Soil Food Web Components Research Articles

Belowground plant inputs exert higher metabolic activities and carbon use efficiency of soil nematodes than aboveground inputs

Soil nematodes are key components of soil food web and, through their metabolic activities, play a crucial role in soil carbon (C) cycling. Aboveground and belowground plant C inputs can directly, or indirectly via soil microbes, modify nematode abundance and community composition. Aboveground and belowground C inputs differ in chemical composition, amounts, and frequency, so we hypothesized that the two input pathways affect nematode communities differently. To assess the relative contributions of aboveground versus belowground inputs to nematode community composition and activity, we subjected grassland soils to four plant input pathways over two consecutive years: no input, only aboveground input (+A), only belowground input (+B), and both aboveground and belowground inputs (+A + B). Nematode metabolic footprints, as estimates of C used in growth/reproduction and C lost by respiration, and C use efficiency (C used/(C used + C lost)) were calculated. We predicted that soils with belowground inputs, which are more directly linked to the soil biota, and which contain a more labile blend of molecules, would support richer and more complex nematode communities, and also favor a bacterial-driven decomposition channel. Accordingly, we showed that + B soils supported higher nematode numbers than + A soils, and that the bacterial decomposition channel was dominant in + B soils, while the fungal decomposition channel dominated in + A soils. Compared with + A soils, +B input system increased nematode structure footprints (the metabolic footprints of nematodes in upper functional guilds) rather than enrichment footprints (the metabolic footprints of enrichment opportunistic nematodes). Moreover, we observed that, compared to + A soils, +B soils had higher growth and respiration rates of bacterivores, omnivores-predators, and total nematodes. Finally, we found higher C use efficiency values for omnivores-predators and total nematodes in + B than in + A soils. We thus conclude that belowground plant-derived resources, by changing the ratio between fungivores and bacterivores, induce a faster carbon turnover rate, and higher metabolic activity of soil nematodes within soil food web, ultimately spurring richer and more efficient soil food web than aboveground inputs.

A case study of soil food web components affected by Fallopia japonica (Polygonaceae) in three natural habitats in Central Europe.

This study determined the effect of the invasive plant Fallopia japonica on soil physical properties, microbial respiration, microbial biomass carbon content, enzymatic activities, and soil nematode communities. We established in total 30 plots in three natural habitats (forest, grassland, wetland) that were either uninvaded or mostly monospecifically invaded by F. japonica. The soil physical and microbial properties differed among the investigated plots, but the differences were observed to be non-significant between the invaded and the uninvaded plots. Non-metric multidimensional scaling based on nematode species diversity indicated that the total number of identified nematode species and their abundance were higher in the uninvaded compare to the invaded plots. Negative effect of F. japonica on omnivores, plant parasites, and root-fungal feeder nematodes was confirmed by their lower abundance in the invaded compared to the uninvaded plots. In the invaded plots, we also confirmed lower Maturity and Channel index, but higher Enrichment index. Our results thus indicated that the invasive plant F. japonica could affect nematode communities, more than physical or microbial properties, regardless of habitat.

Plants mitigate detrimental nitrogen deposition effects on soil biodiversity

Anthropogenic nitrogen (N) deposition is an important component of global change and threatens terrestrial biodiversity. Most previous studies of the consequences of N deposition have focused on plant community responses and found that N deposition decreases plant diversity. However, the effects of N deposition on soil biodiversity and belowground biotic interactions remain poorly understood. We explored the changes in main soil food web components (microbes, nematodes, springtails, and mites) in response to elevated N deposition (60 kg N ha−1yr−1, starting from 2012 to 2014), and whether these changes are altered by the presence of plants (planting of shrubs in 2008) in a two-factorial field mesocosm experiment with 16 equally-sized plots (1 × 2 m). Our results showed that elevated N deposition negatively affected soil bacteria, while fungi showed rather neutral responses. Specifically, N deposition decreased bacteria Shannon's diversity index H′, richness, observed species abundance, bacterial activity, and resulted in a non-significant decrease of the relative abundance of rare bacterial taxa. By contrast, for fungi, only a non-significant decrease of richness was observed with N deposition. Importantly, those N deposition effects mostly occurred in the absence of planted shrubs. Moreover, shrub presence and N deposition also interactively affected the diversity of soil invertebrates, i.e., N deposition had little effect on them in the absence of planted shrubs, but resulted in an increase or a non-significant increase of soil invertebrate diversity in the presence of planted shrubs. Furthermore, N deposition did not affect the biomass/density of any soil food web component and biomass/density ratios related to soil food web structure regardless of absence or presence of planted shrubs; these indices were only affected by the presence of shrubs. Overall, these dissimilar responses of the diversity of soil microorganisms and animals to elevated N deposition indicate that plants are important mediators of N deposition effects on soil biodiversity. Thus, the present results may imply that an intact plant cover may mitigate detrimental N deposition effects on soil biodiversity.

Using structural equation modeling to test established theory and develop novel hypotheses for the structuring forces in soil food webs

Abstract Understanding the structuring elements of soil food webs is a major challenge for ecologists. Given the complex nature of soil food webs, common treatment-based experiments and bivariate data analyses can only capture part of that complexity. Structural equation model (SEM) is a promising multivariate technique that may help understand direct and indirect relationships in complex soil food webs. As bacterial and fungal energy channels, as well as bottom–up and top–down forces operate simultaneously, we used SEM to explore the complex interactions among multiple trophic levels in soil food webs. Further, SEM allowed us to test established theory and to refine and derive hypotheses regarding the structuring forces of soil food webs. We studied detritus-based soil food webs in a 55-year-old subtropical Illicium verum forest ecosystem by sampling different important soil food web components, including main soil microbial groups, soil nematodes, and soil microarthropods. The SEM results confirmed established theory by showing the central role of the fungal energy channel, and bottom–up effects were more important for the structure of the food web than top–down forces in the studied subtropical forest soil. In addition, we could derive some novel and refined hypotheses regarding important feeding links in soil food webs. For instance, we provide hypotheses on feeding preferences of different groups of soil biota and on the strength of bottom–up versus top–down effects in forest soil food webs. Overall, we highlight that SEM provides a framework to understand complex interactions and energy pathways in soil food webs in a multivariate context, test established theory, and propose novel experimental tests.

Selective alteration of soil food web components by invasive giant goldenrod Solidago gigantea in two distinct habitat types

Apart from relatively well‐studied aboveground effects, invasive plant species will also impact the soil food web. So far, most research has been focusing on primary decomposers, while studies on effects at higher trophic levels are relatively scarce. Giant goldenrod Solidago gigantea, native to North America, is a widespread and common invasive species in most European countries. We investigated its impact on plant communities and on multiple trophic levels of the soil food web in two contrasting habitats: riparian zones and semi‐natural grasslands. In 30 pairs of invaded and uninvaded plots, floristic composition, pH, fungal biomass and the densities of 11 nematode taxa were determined by using a quantitative PCR‐based method. In the two habitats, the invader outcompeted both rare and dominant plant species. Belowground, S. gigantea invasion reduced pH, increased overall fungal biomass as well as the density of a single lineage of fungivorous nematodes, the family Aphelenchoididae. The densities of two other, phylogenetically distinct lineages of fungivorous nematodes, Aphelenchidae and Diphtherophoridae, were unaffected by the local increase in fungal biomass. Apparently this plant species induces a local asymmetric boost of the fungal community, and only Aphelenchoididae were able to benefit from this invader‐induced change. The alternative explanation – the results are explained by a subtle, S. gigantea‐induced 0.1–0.2 units decrease of pH – seems unlikely, as pH optima for nematode taxa are relatively broad. Thus, apart from readily observable aboveground effects, the invasive plant species S. gigantea affects fungal biomass as well as a specific part of the fungivorous nematode community in a soil type‐independent manner.

Nitrogen Addition and Warming Independently Influence the Belowground Micro-Food Web in a Temperate Steppe

Climate warming and atmospheric nitrogen (N) deposition are known to influence ecosystem structure and functioning. However, our understanding of the interactive effect of these global changes on ecosystem functioning is relatively limited, especially when it concerns the responses of soils and soil organisms. We conducted a field experiment to study the interactive effects of warming and N addition on soil food web. The experiment was established in 2006 in a temperate steppe in northern China. After three to four years (2009–2010), we found that N addition positively affected microbial biomass and negatively influenced trophic group and ecological indices of soil nematodes. However, the warming effects were less obvious, only fungal PLFA showed a decreasing trend under warming. Interestingly, the influence of N addition did not depend on warming.Structural equation modeling analysis suggested that the direct pathway between N addition and soil food web components were more important than the indirect connections through alterations in soil abiotic characters or plant growth. Nitrogen enrichment also affected the soil nematode community indirectly through changes in soil pH and PLFA. We conclude that experimental warming influenced soil food web components of the temperate steppe less than N addition, and there was little influence of warming on N addition effects under these experimental conditions.

Consequences of exclusion of precipitation on microorganisms and microbial consumers in montane tropical rainforests

The structure and functioning of decomposer systems heavily relies on soil moisture. However, this has been primarily studied in temperate ecosystems; little is known about how soil moisture affects the microfaunal food web in tropical regions. This lack of knowledge is surprising, since the microfaunal food web controls major ecosystem processes. To evaluate the role of precipitation in the structure of soil food web components (i.e., microorganisms and testate amoebae), we excluded water input by rain in montane rainforests at different altitudes in Ecuador. Rain exclusion strongly reduced microbial biomass and respiration by about 50 %, and fungal biomass by 23 %. In testate amoebae, rain exclusion decreased the density of live cells by 91 % and caused a shift in species composition at each of the altitudes studied, with ergosterol concentrations, microbial biomass, and water content explaining 25 % of the variation in species data. The results document that reduced precipitation negatively affects soil microorganisms, but that the response of testate amoebae markedly exceeds that of bacteria and fungi. This suggests that, in addition to food, low precipitation directly affects the community structure of testate amoebae, with the effect being more pronounced at lower altitudes. Overall, the results show that microorganisms and testate amoebae rapidly respond to a reduction in precipitation, with testate amoebae—representatives of higher trophic levels—being more sensitive. The results imply that precipitation and soil moisture in tropical rainforests are the main factors regulating decomposition and nutrient turnover.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-012-2360-6) contains supplementary material, which is available to authorized users.

Do burned areas recover from inside? An experiment with soil fauna in a heterogeneous landscape

The post-fire soil faunal communities are formed both by animals having survived the fire and by colonizers from the surrounding landscape. The relative impact of these processes is largely affected by fire intensity. However, with the same fire intensity, the severity of the fire and, thus, the survival of soil animals could vary depending on environmental heterogeneity. We hypothesized that much fewer soil animals would survive the same fire intensity on shallow, dry soils than on deep, moist soils. To clarify the impact of soil depth and moisture on animal survival after fire, we conducted a burning experiment in the laboratory. Soil samples containing indigenous populations of soil fauna were taken along two transects from the top, slope and foot of two respective rocky outcrops within a mixed coniferous forest in Central Sweden. Half of the samples were burnt and half were left unburnt. Burning depth varied between 24mm (soils from the top of the gradient) to 12mm (slope and foot soils) indicating a difference in flammability. The proportion of animals surviving fire seemed to be fairly independent of burning depth (42 to 62% survival rate). Contribution of eggs which survived fire in the soil to the overall animal abundance restoration was negligible (1–3%).A multi-trophic approach resulted in different sensitivity estimates to artificial burning of various parameters. Abundance and biomass of all fauna groups studied was more sensitive to fire than species richness. Collembolans and macrofauna predators were the groups most tolerant to fire, while oribatid mites and macrofaunal detritivores showed higher mortality after the fire treatment. Despite a more pronounced alteration of the components of soil food-web by burning in the lowland Sphagnum plots, they may be important as refugia, especially for more slowly moving soil-dwelling macro- and microarthropods.

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.

Plant species identity and diversity effects on different trophic levels of nematodes in the soil food web

Previous studies on biodiversity and soil food web composition have mentioned plant species identity, as well as plant species diversity as the main factors affecting the abundance and diversity of soil organisms. However, most studies have been carried out under limitations of time, space, or appropriate controls. In order to further examine the relation between plant species diversity and the soil food web, we conducted a three‐year semi‐field experiment in which eight plant species (4 forb and 4 grass species) were grown in monocultures and mixtures of two, four and eight plant species. In addition there were communities with 16 plant species. We analyzed the abundance and identity of the nematodes in soil and roots, including feeding groups from various trophic levels (primary and secondary consumers, carnivores, and omnivores) in the soil food web.Plant species diversity and plant identity affected the diversity of nematodes. The effect of plant diversity was attributed to the complementarity in resource quality of the component plant species rather than to an increase in total resource quantity. The nematode diversity varied more between the different plant species than between different levels of plant species diversity, so that plant identity is more important than plant diversity. Nevertheless the nematode diversity in plant mixtures was higher than in any of the plant monocultures, due to the reduced dominance of the most abundant nematode taxa in the mixed plant communities. Plant species identity affected the abundances of the lower trophic consumer levels more than the higher trophic levels of nematodes. Plant species diversity and plant biomass did not affect nematode abundance. Our results, therefore, support the hypothesis that resource quality is more important than resource quantity for the diversity of soil food web components and that plant species identity is more important than plant diversity per se.

Soil food web components affect plant community structure during early succession

There is a growing awareness among ecologists of the strong links that exist between above‐ and belowground food webs. So far, the majority of studies have considered these links from the microbial point of view, usually with single plants or very simple plant communities. Here, we report the interactions between two components of the soil food web, root‐feeding insects and arbuscular mycorrhizal (AM) fungi, and their effects on the structure and development of early successional plant communities. We use long‐term field experiments that employ manipulations of these organisms, both singly and in combination. Both groups have strong effects on plant community structure, with root‐feeding insects increasing and AM fungi decreasing plant species richness. Root‐feeding insects appear to accelerate the process of early succession, while AM fungi retard it. There are strong interactions between the insects and fungi. The effect of insects is greatest when AM fungi are present. It is suggested that this is a consequence of plant physiology, rather than any direct interaction between the groups involved. Meanwhile, AM fungi have their greatest effect on plant communities when insects are absent, suggesting that there is some disruption of the symbiosis by the invertebrates. In developing plant communities, the rate and direction of the succession is therefore determined by the relative abundance of these two members of the soil food web. The next challenge will be to understand the roles of other members of the subterranean web in terms of their interactions with insects and AM fungi and effects on plant community development.

The Spatial Distribution of Nematode Trophic Groups Across a Cultivated Ecosystem

In order to better understand the spatial distributions of soil trophic groups and the potential significance of these distributions to ecosystem functioning we initiated a study to describe the within—site variability of nematode feeding groups in a row—crop ecosystem. Soil cores were removed from a 48—ha corn (Zea mays) field in the U.S. Midwest prior to spring planting, and nematodes were identified by phenotypic criteria to four groups: bacterivores, fungivores, omnivores/predators, and plant parasites. Within—site variability was high for all groups; population counts spanned two orders of magnitude, with coefficients of variation ranging from 40—130% (n = 115—138 soil samples). Probability distributions were strongly lognormal. Geostatistical analysis showed that a major part of this variability was spatially dependent; variograms suggest that 70—99% of sample population variance was related to spatial autocorrelation over our geographic range of 6—80 m, except for the parasitic group, for which we detected no autocorrelation to 1200m. Maps of nonparasitic feeding groups across the field showed large multi—hectare areas of low to moderate population densities, with sub—hectare clusters of high—density populations towards one end of the site. Individual feeding groups were only weakly correlated with one another across the field (Kendall's t < 0.363, P < 0.001). Edaphic factors (bulk density, texture, pH, C availability, N availability) could collectively explain <30% of the variability in the nonparasitic groups across the area sampled. Results suggest that important soil food web components are strongly patterned at sub—hectare scales in this site. That this patterning is maintained in an ecosystem subjected to the homogenizing influences of annual soil tillage and a monoculture plant population is remarkable, and suggests that such patterning may be even more common in less—disturbed sites. Inclusion of these patterns in studies of ecosystem processes and soil community dynamics may significantly improve soil trophic models and our understanding of the relationship between soil populations and ecosystem function.