Paul Henning Krogh

Glacial aquitards from the last Pleistocene glaciation cover vast areas of the Northern Hemisphere. Due to their clay-rich composition, these aquitards are considered to protect underlying aquifers from surface contaminations, provided they are not penetrated by widespread deep fractures. In this study we combine ancient DNA (aDNA) with detailed hydrological techniques to investigate contaminant transport in the aquitards - an interdisciplinary approach that was not possible when the current understanding of the dominant role of fractures was established. We show that deep fractures (> 2 m below surface) in the glacial aquitards across Denmark are closed or marginal as pathways of dissolved contaminant transport unless they contain flow channels from deep modern and ancient tree roots. Our study reveals that deep tree roots can compromise natural groundwater protection and accelerate groundwater pollution by opening fractures and generating deep macropores. The results highlight that thorough attention needs to be paid to land-use history and widespread legacy and future soil pollution prior to reforestation and afforestation of aquitards above drinking water resources. We furthermore demonstrate that the protection of aquitard areas with less thickness than historical rooting depths and high degrees of lithological heterogeneity (related to aquitard glacial till types) is crucial to preserve drinking water resources and to restore groundwater quality beneath glacial aquitards.

Earthworms influence infiltration and solute transport in agricultural soils developed on glacial till—a widespread parent material across the Northern Hemisphere. Their burrowing activity enhances soil structure and generally improves water infiltration while reducing surface runoff. However, their burrows can function as preferential flow paths, potentially accelerating the leaching of nitrates and pesticides to groundwater under certain conditions. At the same time, the microbiome of the drilosphere can adsorb, retain and degrade solutes. This study maps the spatial structure of earthworm burrows in a Danish field under winter wheat and annual crop rotation from 2011 to 2014. Macropore connectivity and preferential flow potential were assessed using horizontal excavations and macropore mapping in vertical profiles, Brilliant Blue dye tracing and surface smoke tracer tests. The earthworm community was dominated by anecic species ( Lumbricus herculeus and Aporrectodea longa ), which maintain deep vertical burrows that enhance hydrological connectivity. Stable isotope analysis confirmed distinct ecological categories among the four species, consistent with their feeding types and ecological traits relevant to burrow formation. Following a transition from conventional to reduced tillage, we observed a marked increase in surface‐connected burrows and their hydrological connectivity, indicating recovery of earthworm‐mediated infiltration pathways. The density of Brilliant Blue–stained earthworm burrows—indicating hydrologically active flow paths—remained stable down to 2.0 m depth, whereas unstained burrows declined sharply with depth. This contrast shows that only a subset of earthworm burrows remains functionally connected to infiltrating water at depth, supporting the interpretation that anecic earthworm burrows constitute dominant preferential flow paths. These findings highlight the functional role of earthworm burrows in structured glacial till soils, with important implications for groundwater recharge and solute transport.

Earthworms influence infiltration and solute transport in agricultural soils formed on glacial till - a widespread parent material across the Northern Hemisphere. Their burrowing activity enhances soil structure and generally improves water infiltration while reducing surface runoff. However, their burrows can function as preferential flow paths, potentially accelerating the leaching of nitrates and pesticides to groundwater under certain conditions. At the same time, the microbiome of the drilosphere can adsorb, retain, and degrade solutes. This study maps the spatial structure of earthworm macropores in a Danish field under winter wheat and annual crop rotation from 2011 to 2014. Macropore connectivity and preferential flow potential were assessed using horizontal excavations and macropore mapping in vertical profiles, Brilliant Blue dye tracing, and surface smoke tracer tests. The earthworm community was dominated by anecic species ( Lumbricus terrestris and Aporrectodea longa ) which maintain deep vertical burrows that enhance hydrological connectivity. Stable isotope analysis confirmed their distinct ecological roles and trophic strategies. Following a transition from conventional to reduced tillage, we observed a marked increase in surface-connected macropores and burrow density, indicating recovery of earthworm-mediated infiltration pathways. The density of BB-stained earthworm burrows remained stable down to 2.0 m depth, while unstained burrows declined with depth, supporting the interpretation that earthworm burrows acted as dominant preferential flow paths. These findings highlight the functioning of earthworm burrows in structured glacial till soils, with important implications for groundwater recharge and solute transport.

The most important reason for the lack of a German nationwide and standardised survey of soil organisms is probably the time‐consuming and expensive identification of soil invertebrates. The present contribution should contribute to solving this problem. Earthworms and soil were sampled at 25 sites, the animals were identified morphologically and by community DNA (comDNA) and environmental DNA (eDNA) metabarcoding. The comparison of results showed that comDNA detected more species (3.6 on average) than eDNA (3.0) and morphological identification (2.8). In contrast, eDNA, on average, detected a similar number of species as morphological identification. However, some species appear to have a different probability of being detected by eDNA than others, depending on their abundance, behaviour, biology or body size. All three identification methods can differentiate between sites with different species composition, and the degree of separation can vary depending on the identification method. The relative proportion of eDNA reads shows potential as a surrogate of relative abundance/biomass for endogeic but not for anecic species. The overall aim of the ‘MetaSOL’ project (which the present contribution originated from) was to develop recommendations for efficient and routinely implementable monitoring of soil fauna. The results showed that genetic identification methods are suitable for earthworms. Before genetic identification methods can be introduced into official practice, key preconditions such as comprehensive, well‐curated and quality‐controlled DNA reference databases and method standardisation must be addressed. Robust indices of soil health based on soil organism data need to be developed. The inclusion of further groups in addition to earthworms should be examined.

Soil organic matter (SOM) transformation processes are regulated by the activities of plants, microbes, and fauna. Compared with plants and microbes, effects of soil fauna are less understood because of their high taxonomic and functional diversity, and mix of direct and indirect effect mechanisms. Trait‐based approaches offer a generic perspective to quantify mechanistic relationships between soil fauna and SOM transformations, including decomposition, translocation, and stabilisation of organic carbon. Yet, at present, we lack a consensus concerning relevant key effect traits of soil fauna (i.e. those affecting ecosystem functioning). Here, we address this knowledge gap by focusing on relationships between soil fauna effect traits and SOM transformations. Based on existing literature, we identify key processes linked to SOM transformations, and fauna effect traits universally applicable across taxa and soil types, and discuss the process‐trait links. We define eight SOM transformation processes that are directly affected by soil fauna: (i) litter mass loss, (ii) litter fragmentation, (iii) SOM aggregation in faeces, (iv) SOM aggregation in soil mineral particles, (v) decomposition of faeces, (vi) SOM and mineral translocation, (vii) pore space creation and maintenance and (viii) SOM stabilisation. We link these processes to general effect traits classified into four categories: (a) food selection and ingestion, (b), digestion and excretion, (c) mobility, and (d) body mass and metabolic rate. We also propose proxies when effect trait measurements are laborious. The proposed links between effect traits and SOM transformation processes need to be validated in targeted experiments. We urge researchers to obtain quantitative experimental data, together with metabolic approaches, to integratively quantify soil fauna contributions to soil functioning. Read the free Plain Language Summary for this article on the Journal blog.

The Arctic has warmed nearly four times faster than the global average since 1979, resulting in rapid glacier retreat and exposing new glacier forelands. These forelands offer unique experimental settings to explore how global warming impacts ecosystems, particularly for highly climate‐sensitive arthropods. Understanding these impacts can help anticipate future biodiversity and ecosystem changes under ongoing warming scenarios. In this study, we integrate data on arthropod diversity from DNA gut content analysis—offering insight into predator diets—with quantitative measures of arthropod activity‐density at a Greenland glacier foreland using Structural Equation Modelling (SEM). Our SEM analysis reveals both bottom‐up and top‐down controlled food chains. Bottom‐up control, linked to sit‐and‐wait predator behavior, was prominent for spider and harvestman populations, while top‐down control, associated with active search behavior, was key for ground beetle populations. Bottom‐up controlled dynamics predominated during the early stages of vegetation succession, while top‐down mechanisms dominated in later successional stages further from the glacier, driven largely by increasing temperatures. In advanced successional stages, top‐down cascades intensify intraguild predation (IGP) among arthropod predators. This is especially evident in the linyphiid spider Collinsia holmgreni, whose diet included other linyphiid and lycosid spiders, reflecting high IGP. The IGP ratio in C. holmgreni negatively correlated with the activity‐density of ground‐dwelling prey, likely contributing to the local decline and possible extinction of this cold‐adapted species in warmer, late‐succession habitats where lycosid spiders dominate. These findings suggest that sustained warming and associated shifts in food web dynamics could lead to the loss of cold‐adapted species, while brief warm events may temporarily impact populations without lasting extinction effects.

Not-yet-known not-yet-known not-yet-known unknown The Arctic has warmed nearly four times faster than the global average since 1979, resulting in rapid glacier retreat and exposing new glacier forelands. These forelands offer unique experimental settings to explore how global warming impacts ecosystems, particularly for highly cli-mate-sensitive arthropods. Understanding these impacts can help anticipate future biodiversity and ecosystem changes under ongoing warming scenarios. In this study, we integrate data on arthropod diversity from DNA gut content analysis—offering insight into predator diets—with quantitative measures of arthropod activity-density at a Greenland glacier foreland using Structural Equation Modelling (SEM). Our SEM analysis reveals both bottom-up and top-down controlled food chains. Bottom-up control, linked to sit-and-wait predator behavior, was prominent for spider and harvestman populations, while top-down control, associated with active search behavior, was key for ground beetle populations. Bottom-up controlled dynamics predominated during the early stages of vegetation succession, while top-down mechanisms dominated in later successional stages further from the glacier, driven largely by increasing temperatures. In advanced successional stages, top-down cascades intensify intraguild predation (IGP) among arthropod predators. This is especially evident in the linyphiid spider Collinsia holmgreni, whose diet included other linyphiid and lycosid spiders, reflecting high IGP. The IGP ratio in C. holmgreni negatively correlated with the activity-density of ground-dwelling prey, likely con-tributing to the local decline and possible extinction of this cold-adapted species in warmer, late-succession habitats where lycosid spiders dominate. These findings suggest that sustained warming and associated shifts in food web dynamics could lead to the loss of cold-adapted species, while brief warm events may temporarily impact populations without lasting extinction effects.

Springtails (Collembola) inhabit soils from the Arctic to the Antarctic and comprise an estimated ~32% of all terrestrial arthropods on Earth. Here, we present a global, spatially-explicit database on springtail communities that includes 249,912 occurrences from 44,999 samples and 2,990 sites. These data are mainly raw sample-level records at the species level collected predominantly from private archives of the authors that were quality-controlled and taxonomically-standardised. Despite covering all continents, most of the sample-level data come from the European continent (82.5% of all samples) and represent four habitats: woodlands (57.4%), grasslands (14.0%), agrosystems (13.7%) and scrublands (9.0%). We included sampling by soil layers, and across seasons and years, representing temporal and spatial within-site variation in springtail communities. We also provided data use and sharing guidelines and R code to facilitate the use of the database by other researchers. This data paper describes a static version of the database at the publication date, but the database will be further expanded to include underrepresented regions and linked with trait data.

The protection of soil biodiversity is essential for ensuring soil functioning and provisioning of related ecosystem services, but also the conservation of species per se. For this purpose, it is essential to know what soil organism is where and how it is influenced by environmental, climate and human activities. Numerous studies (there are over 1400 published articles) have been elaborated in Europe regarding soil fauna. A data platform to store data on soil fauna already exists and is currently under further development to become a pan-European data warehouse for soil biodiversity (https://www.eudaphobase.eu/). Though, the data stored in this database until now comprises only a small percent of the available data. Hence, it is evident that a compiled dataset providing an overall understanding of the distribution of several taxa across European biomes and under different land use still is missing.Attempting to fill this gap, we start a new initiative within the framework of the Cost Action EUdaphobase CA18237: the creation of the first European Atlas of Soil Fauna.  We aim to map, summarize and upscale the current knowledge on soil fauna to help in providing support to the scientific community and directions to stakeholders and policymakers. We seek collaboration (data holders and experts) to identify, collect and analyze data of all groups of micro-, meso- and macrofauna in Europe. This presentation will focus on the aims of this initiative and the way we plan to collect data and involve people.