Trophic interactions play a vital role in soil functioning and are increasingly considered as important drivers of the soil microbiome and biogeochemical cycles. In the last decade, novel tools to ...decipher the structure of soil food webs have provided unprecedent advance in describing complex trophic interactions. Yet, the major challenge remains to understand the drivers of the trophic interactions. Evidence suggests that small scale soil physical structure may offer a unifying framework for understanding the nature and patterns of trophic interactions in soils. Here, we review the current knowledge of how restrictions on soil organisms’ ability to sense and access food resources/prey inherent to soil physical structure essentially shape trophic interactions. We focus primarily on organisms unable to deform the soil and create pores themselves, such as bacteria, fungi, protists, nematodes and microarthropods, and consider pore geometry, connectivity and hydration status as main descriptors of the soil physical structure. We point that the soil physical structure appears to mostly limit the sensing and accessibility to food resources/prey, with negative effects on bottom up controls. The main mechanisms are (i) the reduced transport of sensing molecules, notably volatiles, through the soil matrix and (ii) the wide presence of refuges leading to pore size segregation of consumer/predators and food sources/prey in pores of contrasting size. In addition, variations in the connectivity of the soil pores and the water film is suggested as a central aspect driving encounter probability between consumers/predator and food source/prey and hence locally decrease or increase top-down controls. Constraints imposed by the soil physical structure on trophic interactions are thought to be major drivers of the soil diversity and local community assemblage, notably by favoring a variety of adaptations to feed in this dark labyrinth (food specialists/flexible/generalists) and by limiting competitive exclusion through limited encounter probability of consumers. We conclude with possible future ways for an interdisciplinary and more quantitative research merging soil physics and soil food web ecology.
•Soil physical structure constrains sensing and accessibility of food resources.•Size segregation of consumers limits access to resources/prey in small pores.•Limited mobility in soil reduces opportunities of consumer - resource interactions.•Constrained trophic interactions drive the co-existence of the diverse biota in soil.•Future work merging soil physics and soil food web ecology is needed.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
ABSTRACT
Despite the major importance of soil biota in nutrient and energy fluxes, interactions in soil food webs are poorly understood. Here we provide an overview of recent advances in uncovering ...the trophic structure of soil food webs using natural variations in stable isotope ratios. We discuss approaches of application, normalization and interpretation of stable isotope ratios along with methodological pitfalls. Analysis of published data from temperate forest ecosystems is used to outline emerging concepts and perspectives in soil food web research.
In contrast to aboveground and aquatic food webs, trophic fractionation at the basal level of detrital food webs is large for carbon and small for nitrogen stable isotopes. Virtually all soil animals are enriched in 13C as compared to plant litter. This ‘detrital shift’ likely reflects preferential uptake of 13C‐enriched microbial biomass and underlines the importance of microorganisms, in contrast to dead plant material, as a major food resource for the soil animal community.
Soil organic matter is enriched in 15N and 13C relative to leaf litter. Decomposers inhabiting mineral soil layers therefore might be enriched in 15N resulting in overlap in isotope ratios between soil‐dwelling detritivores and litter‐dwelling predators. By contrast, 13C content varies little between detritivores in upper litter and in mineral soil, suggesting that they rely on similar basal resources, i.e. little decomposed organic matter. Comparing vertical isotope gradients in animals and in basal resources can be a valuable tool to assess trophic interactions and dynamics of organic matter in soil.
As indicated by stable isotope composition, direct feeding on living plant material as well as on mycorrhizal fungi is likely rare among soil invertebrates. Plant carbon is taken up predominantly by saprotrophic microorganisms and channelled to higher trophic levels of the soil food web. However, feeding on photoautotrophic microorganisms and non‐vascular plants may play an important role in fuelling soil food webs.
The trophic niche of most high‐rank animal taxa spans at least two trophic levels, implying the use of a wide range of resources. Therefore, to identify trophic species and links in food webs, low‐rank taxonomic identification is required. Despite overlap in feeding strategies, stable isotope composition of the high‐rank taxonomic groups reflects differences in trophic level and in the use of basal resources. Different taxonomic groups of predators and decomposers are likely linked to different pools of organic matter in soil, suggesting different functional roles and indicating that trophic niches in soil animal communities are phylogenetically structured.
During last two decades studies using stable isotope analysis have elucidated the trophic structure of soil communities, clarified basal food resources of the soil food web and revealed links between above‐ and belowground ecosystem compartments. Extending the use of stable isotope analysis to a wider range of soil‐dwelling organisms, including microfauna, and a larger array of ecosystems provides the perspective of a comprehensive understanding of the structure and functioning of soil food webs.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Ecology Letters (2012) 15: 468–474
Biodiversity is a major determinant of ecosystem functioning. Species‐rich communities often use resources more efficiently thereby improving community performance. ...However, high competition within diverse communities may also reduce community functioning. We manipulated the genotypic diversity of Pseudomonas fluorescens communities, a plant mutualistic species inhibiting pathogens. We measured antagonistic interactions in vitro, and related these interactions to bacterial community productivity (root colonisation) and ecosystem service (host plant protection). Antagonistic interactions increased disproportionally with species richness. Mutual poisoning between competitors lead to a ‘negative complementarity effect’, causing a decrease in bacterial density by up to 98% in diverse communities and a complete loss of plant protection. The results emphasize that antagonistic interactions may determine community functioning and cause negative biodiversity–ecosystem functioning relationships. Interference competition may thus be an additional key for predicting the dynamics and performance of natural assemblages and needs to be implemented in future biodiversity models.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Stability is a crucial ecosystem feature gaining particular importance in face of increasing anthropogenic stressors. Biodiversity is considered to be a driving biotic force maintaining stability, ...and in this study we investigate how different indices of biodiversity affect the stability of communities in varied abiotic (composition of available resources) and biotic (invasion) contexts.
We set up microbial microcosms to study the effects of genotypic diversity on the reliability of community productivity, defined as the inverse of the coefficient of variation of across-treatment productivity, in different environmental contexts. We established a bacterial diversity gradient ranging from 1 to 8 Pseudomonas fluorescens genotypes and grew the communities in different resource environments or in the presence of model invasive species. Biodiversity significantly stabilized community productivity across treatments in both experiments. Path analyses revealed that different aspects of diversity determined stability: genotypic richness stabilized community productivity across resource environments, whereas functional diversity determined stability when subjected to invasion.
Biodiversity increases the stability of microbial communities against both biotic and abiotic environmental perturbations. Depending on stressor type, varying aspects of biodiversity contribute to the stability of ecosystem functions. The results suggest that both genetic and functional diversity need to be preserved to ensure buffering of communities against abiotic and biotic stresses.
Full text
Available for:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Land-use intensification in the tropics plays an important role in meeting global demand for agricultural commodities but generates high environmental costs. Here, we synthesize the impacts of ...rainforest conversion to tree plantations of increasing management intensity on carbon stocks and dynamics. Rainforests in Sumatra converted to jungle rubber, rubber, and oil palm monocultures lost 116 Mg C ha
, 159 Mg C ha
, and 174 Mg C ha
, respectively. Up to 21% of these carbon losses originated from belowground pools, where soil organic matter still decreases a decade after conversion. Oil palm cultivation leads to the highest carbon losses but it is the most efficient land use, providing the lowest ratio between ecosystem carbon storage loss or net primary production (NPP) decrease and yield. The imbalanced sharing of NPP between short-term human needs and maintenance of long-term ecosystem functions could compromise the ability of plantations to provide ecosystem services regulating climate, soil fertility, water, and nutrient cycles.
ABSTRACT
Soil organisms drive major ecosystem functions by mineralising carbon and releasing nutrients during decomposition processes, which supports plant growth, aboveground biodiversity and, ...ultimately, human nutrition. Soil ecologists often operate with functional groups to infer the effects of individual taxa on ecosystem functions and services. Simultaneous assessment of the functional roles of multiple taxa is possible using food‐web reconstructions, but our knowledge of the feeding habits of many taxa is insufficient and often based on limited evidence. Over the last two decades, molecular, biochemical and isotopic tools have improved our understanding of the feeding habits of various soil organisms, yet this knowledge is still to be synthesised into a common functional framework. Here, we provide a comprehensive review of the feeding habits of consumers in soil, including protists, micro‐, meso‐ and macrofauna (invertebrates), and soil‐associated vertebrates. We have integrated existing functional group classifications with findings gained with novel methods and compiled an overarching classification across taxa focusing on key universal traits such as food resource preferences, body masses, microhabitat specialisation, protection and hunting mechanisms. Our summary highlights various strands of evidence that many functional groups commonly used in soil ecology and food‐web models are feeding on multiple types of food resources. In many cases, omnivory is observed down to the species level of taxonomic resolution, challenging realism of traditional soil food‐web models based on distinct resource‐based energy channels. Novel methods, such as stable isotope, fatty acid and DNA gut content analyses, have revealed previously hidden facets of trophic relationships of soil consumers, such as food assimilation, multichannel feeding across trophic levels, hidden trophic niche differentiation and the importance of alternative food/prey, as well as energy transfers across ecosystem compartments. Wider adoption of such tools and the development of open interoperable platforms that assemble morphological, ecological and trophic data as traits of soil taxa will enable the refinement and expansion of the multifunctional classification of consumers in soil. The compiled multifunctional classification of soil‐associated consumers will serve as a reference for ecologists working with biodiversity changes and biodiversity–ecosystem functioning relationships, making soil food‐web research more accessible and reproducible.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Plant diversity strongly influences ecosystem functions and services, such as soil carbon storage. However, the mechanisms underlying the positive plant diversity effects on soil carbon storage are ...poorly understood. We explored this relationship using long-term data from a grassland biodiversity experiment (The Jena Experiment) and radiocarbon ((14)C) modelling. Here we show that higher plant diversity increases rhizosphere carbon inputs into the microbial community resulting in both increased microbial activity and carbon storage. Increases in soil carbon were related to the enhanced accumulation of recently fixed carbon in high-diversity plots, while plant diversity had less pronounced effects on the decomposition rate of existing carbon. The present study shows that elevated carbon storage at high plant diversity is a direct function of the soil microbial community, indicating that the increase in carbon storage is mainly limited by the integration of new carbon into soil and less by the decomposition of existing soil carbon.
We explore empirical and theoretical evidence for the functional significance of plant-litter diversity and the extraordinary high diversity of decomposer organisms in the process of litter ...decomposition and the consequences for biogeochemical cycles. Potential mechanisms for the frequently observed litter-diversity effects on mass loss and nitrogen dynamics include fungi-driven nutrient transfer among litter species, inhibition or stimulation of microorganisms by specific litter compounds, and positive feedback of soil fauna due to greater habitat and food diversity. Theory predicts positive effects of microbial diversity that result from functional niche complementarity, but the few existing experiments provide conflicting results. Microbial succession with shifting enzymatic capabilities enhances decomposition, whereas antagonistic interactions among fungi that compete for similar resources slow litter decay. Soil-fauna diversity manipulations indicate that the number of trophic levels, species identity, and the presence of keystone species have a strong impact on decomposition, whereas the importance of diversity within functional groups is not clear at present. In conclusion, litter species and decomposer diversity can significantly influence carbon and nutrient turnover rates; however, no general or predictable pattern has emerged. Proposed mechanisms for diversity effects need confirmation and a link to functional traits for a comprehensive understanding of how biodiversity interacts with decomposition processes and the consequences of ongoing biodiversity loss for ecosystem functioning.
Full text
Available for:
BFBNIB, DOBA, INZLJ, IZUM, KILJ, NMLJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK, ZRSKP
Ecology Letters (2012) 15: 436–443
Understanding effects of species loss in complex food webs with multiple trophic levels is complicated by the idiosyncrasy of the predator effects on lower trophic ...levels: direct and indirect effects intermingle and may increase, decrease or not affect ecosystem functioning. We introduce a reductionist approach explaining a predator’s trophic effect only by empirically well‐founded body‐mass constraints on abundance, diet breadth and feeding strength. We demonstrate that this mechanistic concept successfully explains the positive, negative and neutral net effects of predators on decomposers in a litter microcosm experiment. This approach offers a new perspective on the interplay of complex interactions within food webs and is easily extendable to include phylogenetic and other body‐mass independent traits. We anticipate that allometry will substantially improve our understanding of idiosyncratic predator effects in experiments and the consequences of predator loss in natural ecosystems.
Full text
Available for:
BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Soil microarthropods are essential for nutrient cycling in forest ecosystems as they are integral components of decomposer food webs. They channel carbon and nutrients from leaf litter and roots to ...higher trophic levels; however, knowledge on the relative importance of different channels and on their variation with forest type is lacking. Although the importance of root‐derived inputs for sustaining soil food webs is increasingly recognized, the pathways by which they are channeled to higher trophic levels are little understood. For the channeling, ectomycorrhizal fungi may play a significant role, but until now methods allowing to separate the contribution of ectomycorrhizal and saprotrophic fungi to the nutrition of soil animal communities are lacking. Using dual analysis of 15N and 13C in amino acids (AAs), we investigated trophic positions and basal resources of two major groups of soil microarthropods, Collembola and Oribatida, in beech and spruce forests in Germany. By applying a 13C fingerprinting approach and Bayesian mixing models, we separated in a first step the relative contribution of bacteria, fungi, and plants to the nutrition of soil microarthropods. As fungi were identified as the major food source, in a second step we attempted to separate the contribution of ectomycorrhizal vs. saprotrophic fungi. For the first time, we provide direct evidence that soil microarthropods mainly rely on saprotrophic fungi, whereas ectomycorrhizal fungi are consumed by only few species. While trophic niches of Collembola and Oribatida species generally varied little between beech and spruce forests, plant detritus as basal resource of soil microarthropods was somewhat more important in beech forests, whereas in spruce forests microbial resources dominated. Overall, the dual analysis of carbon and nitrogen in AAs provided insight into food web structure of soil microarthropods in unprecedented detail, and for the first time allowed to estimate the relative importance of mycorrhizal and saprotrophic fungi for soil food web nutrition, a long‐standing riddle in soil food web ecology. The technique provides the perspective for a comprehensive understanding of the trophic structure and energy channeling in soil food webs.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
PDF
