Chantal Herzog

Societal Impact Statement: Soil is a vital resource for life on Earth. Although many soils are degraded, societal awareness of soil‐related issues is limited. To highlight soil as a key resource and fascinating study system, we initiated a citizen science project together with 1000 participants investigating soil health across Switzerland using buried cotton underpants. The degree of underpants decomposition was interpreted as a measure for soil biological activity and soil health. This work demonstrates that underwear decomposition varied widely across land‐use types and that buried underwear is a simple and easily interpretable tool to gain a first impression of soil health. Summary: Soil is a vital resource for plant growth. It hosts a major part of global species diversity and is key for ecosystem functioning and it forms the basis of human civilization. Yet soils are rapidly degrading, threatening food production, biodiversity, and Earth system functioning. This threat is largely unrecognized in broader society, hindering effective steps towards soil protection. In a citizen science approach, involving 1000 citizen scientists burying more than 2000 cotton underpants and 12,000 teabags across 1000 locations, we aimed to assess soil biological activity and soil health in Switzerland. Decomposition of underpants and tea bags was considered as proxies for soil biological activity and soil health. Underwear decomposition differed greatly across land‐use types, indicating significant variation in soil biological activity among sites. Land use was the most important predictor of decomposition processes, followed by soil chemical parameters. Private gardens, the land‐use type with the highest levels of soil organic carbon and nutrients, exhibited the greatest biological activity, whereas lawns showed the lowest, with croplands and grasslands falling in between. Overall, this project demonstrates that underpants can serve as a simple and easy‐to‐interpret tool, providing informative insights into the state of soils. Moreover, they are perfectly suited to raise attention in society for soil. We propose the “Underpants Index” as an accessible, engaging method to assess soil decomposition processes and soil fertility and raise public awareness for soil as a vital, yet threatened resource.

Introduction: Improving soil health while maintaining crop yield is a key challenge for farmers. So far, only a few studies assessed the effects of compost and solid digestate application on soil health and plant yield under practical on‐farm conditions across both organic and conventional cropping systems.Materials and Methods: This study examined 56 arable fields in Switzerland, managed either conventionally (n = 39) or organically (n = 17) by individual farmers. Fields were categorised based on their fertilisation history: standard fertilisation (n = 21), including livestock manure, slurry, and mineral fertilisers (reference), or with additional compost (n = 26) or solid digestate (n = 9) amendments. Soil health was assessed based on eight chemical, biological, and physical soil health indicators.Results: Compost use, but not solid digestate use, was associated with enhanced average soil health ( + 31% over reference fields), driven by increases in basal respiration ( + 45%), cation exchange capacity ( + 42%), fungal richness ( + 18%), and marginally higher soil organic carbon stocks ( + 28%). These differences were consistent across management systems, despite site variability. Clay content and extended periods of crop cover also positively influenced soil health. Wheat yields were 21% lower under organic management but unaffected by compost or digestate use.Conclusion: These findings suggest that using compost alongside practices like extended periods of crop cover can effectively promote soil health while maintaining yields in practical farming scenarios, offering a means to balance multiple sustainability goals simultaneously.

Protists, a crucial part of the soil food web, are increasingly acknowledged as significant influencers of nutrient cycling and plant performance in farmlands. While topographical and climatic factors are often considered to drive microbial communities on a continental scale, higher trophic levels like heterotrophic protists also rely on their food sources. In this context, bacterivores have received more attention than fungivores. Our study explored the connection between the community composition of protists (specifically Rhizaria and Cercozoa) and fungi across 156 cereal fields in Europe, spanning a latitudinal gradient of 3000 km. We employed a machine‐learning approach to measure the significance of fungal communities in comparison to bacterial communities, soil abiotic factors, and climate as determinants of the Cercozoa community composition. Our findings indicate that climatic variables and fungal communities are the primary drivers of cercozoan communities, accounting for 70% of their community composition. Structural equation modelling (SEM) unveiled indirect climatic effects on the cercozoan communities through a change in the composition of the fungal communities. Our data also imply that fungivory might be more prevalent among protists than generally believed. This study uncovers a hidden facet of the soil food web, suggesting that the benefits of microbial diversity could be more effectively integrated into sustainable agriculture practices.

Soil fungi are a key constituent of global biodiversity and play a pivotal role in agroecosystems. How arable farming affects soil fungal biogeography and whether it has a disproportional impact on rare taxa is poorly understood. Here, we used the high-resolution PacBio Sequel targeting the entire ITS region to investigate the distribution of soil fungi in 217 sites across a 3000 km gradient in Europe. We found a consistently lower diversity of fungi in arable lands than grasslands, with geographic locations significantly impacting fungal community structures. Prevalent fungal groups became even more abundant, whereas rare groups became fewer or absent in arable lands, suggesting a biotic homogenization due to arable farming. The rare fungal groups were narrowly distributed and more common in grasslands. Our findings suggest that rare soil fungi are disproportionally affected by arable farming, and sustainable farming practices should protect rare taxa and the ecosystem services they support.

Organic carbon and aggregate stability are key features of soil quality and are important to consider when evaluating the potential of agricultural soils as carbon sinks. However, we lack a comprehensive understanding of how soil organic carbon (SOC) and aggregate stability respond to agricultural management across wide environmental gradients. Here, we assessed the impact of climatic factors, soil properties and agricultural management (including land use, crop cover, crop diversity, organic fertilization, and management intensity) on SOC and the mean weight diameter of soil aggregates, commonly used as an indicator for soil aggregate stability, across a 3000 km European gradient. Soil aggregate stability (−56%) and SOC stocks (−35%) in the topsoil (20 cm) were lower in croplands compared with neighboring grassland sites (uncropped sites with perennial vegetation and little or no external inputs). Land use and aridity were strong drivers of soil aggregation explaining 33% and 20% of the variation, respectively. SOC stocks were best explained by calcium content (20% of explained variation) followed by aridity (15%) and mean annual temperature (10%). We also found a threshold‐like pattern for SOC stocks and aggregate stability in response to aridity, with lower values at sites with higher aridity. The impact of crop management on aggregate stability and SOC stocks appeared to be regulated by these thresholds, with more pronounced positive effects of crop diversity and more severe negative effects of crop management intensity in nondryland compared with dryland regions. We link the higher sensitivity of SOC stocks and aggregate stability in nondryland regions to a higher climatic potential for aggregate‐mediated SOC stabilization. The presented findings are relevant for improving predictions of management effects on soil structure and C storage and highlight the need for site‐specific agri‐environmental policies to improve soil quality and C sequestration.

Phosphorus (P) acquisition is key for plant growth. Arbuscular mycorrhizal fungi (AMF) help plants acquire P from soil. Understanding which factors drive AMF-supported nutrient uptake is essential to develop more sustainable agroecosystems. Here we collected soils from 150 cereal fields and 60 non-cropped grassland sites across a 3,000 km trans-European gradient. In a greenhouse experiment, we tested the ability of AMF in these soils to forage for the radioisotope ³³P from a hyphal compartment. AMF communities in grassland soils were much more efficient in acquiring ³³P and transferred 64% more ³³P to plants compared with AMF in cropland soils. Fungicide application best explained hyphal ³³P transfer in cropland soils. The use of fungicides and subsequent decline in AMF richness in croplands reduced ³³P uptake by 43%. Our results suggest that land-use intensity and fungicide use are major deterrents to the functioning and natural nutrient uptake capacity of AMF in agroecosystems.

Phosphorus (P) acquisition is key for global food production. Arbuscular mycorrhizal fungi (AMF) help plants acquire P and are considered key for the design of sustainable agroecosystems. However, how the functioning of AMF varies across agricultural soils and responds to management practices is still unknown. Here, we collected soils from 150 cereal fields and 60 non-cropped grassland sites across Europe, and in a greenhouse experiment, we tested the ability of AMF in these soils to forage for radioisotope-labelled ³³P from a hyphal compartment. Hyphal-mediated P uptake was 64% higher in non-cropped grassland compared to cropland soils. Soil pH and organic carbon best explained the hyphal-mediated P uptake in the grasslands, while the use of fungicide in croplands reduced P uptake in the croplands by 43%. Our results suggest that land-use intensity and fungicide use are major deterrents to the natural capacity of AMF to contribute to sustainable crop production.