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Abstract Fauna is highly abundant and diverse in soils worldwide, but surprisingly little is known about how it affects soil organic matter stabilization. Here, we review how the ecological strategies of a multitude of soil faunal taxa can affect the formation and persistence of labile (particulate organic matter, POM) and stabilized soil organic matter (mineral-associated organic matter, MAOM). We propose three major mechanisms - transformation, translocation, and grazing on microorganisms - by which soil fauna alters factors deemed essential in the formation of POM and MAOM, including the quantity and decomposability of organic matter, soil mineralogy, and the abundance, location, and composition of the microbial community. Determining the relevance of these mechanisms to POM and MAOM formation in cross-disciplinary studies that cover individual taxa and more complex faunal communities, and employ physical fractionation, isotopic, and microbiological approaches is essential to advance concepts, models, and policies focused on soil organic matter and effectively manage soils as carbon sinks, nutrient stores, and providers of food.

Recent research on earthworms has shed light on their global distribution, with high alpha richness in temperate zones and high beta diversity in tropical areas. Climate and agricultural practices, notably plowing and conservation methods, were shown to strongly influence earthworm communities. However, data gaps persist in regions like North Australia, Asia, Russia, and Africa, limiting our understanding of earthworm distribution and their responses to global changes. Understanding changes within earthworm communities is crucial given their profound influence on ecosystem functions such as soil structure, nutrient dynamics, and plant growth. Classifying earthworms into functional groups remains complex, prompting the adoption of a trait-based approach for a more comprehensive classification, but there is no representative global data on earthworm traits. To address these knowledge gaps, the Soil BON Earthworm initiative aims at creating a global community of earthworm experts, standardizing sampling methods and databases, collecting time series data on earthworm communities, and modeling future earthworm distributions under different climate scenarios. The initiative aims to address key questions, such as the dynamic of earthworm communities over time and their response to environmental factors and anthropogenic influences, their impact on ecosystem functioning, and the redefinition of functional groups based on traits. The consortium invites researchers worldwide to contribute to this endeavor and encourages the resampling of study sites, to expand currently limited time series datasets. To facilitate data collection, standardized protocols and data templates are proposed, ensuring data quality and interoperability. Furthermore, the initiative intends to make use of citizen science in expanding observations and improving taxonomic coverage, highlighting platforms like iNaturalist for community engagement. Soil BON Earthworm seeks to unite global expertise and foster collaborative research to address critical gaps in understanding earthworm ecology and its implications for ecosystems at a global scale.

Earthworms can stimulate microbial activity and hence greenhouse gas (GHG) emissions from soils. However, the extent of this effect in the presence of plants and soil moisture fluctuations, which are influenced by earthworm burrowing activity, remains uncertain. Here, we report the effects of earthworms (without, anecic, endogeic, both) and plants (with, without) on GHG (CO2, N2O) emissions in a 3-month greenhouse mesocosm experiment simulating a simplified agricultural context. The mesocosms allowed for water drainage at the bottom to account for the earthworm engineering effect on water flow during two drying-wetting cycles. N2O cumulative emissions were 34.6% and 44.8% lower when both earthworm species and only endogeic species were present, respectively, and 19.8% lower in the presence of plants. The presence of the endogeic species alone or in combination with the anecic species slightly reduced CO2 emissions by 5.9% and 11.4%, respectively, and the presence of plants increased emissions by 6%. Earthworms, plants and soil water content interactively affected weekly N2O emissions, an effect controlled by increased soil dryness due to drainage via earthworm burrows and mesocosm evapotranspiration. Soil macroporosity (measured by X-ray tomography) was affected by earthworm species-specific burrowing activity. Both GHG emissions decreased with topsoil macropore volume, presumably due to reduced moisture and microbial activity. N2O emissions decreased with macropore volume in the deepest layer, likely due to the presence of fewer anaerobic microsites. Our results indicate that, under experimental conditions allowing for plant and earthworm engineering effects on soil moisture, earthworms do not increase GHG emissions, and endogeic earthworms may even reduce N2O emissions.

Understanding global biodiversity change, its drivers, and the ecosystem consequences requires a better appreciation of both the factors that shape soil macrofauna communities and the ecosystem effects of these organisms. The project “sOilFauna” was funded by the synthesis center sDiv (Germany) to address this major gap by forming a community of soil ecologists, identifying the most pressing research questions and hypotheses, as well as conducting a series of workshops to foster the global synthesis and hypothesis testing of soil macrofauna. The overarching goal is to analyze the most comprehensive soil macrofauna database - the MACROFAUNA database - which collates abundance data of 17 soil invertebrate groups assessed with a standardized method at 7180 sites around the world, and seeks to foster the collection of future data. In a recent kick-off workshop in May 2022, the first research priorities and collaboration guidelines were determined. Here, we summarize the main outcomes of this workshop and highlight the benefits of creating an open global community of soil ecologists providing standardized soil macrofauna data for future research, evaluation of ecosystem health, and nature protection.

• Millipedes and woodlice were sampled in pure and mixed forest stands across Europe. • Macroarthropod species richness was generally higher in mixed forest stands. • Broadleaf-conifer mixtures in Eastern Europe were very favourable to macroarthropods. • Resource heterogeneity in the litter layer of mixed stands likely plays a key role. Impacts of tree diversity on forest soil fauna remain poorly known. Two groups of functionally similar invertebrates, millipedes (Diplopoda) and woodlice (Isopoda, Oniscidea), were sampled at 61 forest plots in four European countries (Finland, Poland, Romania, Italy) in order to (1) compare the abundance and species richness of these saprophagous macroarthropods in monospecific and mixed, three-species forest stands, and (2) determine whether the influence of tree species richness depends or not on tree identity. The results showed that, in boreal forests of Finland, regional conditions (temperature, soil) proved to be very unfavourable for millipedes and woodlice, which prevented further comparisons. In the other three regions, tree species richness had no consistent effect on macroarthropod abundance but there was a clear trend towards a higher macroarthropod species richness in mixed stands compared to monospecific stands. This resulted mainly from a significantly positive response of millipedes to tree mixtures in hemiboreal forests of Poland and mountainous forests of Romania, and from a slightly positive response of woodlice in thermophilous forests of Italy. However, not all tree mixtures were equally favourable for saprophagous macroarthropods. An examination of tree identity in mixed stands showed that mixtures including hornbeam and conifers in Poland and those including beech and spruce in Romania had significantly higher macroarthropod species richness than the other mixed stands. Moreover, macroarthropod species richness was higher in these broadleaf-conifer mixtures than in monospecific stands of either component tree species, which supports the hypothesis of a greater diversity of resources for macroarthropods in these mixed stands. This hypothesis was not supported in Italy, where macroarthropod species richness was highest in monospecific chestnut stands. The positive influence of conifer presence on the species richness of saprophagous macroarthropods in Eastern European forests contrasts with results of previous studies, which calls for further research to explain those differences. [ABSTRACT FROM AUTHOR]

Earthworms can stimulate microbial activity and hence greenhouse gas (GHG) emissions from soils. However, the extent of this effect in the presence of plants and soil moisture fluctuations, which are influenced by earthworm burrowing activity, remains uncertain. Here, we report the effects of earthworms (without, anecic, endogeic, both) and plants (with, without) on GHG (CO2, N2O) emissions in a 3-month greenhouse mesocosm experiment simulating a simplified agricultural context. The mesocosms allowed for water drainage at the bottom to account for the earthworm engineering effect on water flow during two drying-wetting cycles. N2O cumulative emissions were 34.6% and 44.8% lower when both earthworm species and only endogeic species were present, respectively, and 19.8% lower in the presence of plants. The presence of the endogeic species alone or in combination with the anecic species slightly reduced CO2 emissions by 5.9% and 11.4%, respectively, and the presence of plants increased emissions by 6%. Earthworms, plants and soil water content interactively affected weekly N2O emissions, an effect controlled by increased soil dryness due to drainage via earthworm burrows and mesocosm evapotranspiration. Soil macroporosity (measured by X-ray tomography) was affected by earthworm species-specific burrowing activity. Both GHG emissions decreased with topsoil macropore volume, presumably due to reduced moisture and microbial activity. N2O emissions decreased with macropore volume in the deepest layer, likely due to the presence of fewer anaerobic microsites. Our results indicate that, under experimental conditions allowing for plant and earthworm engineering effects on soil moisture, earthworms do not increase GHG emissions, and endogeic earthworms may even reduce N2O emissions., Competing Interests: The authors have declared that no competing interests exist., (Copyright: © 2024 Ganault et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.)