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Although soil ecology has benefited from recent advances in describing the functional and trophic traits of soil organisms, large scale reconstruction of soil food webs is still hampered by the issues that (1) most of the data about trophic interactions and diets are distributed across heterogeneous repositories, (2) no well-established terminology exists for describing trophic groups, feeding processes, and resource types, and (3) large heterogeneity exists in the trophic classification of different soil organisms, or such classifications are lacking. Here, we propose the Soil Food Web Ontology as a new formal framework for representing knowledge on trophic ecology of soil organisms. This ontology captures the semantics of trophic concepts, including consumer-resource interactions, diets and feeding processes, and provides a formalisation of trophic group definitions. This ontology can be used to add semantic annotations to trophic data, thus facilitating the integration of heterogeneous datasets. It also provides lexical resources that can support the development of information extraction tools for the creation of literature-based datasets. Finally, it enables automatic and consistent classification of soil organisms based on their trophic relationships. We argue that, by harmonising the terminology and underlying concepts of soil trophic ecology, our ontology allows a better use of available information on the feeding habits of soil organisms and sounder trophic classifications, thus facilitating the reconstruction of soil food webs and making food web research more accessible, reusable and reproducible.

The molecular composition of soil organic matter (SOM) contains key information on the persistence of soil carbon (C) in relation to changes in vegetation and environmental factors. Depending on the ecosystem, analytical method and specific objectives, many SOM fractions and numerous fractionation schemes have been proposed to study soil C. However, the molecular composition and environmental significance of those different SOM fractions have not yet been compared systematically. We use a reverse fit approach to fill this knowledge gap: i.e. we chose a study area with a well-known land history to assess which information is stored in the most frequently analysed SOM fractions. The Gaume forest (Belgium) is an ancient deciduous forest (>200 years) in which small stands were converted to Norway spruce (Picea abies) 40–50 years ago. Those stands are located along a lithological gradient in soil buffering capacity. We investigated the molecular composition of bulk mineral soil samples and five organic SOM fractions by pyrolysis gas chromatography/mass spectrometry (Py-GC/MS). The SOM fractions included particulate OM (POM), water extractable OM (WEOM), and fractions obtained by alkaline extraction, including the traditionally used humic acid (HA) and fulvic acid (FA) and the total base extract from which they are obtained (BE). Our results indicate that pyrolysates of bulk mineral soil did not prove useful to reflect environmental changes after forest stand conversion. Principal Component Analysis indicated that within each organic fraction similar changes occurred when comparing soil depths, degree of SOM decomposition, litter inputs, and soil buffering capacity. However, only the HA and BE appeared successful in capturing all these processes: the degree of SOM decomposition was not expressed in pyrolysates from the POM, differences in litter input between forest stand types were not evidenced in the WEOM, and effects of buffering capacity were not demonstrated in the WEOM and FA fractions. Thus, the molecular composition of different SOM fractions can be used complementary to each other to study environmental and ecological effects of forest stand conversion on soil C dynamics. ispartof: Geoderma vol:430 status: Published online

International audience; Classifying organisms has a wide use and a long history in ecology. However, the meaning of a ‘group of organisms’ and how to group organisms is still the subject of much theoretical and empirical work. Achieving this long quest requires simplifying the complexity of species niches for which relevant morphological, behavioural, biochemical or life-history traits are often used as relevant proxies. Soil fauna is highly diverse and many classifications have been proposed to synthesize both the response of soil organisms to their environment and their effect on soil functioning. Here, we provide a critical overview of the characteristics and limitations of the existing classifications in soil ecology, and propose clarifications and alternatives to current practices. We summarise the similarities and differences in how classifications have been created and used in soil ecology. We propose a harmonization of the current concepts by properly defining ‘guilds’, ‘functional groups’ and ‘trophic groups’ as subcategories of ‘ecological groups’, with different purposes and distinguishing criteria. Finally, based on these concepts, we suggest a common framework to define classifications based on functional traits that allows a better and unified understanding of changes in soil biodiversity and ecosystem functioning.

Despite that all above-ground life depends on that below-ground, the knowledge on soil biodiversity is still very limited. Conservation activities have been often postponed due to missing data. For example, this was the case in previous EU Common Agricultural Policy agreements, when environmental indicators for soil biodiversity were meant to be integrated comparable to biodiversity indicators in place. While massive sequencing of soil biodiversity contributes significantly to shredding light on below-ground life, sampling methods are not adequately harmonised, preventing the establishment of reliable quantifiable conservation targets. In addition, broad-scale studies are often biased towards microorganisms and hence, in the majority of the published literature, eukaryotes, and more specifically, animals and protists, are neglected. Therefore, in a first EU-wide study of 885 sites, we investigated the response of environmental factors (i.e., soil properties, biogeographical location and climate) and land cover (cropland, grassland and woodland) on soil eukaryotic diversity. The LUCAS survey and previously published studies provided soil, climate and land cover data. DNA metabarcoding of the 18S genes allowed us to assess the diversity of animals, protists and fungi, but due to their high variability in body size, we grouped them into micro-, meso- and macrofauna. For the bioinformatics analyses, we clustered sequence reads into amplicon sequence variants using DADA2 and thereafter, we assessed alpha and beta diversity and the relationships between eukaryotic diversity and environmental drivers using regression, ordination, and variance partitioning analyses. Our results allowed us to identify potential indicator species for EU soils, representing the effect of different drivers on eukaryotic diversity. These findings will help to understand the links between soil abiotic and biotic patterns at large scale, pathing the way for quantifiable goals to be included in conservation activities and policies.

Biochar is proposed as an option to sequester carbon (C) in soils and promote other soil-based ecosystem services. However, its impact on soil biota from micro to macroscale remains poorly understood. We investigated biochar effects on the soil biota across the soil food web, on plant community composition and on biomass production. We conducted a field experiment in a nature restoration grassland testing four treatments: two biochar types (herbaceous feedstock pyrolyzed at 400 °C or 600 °C – hereafter B400 and B600), and a positive (i.e. unpyrolysed biochar feedstock, hereafter Hay) and negative (no addition) control. Responses of plants and soil biota were evaluated one and three years after establishing the treatments. Soil pH and K concentrations increased significantly in the B600 treatment. Mite abundances were significantly higher in B400 whereas nematode abundances were highest in Hay (1st year) and lowest in B400 (3rd year). Other soil fauna groups (enchytraeids and earthworms) varied more between years than between treatments. Legume cover increased significantly in the biochar treatments but this effect was transient. Legumes, grasses and primary productivity also showed a statistically significant Treatment x Year interaction due to transitory effects that were no longer present by the 3rd year. Our results suggest that biochar produced from meadow cuttings and applied at the 10 t/ha rate cause transitory impacts on soil biota abundance and plant communities over the 3-year timeframe used for this experiment. Therefore, this type of biochar could potentially be used for soil carbon sequestration, with minimal impacts on soil biota abundance or diversity, within the groups studied here, or plant biodiversity and productivity. Further research is required to investigate the longer-term impacts of this potential soil C storage sink.