure in semi-natural grassland ecosystems of different productivity

The main objective was to describe the effects of plant litter on SOC and on soil microbial activity and structure in extensively managed grasslands in Central Germany that vary in biomass production and plant community composition. The decomposition of shoot and root litter was studied in an incubation experiment. Labile C and N were isolated by hot water extraction (CHWE, NHWE), while functional groups of microbes were identified by PLFA analysis and microbial activity was measured using a set of soil exo-enzymes. The plant community composition, particulary legume species affected SOC dynamics and below-ground microbial processes, especially via roots. This was reflected in about 20% lower decomposition of root litter in low productivity grassland soil. The CHWE soil pool was found to be a key driver of the below-ground food web, controlling soil microbial processes. Below-ground responses appear to be related to the presence of legume species, which affected the microbial communities, as well as the ratio between fungal and bacterial biomass and patterns of soil enzyme activity. Low productivity fungal-dominated grasslands with slow C turnover rates may play an important role in SOC accumulation. The approach used here is of particular importance, since associated biological and biochemical processes are fundamental to ecosystem functioning.

Impacts of emerald ash borer-induced tree mortality on leaf litter arthropods and exotic earthworms

a b s t r a c t Because leaf litter occurs at the interface between the soil and atmosphere, the invertebrates inhab- iting it represent important linkages between above- and below-ground food webs. The responses of these organisms to forest disturbance brought about by invasive species should therefore have far- reaching ecological effects. The purpose of this study was to explore how canopy gap formation (gap fraction 1-10%) and fallen ash trees (logs) resulting from extensive (>99%) ash tree mortality caused by the invasive emerald ash borer (EAB) affect the distributions of litter-dwelling arthropods and earth- worms. These organisms did not vary in abundance across the gap fraction range studied but, as predicted from the literature, many taxa (e.g., Aranea, Coleoptera, Collembola, Diplopoda, Isopoda, Opiliones and exotic earthworms) were more abundant next to logs than away from them. Contrary to expectations, arthropods did not become more concentrated next to logs as canopy openness increased, with isopods exhibiting the opposite response. These results suggest litter-dwelling arthropods in EAB-infested forests are favored by inputs of ash wood to the forest floor but are largely unaffected by increases in canopy openness, at least across the gap fraction range studied. Published by Elsevier GmbH.

The action of an animal ecosystem engineer: Identification of the main mechanisms of earthworm impacts on soil microarthropods

Non-trophic interactions are shaping soil food web structure and functions. Particularly, the action of ecosystem engineers, such as earthworms, are likely to fundamentally impact the abiotic and biotic properties of their environment. The present study aimed to identify the main mechanisms through which earthworms belonging to varying ecological groups – epigeic, endogeic and anecic species – affect soil microarthropods by reviewing the literature on this topic and by performing meta-analyses. Earthworm ecological groups differed considerably in their impacts on microarthropods, whereas effects did not vary significantly between microarthropod taxa at the habitat scale. Inconsistent impacts of epigeic species on soil microarthropods are most likely due to differences in earthworm densities. Effects can thus be positive in the case of moderate densities or negative in the case of high densities and associated distinct changes in the physical structure of the upper soil organic layers. By contrast, impacts of endogeic earthworms appeared to be mainly negative and were primarily due to competition with microarthropods for food resources. Consequently, negative impacts on soil microarthropods intensified with increasing earthworm density and biomass. This interaction between endogeic earthworms and microarthropods is better referred to as amensalism due to the competitive predominance of earthworms. Impacts of anecic earthworm species differed significantly from that of endogeic ones; they were neutral at the habitat scale and positive on the microhabitat scale. Moreover, impacts were independent of earthworm densities due to the quasi-territorial behaviour of anecic earthworms. Positive effects were mainly attributed to the formation of stable microhabitats by anecic species; namely burrows/middens, rich in nutrients and microorganisms. The present study points to the relevance of the non-trophic biotic interactions that drive the composition of belowground food webs by identifying the most essential mechanisms underlying the impacts of animal ecosystem engineers on soil microarthropods. Moreover, as earthworms emerge as important biological invaders, the results of the present study may help to fully appreciate, estimate and model the consequences of this momentous global change phenomenon. Particularly, the spread of exotic epigeic and endogeic earthworm species likely threatens soil microarthropod density, diversity and functions.

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

AbstractCarbon and nitrogen stable isotope ratios (δ13C and δ15N) 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 δ13C values are similar (<1‰ difference) to those of their diet, while consumer δ15N 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 δ13C and δ15N in belowground systems, e.g., enrichment of13C and15N in soil organic matter (likely related to soil microbial metabolism). These enrichments should be associated with the high13C (~3‰) enrichment in belowground consumers relative to litter and soil organic matter and with the large variation in δ15N (~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 δ13C and δ15N analyses of terrestrial food webs. Meanwhile, by measuring the δ13C of key predators, the linkage between above‐ and belowground systems could be estimated based on observed differences in δ13C of primary producers, detritivores and predators. Furthermore, radiocarbon (14C) measurements will allow the direct estimation of the dependence of predators on the belowground systems.

Changes in herbivore control in arable fields by detrital subsidies depend on predator species and vary in space.

Prey from the decomposer subsystem may help sustain predator populations in arable fields. Adding organic residues to agricultural systems may therefore enhance pest control. We investigated whether resource addition (maize mulch) strengthens aboveground trophic cascades in winter wheat fields. Evaluating the flux of the maize-borne carbon into the food web after 9 months via stable isotope analysis allowed differentiating between prey in predator diets originating from the above- and belowground subsystems. Furthermore, we recorded aphid populations in predator-reduced and control plots of no-mulch and mulch addition treatments. All analyzed soil dwelling species incorporated maize-borne carbon. In contrast, only 2 out of 13 aboveground predator species incorporated maize carbon, suggesting that these 2 predators forage on prey from the above- and belowground systems. Supporting this conclusion, densities of these two predator species were increased in the mulch addition fields. Nitrogen isotope signatures suggested that these generalist predators in part fed on Collembola thereby benefiting indirectly from detrital resources. Increased density of these two predator species was associated by increased aphid control but the identity of predators responsible for aphid control varied in space. One of the three wheat fields studied even lacked aphid control despite of mulch-mediated increased density of generalist predators. The results suggest that detrital subsidies quickly enter belowground food webs but only a few aboveground predator species include prey out of the decomposer system into their diet. Variation in the identity of predator species benefiting from detrital resources between sites suggest that, depending on locality, different predator species are subsidised by prey out of the decomposer system and that these predators contribute to aphid control. Therefore, by engineering the decomposer subsystem via detrital subsidies, biological control by generalist predators may be strengthened.

The structure of a food web in a tropical rain forest in Malaysia based on carbon and nitrogen stable isotope ratios

Abstract:Carbon and nitrogen stable isotope ratios (δ13C and δ15N) have been used to study the structure of food webs. However, few studies have examined how a terrestrial food web can be depicted by this technique. We measured δ13C and δ15N in various consumers of four trophic groups (detritivores, herbivores, omnivores and predators), including vertebrates and invertebrates (14 orders, ≥24 families), as well as canopy and understorey leaves in a tropical rain forest in Malaysia. We found that δ13C and δ15N of the consumers differed significantly among the trophic groups. The predators had significantly higher δ13C than the herbivores, and were similar in δ13C to the detritivores, suggesting that most predators examined depend largely on below-ground food webs. δ15N was higher in predators than detritivores by about 3‰. The comparison of δ13C in plant materials and herbivores suggests that most herbivores are dependent on C fixed in the canopy layers. The vertebrates had significantly higher δ15N and δ13C than the invertebrates of the same trophic group, likely reflecting differences in the physiological processes and/or feeding habits. This study indicates that stable isotope techniques can help better understanding of the terrestrial food webs in terms of both trophic level and the linkage of above- and below-ground systems.

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.

Long-term impacts of high-input annual cropping and unfertilized perennial grass production on soil properties and belowground food webs in Kansas, USA

Soil ecosystem properties and processes which simultaneously maintain native fertility and sustain plant yields are of principal interest in sustainable agriculture. Native prairies in Kansas are relevant in this context, as some have been annually harvested for hay for over 75 years with no fertilization or detectable decline in yield or soil fertility. In contrast, annual crop production has resulted in significant reductions in soil fertility and now requires intensive inputs to maintain yields. Soil food webs were compared between hayed native grasslands and adjacent annual croplands in order to determine the long-term effects of these two production systems on soil ecosystem properties. Soil chemical and physical properties, bacterial and nematode community structure and abundance were measured across five paired sites at six depth intervals to 1 m. Soil organic carbon, total nitrogen, and water stable aggregates were all significantly greater in perennial grasslands than in annual croplands to a depth of 60 cm. Microbial biomass carbon was also greater in grasslands than in croplands, and shifts in δ 13C indicated greater input of new carbon at lower depths in grasslands relative to annual croplands. Bacterial and nitrogen fixing communities in croplands and grasslands were significantly different in the surface 40 cm and nematode community differences persisted through 1 m. Nematode community indices suggested enhanced fungal decomposition pathways, fewer plant-feeding nematodes, and greater food web complexity and stability in grassland soils than in annual cropland soils. These data indicate that perennial grasslands in Kansas, even when annually harvested for decades, support higher levels of soil fertility and structure and more complex biological communities than annual cropping systems.

Microbial community response to transition from conventional to conservation tillage in cotton fields

In the southeastern United States, conservation tillage techniques are used to conserve soil nutrients and structure, providing habitat and substrate for biota, which are largely responsible for the mineralization of nutrients in the soil. A deterrent for growers considering the transition to conservation tillage is the delay in soil response (e.g. increased soil carbon, efficient nutrient cycling, impacts on yield) associated with the equilibration of the soil food web. The objective of this study was to determine if the microbial community composition and biomass changed with transition to conservation tillage. Soils sampled from five sites, representing a chronosequence of conservation tillage (from conventionally tilled to 30 year no-till), were collected for fatty acid analysis. Microbial communities were significantly different among sites. Fungi, characterized by 18:2ω6, 18:1ω9, and 18:3ω6 c fatty acids, were typically lowest in the conventionally tilled soil, probably due to repeated disruption of the fungal hyphae associated with tillage. In all soils, soil nutrient concentrations, moisture and microarthropod abundance were correlated with microbial structure. Plots in conservation tillage were significantly different from the conventionally tilled plots, but did not exhibit a clear linear pattern across the chronosequence. This evidence that belowground food webs can respond quickly to a cessation in tillage suggests that the delay in soil response may be due more to the time required to build organic matter than to a slow response by the biota.

Scale and density dependent relationships among roots, mycorrhizal fungi and collembola in grassland and forest

Belowground foodwebs, which dominate energy flow in temperate systems, may yield insights into general foodweb function: do consumers drive resource levels or simply respond to them? Controversy exists about the relationships among fine roots, mycorrhizal fungi and invertebrate grazers because of the difficulty of studying belowground processes. We used contrasting temporal scales to examine relationships among roots, fungi and collembola in the top 90 cm of soil in native grassland and forest over three years, with data collected from minirhizotrons. Root production was generally similar between habitats, but fungal production and collembola density were mostly significantly greater in forest than grassland. All variables decreased significantly with increasing depth, except for forest mycorrhizal fungi. This exception suggests that forest mycorrhizae are especially important for nutrient uptake at greater soil depths. Relationships among roots, mycorrhizal fungi and collembola varied with the temporal scale considered. Over the entire growing season, collembola were significantly positively correlated with root production in forest, and with both fungal and root production in grassland, reflecting the broad positive associations between these groups. In contrast, over the shortest sampling interval (i.e. two weeks), strong negative relationships suggested that collembola grazing decreased fungal and root production. Data from all dates and depths considered together revealed that both fungal and root production were significantly higher at intermediate collembola density than at low or very high density. In summary, belowground consumers appear to drive resource levels at short time scales, but respond to broad variation in resource availability at annual time scales.

Above and below ground impacts of terrestrial mammals and birds in a tropical forest

Understanding the impact of losing trophic diversity has global significance for managing ecosystems as well as important theoretical implications for community and ecosystem ecology. In several tropical forest ecosystems, habitat fragmentation has resulted in declines and local extinctions of mammalian and avian terrestrial insectivores. To assess the ability of a tropical rainforest community in Ivory Coast to resist perturbation from such loss of trophic diversity, I traced feedbacks in above and below ground communities and measured changes in nutrient levels and herbivory rates in response to an experimental exclosure of avian and mammalian terrestrial insectivores. I present evidence that loss of this functional group may result in increased tree seedling herbivory and altered nutrient regimes through changes in the abundance and guild structure of invertebrates. Exclusion of top predators of the forest floor resulted in increased seedling herbivory rates and macro‐invertebrate (>5 mm) densities with strongest effects on herbivorous taxa, spiders and earthworms. Densities of microbivores including Collembola, Acarina and Sciaridae showed the opposite trend as did levels of inorganic phosphorus in the soil. Results were evaluated using path analysis which supported the presence of a top down trophic cascade in the detrital web which ultimately affected turnover of phosphorus, a limiting nutrient in tropical soils. Results illustrate the potential importance of vertebrate predators in both above and belowground food webs despite the biotic diversity and structural heterogeneity of the rainforest floor.

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.

Soil food web structure during ecosystem development after land abandonment

The re-establishment of natural species rich heathlands on abandoned agricultural land is a common land use change in North-West Europe. However, it can take several decades to re-establish natural species rich heathland vegetation. The development rate has found to depend both on soil food web composition and on soil processes. We measured the soil food web composition in three ex-arable fields abandoned 2, 9 and 22 years ago and in a heathland. To characterize food structure, we defined four trophic levels and a root, fungal and bacterial energy channel. We hypothesized that with increasing time since abandonment, i.e. field age, (1) the basic resource level biomass, i.e. soil organic matter (SOM) and roots, will increase and thereby also that of biomasses at higher trophic levels, (2a) the root energy channel biomass will decrease, (2b) the fungal energy channel biomass will increase, and (2c) the bacterial energy channel biomass will not change. The results showed that biomasses at the basic resource level and at the first trophic level indeed increased with field age, but not the biomasses at higher trophic levels. It is not clear what the cause of the lack of increase in higher trophic levels was, possibly top-down or bottom-up forces played a role. The root energy channel biomass decreased and the fungal channel increased as hypothesized, but the bacterial channel biomass also increased with field age. The increase of the bacterial channel biomass contradicted the hypothesis, but agreed with the observed increase in SOM quantity and lack of decrease in SOM quality. On overall, results show that changes in belowground food webs lag behind changes of the aboveground vegetation. Such time lags may hamper secondary vegetation succession. Understanding those time lags may therefore help to develop management schemes improving land conversion processes.