Effects of agricultural management on nematode–mite assemblages: Soil food web indices as predictors of mite community composition

Biological indicators based on abundances of soil organisms are powerful tools for inferring functional and diversity changes in soils affected by agricultural perturbations. Field plots, combining organic and conventional practices with no tillage, conservation tillage and standard tillage maintained different nematode assemblages and soil food webs. Soil food web indices based on nematode assemblages were reliable predictors of the trophic composition of functional characteristics of soil mite assemblages. Bacterial-feeding and predatory nematodes, together with predatory mites, were abundant in the organic-no till treatments and were associated with high values of the Enrichment and the Structure Index based on nematode assemblages. Conventional-Standard tillage treatments had high abundances of fungal- and plant-feeding nematodes and algivorous mites, associated with high values of the Basal and Channel Index. This study validates the hypothesis that nematode-based soil food web indices are useful indicators of other soil organisms such as mites, with similar functional roles and environmental sensitivities.     

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.     

Urbanization alters the functional composition,but not taxonomic diversity,of the soil nematode community

We evaluated the response of riparian forest soil nematode community structure to the physico-chemical environment associated with urban land use. Soils were sampled seasonally between December 2000 and October 2002 along an urban—rural transect in Asheville, North Carolina. We characterized the taxonomic (to genus) and functional composition (trophic groups) of the nematode community of forest soils, as well as several nematode ecological indicators (maturity index, channel index, weighted faunal index). The diversity of nematode genera was not affected by urban land use. However, there tended to be functional differences in the nematode communities along the land use gradient. The urban soils tended to have lower abundances of predatory and omnivorous nematodes. Differences in channel index scores indicated that there was less fungal dominance in the soil food webs of the urban soils. Our results indicate that the functional composition of the soil food web is an important component of soil biodiversity that can be affected by land use practices. This study was conducted in a relatively small city; hence the influence of pollutants on the soil environment was not as great as in larger cities. Correspondingly, the impact on the soil nematode community was not very severe. The utilization of the nematode community assemblage as an indicator of soil conditions should be further explored in urban places of differing magnitudes of environmental effects.     

Community composition,diversity and metabolic footprints of soil nematodes in differently-aged temperate forests

Soil nematode communities can provide important information about soil food web structure and function. However, how soil nematode communities and their metabolic footprints change over time in temperate forests is not well known. We examined the changes in the composition, diversity and metabolic footprints of soil nematode communities in three differently-aged (young, mid and old) forests of the Changbai Mountains, China. Carbon flows through different nematode trophic groups were also quantified based on nematode biomasses. The results showed that the highest abundance and diversity of total nematodes was found in the mid forest. Nematode communities were characterized by the replenishment in abundance but not the replacement of dominant genera. A low enrichment footprint in the young forest suggests a decline in available prey, while a high enrichment footprint in the mid forest indicates an increase in resource entry into soil food web. The relationship between the carbon flows of omnivores-predators and fungivores was stronger than that among other trophic groups. Our study shows that bottom-up effects of the vegetation, the soil environment and the connectedness of nematode trophic groups are all important driving forces for nematode community structure in temperate forests.     

温室土壤食物网静态和动态结构特征

Soil food web structure is fundamental to ecosystem process and function; most studies on soil food web structure have focused on agro-ecosystems under different management practices and natural terrestrial ecosystems, but seldom on greenhouses. This study explored the static and temporal variability of soil food structure in two greenhouses of Shandong Province, North China over a two-year period. The static properties were measured directly by surveying functional group composition and a series of parameters portraying the species properties, link properties, chain properties and omnivory properties of the web, as well as indirectly through calculation of nematode indices, enrichment index （EI）, structure index （SI）, and channel index （CI）. The dynamic variability of greenhouse soil food structure was described by the dynamics of functional groups, Bray-Curtis （BC） similarity and cluster analysis. The results showed that the greenhouse soil food web contained 14 functional groups, with microbes having the highest mean biomass, followed by protozoa. Of the three functional groups of protozoa, flagellates were the dominant group on most sampling dates, amoebae only became the dominant group during the summer, while ciliates were the least prevalent group. All nematodes were assigned into one of the four functional groups, bacterivorous, fungivorous, herbivorous and omnivorous, and the fungivorous nematodes had the lowest mean biomass. Mites were assigned into three functional groups and the omnivorous noncryptostigmatic mites were the dominant group. All the functional groups showed significant seasonal changes. The soil food web connectance was 0.15, the maximum food chain length was 5, and the average food chain length was 3.6. The profiles of the EI and SI showed that the food web was resource- depleted with minimal structure. The results of CI indicated that the bacterial decomposition pathway was the dominant pathway in the food web of the greenhouse soils studied and the results of BC similarity showed that the soil food web had higher variability and instability over time. The cluster analysis showed that the functional groups located at high trophic levels with low biomass were in a cluster, whereas those at low trophic levels with high biomass were closer. Compared with the food web structure of agroecosystem and natural terrestrial ecosystem soils, the structure of greenhouse soil food web was simple and unstable, which was likely driven by high agricultural intensification, particularly over application of fertilizers.     

Functional shifts of grassland soil communities in response to soil warming

In terrestrial ecosystems most carbon (C) occurs below-ground, making the activity of soil decomposer organisms critical to the global carbon cycle. Temperate grassland ecosystems, contain large, diverse and active soil meso- and macrofauna decomposer communities. Understanding the effects of climate change on their ecology offers a first step towards meaningful predictions of changes in soil organic carbon mineralisation.We examined the effects of soil warming on the abundance, diversity and ecology of temperate grassland soil fauna functional groups, ecosystem net CO₂ flux and respiration and plant above- and below-ground productivity in a 2-year plant-soil mesocosm experiment. Low voltage heating cable mounted on a framework of stainless steel mesh provided a constant 3.5 °C difference between control and warmed mesocosm soils.Results showed that this temperature increment had little effect on soil respiration and above-ground plant biomass. There was, however, a significant effect on the soil fauna due to warmer conditions and increased root growth, with significant decreases in the numbers in the large oligochaete groups and Prostigmata mites and the re-distribution of enchytraeids to deeper soil layers. Functional groups exhibited individualistic responses to soil warming, with the total disappearance of epigeic species in the case of the ecosystem engineers and an increased diversity of fungivorous mites that, together, produced significant changes in the composition and trophic structure of the fauna community.The observed switch towards a fungal driven food web has important implications for the fate of soil organic carbon in temperate ecosystems subjected to sustained warming. Accordingly, soil biology needs to be properly incorporated in C models to make better predictions of the fate of SOC under warmer scenarios.     

Soil nematode fauna of afforested mine sites: genera distribution,trophic structure and functional guilds

In an afforested coal mining site at Berzdorf, Germany, the soil nematode community was investigated on the basis of genera distribution, trophic structure, and functional guilds. To assess food web complexity and nutritional status of afforested mine soils, the “weighted faunal analysis” was applied and found to be a useful diagnostic tool to describe the poorly developed, basal or matured food web characters of the Berzdorf mine soils. The tree species used in afforestation was a crucial factor. A comparison between nine mine soils of different age (4–46 years) and afforestation (poplar, alder, pine, mixed deciduous, mixed coniferous) showed particularly poorly structured food webs in poplar afforestations, whilst alder and mixed deciduous sites presented more favourable conditions in terms of soil succession and food web maturity. The results revealed no continuous increase in the complexity of food web interactions from younger successional stages to older stages. However, it was demonstrated that food web development at Berzdorf followed distinct shifts in soil primary succession. Correspondences of nematode community data and environmental parameters are discussed.     

The fat that matters: Soil food web analysis using fatty acids and their carbon stable isotope signature

Chemical taxonomy based upon the composition of lipids is widely applied to investigate microbial communities and fatty acids have recently been employed to connect soil microbial and faunal food webs as well as to elucidate functional groups at higher trophic levels. The additional use of compound-specific isotopic analysis of ¹³C/¹²C ratios in fatty acids allows assessing specific trophic links and belowground carbon fluxes. In this review systematic patterns and processes underlying variations in the composition of fatty acids and their ¹³C/¹²C ratio are described. The emphasis is on biomarker fatty acids, their incorporation and modification, effects of pool size, and analytical methods. Further development of the application of fatty acid profiling to soil ecology should include both advances in experimental research and growth of theory. Accordingly, areas in which future experimentation can lead to progress in soil food web analysis are identified. Overall, combining fatty acid biomarker and their isotopic ratios will allow detailed insight into belowground trophic interactions.     

Cover crops alter the soil nematode food web in banana agroecosystems

Cover crops are increasingly being used in agriculture, primarily for weed or erosion management. The addition of cover crops increases the primary productivity of the system and diversifies basal resources for higher trophic levels. How increases in the quality and quantity of basal resources affect bottom-up and top-down control remains a key question in soil food web ecology. We evaluated the response of the nematode community to the introduction of cover crops between rows of a banana plantation. We measured changes in nematode food web structure and inferred the prevalence of bottom-up and top-down effects on the abundance of phytophagous nematodes (i.e., plant-feeding and root-hair-feeding species) 1.5 years after plots with cover crops (Poaceae or Fabaceae species) or bare soil were established. The addition of a cover crop greatly affected the structure and the abundance of the soil nematode community 1.5 years after planting. The abundance of all trophic groups except for plant-feeding nematodes tended to increase with the addition of cover crops. The Shannon–Weaver diversity index and the enrichment index increased with the addition of cover crops, indicating that opportunistic, bacterivorous and fungivorous nematodes benefited from the added resources. Plant-feeding nematodes were least abundant in plots with Poaceae cover crops, while bacterivorous, omnivorous, and root-hair-feeding nematodes were more abundant with Fabaceae cover crops than with bare soil, indicating that cover crop identity or quality greatly affected soil food web structure. Bottom-up effects on all trophic groups other than plant-feeding nematodes were evident with Poaceae cover crops, suggesting an top-down control of plant-feeding nematodes by omnivorous nematodes. Conversely, plant-feeding nematodes were evidently not suppressed in Fabaceae cover crops, perhaps because bottom-up effects on omnivorous nematodes were weaker (hence, top-down control by omnivorous nematodes was weaker), and because Fabaceae cover crops probably served as good hosts for some plant-feeding nematodes.     

Soil community changes during secondary succession to naturalized grasslands

Succession to a naturalized grassland from former agricultural land and pastures is accompanied by changes in plant biodiversity and in the soil community. These changes are the result of a reduction or elimination of management, fertilizer applications and of grazing by large herbivores. We review soil biology studies on agricultural land that are in various successional stages towards naturalized grasslands, where interactions between plant species composition changes and the soil ecology affect each other. In many chronosequence studies, the soil microbial community tends to shift towards a less bacterial, and more fungal dominated food web energy channel following a reduction in fertilizer inputs and grazing intensity. Whereas changes in microarthropod communities are obscured, nematode trophic functional group (ecological guild) changes respond to both plant and soil community changes. There are opportunities to further study the feedback interactions between roots and soil organisms in grasslands. A better understanding of the molecular feedback mechanisms would be beneficial in long-term grassland management.     

Colonisation of newly established habitats by soil decomposer organisms: the effect of habitat corridors in relation to colonisation distance and habitat size

The aim of the present 2.5-year-long field experiment was to explore the ability of various members of the detrital food web to colonise newly established habitat patches in field conditions, either in the presence or absence of habitat corridors. Patch size and distance to the “mainland” (colonisation source) were manipulated to explore the scale dependency of the corridor effects. Sterilised humus patches, embedded in mineral soil regarded as uninhabitable (or non-preferred) matrix for the soil organisms, functioned as newly established habitats. Intact forest soil served as the source of colonisers. Three kinds of patches were established: large ones situated at relatively long distance from the colonisation source, and small ones situated either at long or short distance from the source. Corridors, when present, connected the patches to the intact forest soil. The presence of corridors consistently increased the species richness of soil fungi in the patches. Also bacterial species richness, number of microarthropod taxa and abundance of enchytraeid worms were sometimes increased by the presence of corridors. The response of bacterial species was assumed to be indirect, caused by the response of enchytraeid worms functioning as dispersal agents for the bacteria. Colonisation of the patches by soil organisms was virtually independent of patch size and colonisation distance, both in the presence and absence of corridors. This can be related to the resource-poor or otherwise sub-optimal conditions in the initially sterile humus soil, restricting the number of species able to establish themselves in the patches and/or utilise the corridors. Nevertheless, the results indicate that habitat corridors can facilitate the colonisation of unpopulated habitats by organisms with greatly different life history traits.     

Effect of pesticide application on soil microorganisms

Modern agriculture largely relies on the extensive application of agrochemicals, including inorganic fertilizers and pesticides. Indiscriminate, long-term and over-application of pesticides have severe effects on soil ecology that may lead to alterations in or the erosion of beneficial or plant probiotic soil microflora. Weathered soils lose their ability to sustain enhanced production of crops/grains on the same land. However, burgeoning concern about environmental pollution and the sustainable use of cropping land have emphasized inculcation of awareness and the wider application of tools, techniques and products that do not pollute the environment at all or have only meager ecological concerns. This review covers the types of, concerns about and current issues regarding the extensive application of agrochemicals, in particular pesticides, on a variety of microorganisms integrated in successive food chains in the soil food web.     

The feeding ecology of earthworms – A review

Current knowledge of earthworm feeding ecology is reviewed, with particular reference to food selection, ingestion, digestion and assimilation, and the use of novel techniques to advance understanding of the functional significance of these processes.

Traditional research methods including direct observation of feeding behaviour, gut content analysis, choice tests, and litter bags have provided a wealth of information on earthworm feeding. However, there is a lack of the mechanistic, quantitative information required to characterise adequately their functional role in soil ecosystem processes such as soil C sequestration and loss, decomposition of organic residues, the maintenance of soil structure and trophic interactions with plants and microorganisms.

Stable isotope ratio analysis of light elements (C, N, and S) offers a powerful research tool to reveal and quantify trophic relationships of earthworms in soil food webs, while molecular techniques can further enhance understanding of the interactions between earthworms and microorganisms and their functional significance.     

Carbon fluxes in soil food webs of increasing complexity revealed by C labelling and C natural abundance

Soil food webs are mainly based on three primary carbon (C) sources: root exudates, litter, and recalcitrant soil organic matter (SOM). These C sources vary in their availability and accessibility to soil organisms, which could lead to different pathways in soil food webs. The presence of three C isotopes (¹²C, ¹³C and ¹⁴C) offers an unique opportunity to investigate all three C sources simultaneously. In a microcosm experiment we studied the effect of food web complexity on the utilization of the three carbon sources. We choose an incomplete three factorial design with (i) living plants, (ii) litter and (iii) food web complexity. The most complex food web consisted of autochthonous microorganisms, nematodes, collembola, predatory mites, endogeic and anecic earthworms. We traced C from all three sources in soil, in CO₂ efflux and in individual organism groups by using maize grown on soil developed under C₃ vegetation and application of ¹⁴C labelled ryegrass shoots as a litter layer. The presence of living plants had a much greater effect on C pathways than food web complexity. Litter decomposition, measured as ¹⁴CO₂ efflux, was decreased in the presence of living plants from 71% to 33%. However, living plants increased the incorporation of litter C into microbial biomass and arrested carbon in the litter layer and in the upper soil layer. The only significant effect of food web complexity was on the litter C distribution in the soil layers. In treatments with fungivorous microarthropods (Collembola) the incorporation of litter carbon into mineral soil was reduced. Root exudates as C source were passed through rhizosphere microorganisms to the predator level (at least to the third trophic level). We conclude that living plants strongly affected C flows, directly by being a source of additional C, and indirectly by modifying the existing C flows within the food web including CO₂ efflux from the soil and litter decomposition.     

Functional implications of soil fauna diversity in boreal forests

This paper is a review of recent experiments dealing with the role of soil fauna in decomposition, mineralisation and primary production in coniferous forest soils. The experiments have been grouped according to the degree and nature of the ‘diversity gradient' between the ‘more diverse' community and its control: single animal species or an uncontrolled mixture of species versus microbiota only, several known animal species of the same trophic group versus one species only (species diversity), two or more functional groups versus one only, and food chains with predators versus microbes and microbivores only. The evidence available at present suggests that taxonomic diversity and predation have no consistent effects on the process rates in soil, while adding to the ‘functional' or ‘trophic group diversity' results in a more predictable enhancement in mineralisation. Especially the enchytraeid Cognettia sphagnetorum seems to be a keystone species in boreal forest soils. However, there are only few experiments in which species diversity per se has been taken as a separate factor, without a simultaneous change in the number of trophic groups or in total decomposer biomass.     

Trophic interactions in centipedes (Chilopoda,Myriapoda) as indicated by fatty acid patterns: Variations with life stage,forest age and season

Fatty acid (FA) analysis investigates changes in the relative contribution of prey from major energy channels in decomposer food webs for predator nutrition. Adopting this approach we investigated whether the trophic niche of centipedes, as major invertebrate predators in forest soil food webs, changes with maturation, season or forest age. Generally, each of the four centipede species studied differed significantly in their FA composition suggesting trophic niche differentiation. FA profiles differed more strongly in the two geophilomorph (Strigamia acuminata and Geophilus ribauti) than in the two lithobiomorph species (Lithobius crassipes and Lithobius mutabilis) suggesting that in particular the former feed on markedly different prey. FA profiles changed during post-embryonic development in each of the four centipede species. Differences were most pronounced in the two lithobiomorph species shifting to predominantly fungal feeding prey. Further, FA profiles varied with season indicating that centipedes exploit more prey out of the bacterial channel in autumn. FA profiles of centipedes varied little with forest age suggesting that soil food webs are remarkably invariant across different forest ecosystems. The results indicate that FA composition of second order consumers closely reflects changes in diet of prey species and composition of basal resources. The study proved FA profiles as powerful tool to gain insight into critical characteristics of soil food web stability, i.e., compartmentalisation and the relative importance and variability of energy channels.     

Abiotic controls on the functional structure of soil food webs

Summary The hypothesis that the trophic structure of soil food webs changes as a result of the abiotic environment was examined by reviewing studies of soil biota. In dry soils with a water potential below –1.5 MPa, most bacteria, protozoans, and many species of nematodes are not active. These taxa persist in the soil in a state of anhydrobiosis. Because soil fungi grow at soil water potentials of –6.0 to –8.0 MPa, soil food webs in dry environments appear to be fungal-based and fungal grazers in dry environments appear to be predominantly fungiphagous mites. There is indirect evidence that some species of fungiphagous mites remain inactive in dry soils in a state of cryptobiosis. In habitats where there is insufficient vegetative cover to shade and modify the soil surface, the functional soil food web consists of fungi and a few taxa of soil acari for extended periods of time.     

Transitory dynamic effects in the soil invertebrate community in a temperate deciduous forest: Effects of resource quality

The importance and strength of bottom-up forces in terrestrial soil systems are poorly understood. In contrast to aquatic systems, where trophic cascades and top-down forces dominate, it has been postulated that terrestrial systems are regulated mainly by bottom-up forces. We set up a 17 month field experiment to study the effects of addition of resources of different quality (wood, wheat bran, pet food, and glucose+phosphorous+nitrogen) on the soil micro-, meso- and macrofauna as well as on microbial biomass, ergosterol content and abiotic parameters (soil pH, water content, carbon and nitrogen content) in a beech forest (Fagus sylvatica) on sandstone. We hypothesized that bottom-up effects will be strong on lower trophic levels resulting in increased biomass of bacteria and fungi, and that this increase will propagate to higher trophic levels (microbivorous invertebrates, predators) but with decreasing intensity due to dampening of bottom-up forces at higher trophic levels by high connectivity, trophic-level-omnivory and generalist feeding. The results of the study in general did not support these hypotheses. Microbial biomass only moderately increased after resource addition, and while densities of several animal groups increased (lumbricids, nematodes, collembolans, gamasid mites, staphylinid beetles), densities of other groups declined (oribatid mites, prostigmate mites, lithobiids), and a large number of taxa remained unaffected (enchytraeids, diplopods, uropodine mites, pseudoscorpions, spiders). We conclude that (a) bottom-up forces are of limited importance in the soil system of temperate deciduous forests, (b) large primary decomposers, such as earthworms, do not depend on microorganisms as food but consume organic matter directly, (c) the link between microorganisms and microbivores, such as collembolans, is weak since collembolan density increased even though microbial biomass was unaffected, (d) habitat modification by ecosystem engineers, such as earthworms, is more important than resource availability for a number of soil invetebrates including prostigmate mites and centipedes, and (e) the soil food web in general is rather resistant responding little to changes in resource supply. The results also suggest that species which commonly are assigned to single trophic groups, such as collembolans, differently respond to changes of the base of the food web. Increased fungal biomass led to an increase in the density of Folsomia quadrioculata s.l. and Isotomiella minor, whereas the increased bacterial biomass was accompanied by an increase in density of Ceratophysella denticulata and Isotoma notabilis.