Alain Valzano-Held

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.

Arbuscular mycorrhizal (AM) fungi are often regarded as a secondary root system for plants, supporting them with nutrients and water, thus playing an important role in agriculture. Understanding how AM fungal communities react to multiple agricultural management practices is vital for developing agricultural management systems that provide high levels of soil ecosystem services. To this goal, the present study assessed the effect of tillage practices on AM fungal communities in seven long-term agricultural experiments in Europe. Soil samples were collected from plots subjected to standard-tillage (mouldboard ploughing), reduced-tillage, and no-tillage practices. On average, no-tillage enhanced soil AM fungal richness and Shannon diversity by 23% and 21% relative to standard-tillage, respectively, with substantial site-specific variation across the seven study sites. No-tillage significantly increased soil AM fungal α-diversity at four of the seven sites, while reduced tillage induced significant increases in soil AM fungal α-diversity at two sites. Furthermore, the responses of dominant AM fungal taxa to tillage practices were site-specific, and soil pH and organic phosphorus were identified as key factors shaping AM fungal community structure. Overall, these findings demonstrate that tillage practices exert a strong influence on soil AM fungal communities.

Alternative solutions to mineral fertilizers and pesticides that reduce the environmental impact of agriculture are urgently needed. Arbuscular mycorrhizal fungi (AMF) can enhance plant nutrient uptake and reduce plant stress; yet, large-scale field inoculation trials with AMF are missing, and so far, results remain unpredictable. We conducted on-farm experiments in 54 fields in Switzerland and quantified the effects on maize growth. Growth response to AMF inoculation was highly variable, ranging from −12% to +40%. With few soil parameters and mainly soil microbiome indicators, we could successfully predict 86% of the variation in plant growth response to inoculation. The abundance of pathogenic fungi, rather than nutrient availability, best predicted (33%) AMF inoculation success. Our results indicate that soil microbiome indicators offer a sustainable biotechnological perspective to predict inoculation success at the beginning of the growing season. This predictability increases the profitability of microbiome engineering as a tool for sustainable agricultural management.

Plant microbiomes play crucial roles in nutrient cycling and plant growth, and are shaped by a complex interplay between plants, microbes, and the environment. The role of bacteria as mediators of the 400-million-year-old partnership between the majority of land plants and, arbuscular mycorrhizal (AM) fungi is still poorly understood. Here we test whether AM hyphae-associated bacteria influence the success of the AM symbiosis. Using partitioned microcosms containing field soil, we discovered that AM hyphae and roots selectively assemble their own microbiome from the surrounding soil. In two independent experiments, we identified several bacterial genera, including Devosia , that are consistently enriched on AM hyphae. Subsequently, we isolated 144 pure bacterial isolates from a mycorrhiza-rich sample of extraradical hyphae and isolated Devosia sp. ZB163 as root and hyphal colonizer. We show that this AM-associated bacterium synergistically acts with mycorrhiza on the plant root to strongly promote plant growth, nitrogen uptake, and mycorrhization. Our results highlight that AM fungi do not function in isolation and that the plant-mycorrhiza symbiont can recruit beneficial bacteria that support the symbiosis.

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.