Rutger A. Wilschut

Nitrogen (N) is crucial for plant growth, but its overuse harms biodiversity. Increasing soil biodiversity might provide the means to reduce N inputs, but experimental evidence for this paradigm shift is limited. Using microbiome predators (protists and nematodes) that shape microbiome composition and participate in N cycling, we examined how interactions between their diversity and N addition affect Cannabis sativa growth. The addition of microbiome predators increased plant biomass by up to 53%, irrespective of diversity level, with effects reaching up to 60% under higher microbiome predator diversity. This biomass increase was primarily associated with changes in bacterial community composition and enriching functions related to carbon and N cycling. In contrast, N input played a greater role in determining plant and soil nutrient content. These findings suggest that microbiome predators determine plant biomass in a diversity and N‐specific manner, showing the pivotal role of soil biodiversity in enhancing plant performance and serving as promising tools to mitigate N inputs.

Summary: Ecosystems world‐wide are experiencing an accelerating accumulation of exotic plant species, posing serious threats to biodiversity and ecosystem functioning. While soil microbially mediated indirect interactions play a crucial role in shaping plant community composition, their contribution to accumulation of exotics remains poorly understood, especially under changing environmental conditions. Here, we examined how native and exotic plants impact the growth and competitiveness of later‐arriving native and exotic plants through soil microbial legacies generated in different environmental conditions. We found that soil microbial legacies generated by exotics under well‐watered conditions conferred a competitive advantage to later‐arriving exotics over natives, but legacies generated by natives had no such effect. This indirect advantage for exotics was linked to a higher relative abundance of pathogens in soils conditioned by exotics, which was negatively correlated with native growth and positively associated with the competitive effects of exotics on natives. However, drought eliminated this pathogen‐mediated indirect advantage among exotics by preventing pathogen accumulation. This study highlights the critical importance of considering soil microbially mediated indirect interactions between native and exotic species for accurately predicting the accumulation of exotic plant species under climate change.

Background: Antibiotic pollution, caused by extensive antibiotic applications in industrial farming, threatens human and animal health, but may also affect plant functioning. However, it remains unknown whether antibiotics affect plant performance directly, or indirectly via effects on soil biota. Potential soil-community-mediated antibiotic effects may be most pronounced under abiotic stress, as plant-stress alleviation by soil biota may be compromised. Methods: We conducted a greenhouse experiment to examine how two commonly applied antibiotics (tetracycline and amoxicillin) alter the performance of six grassland plant species in the presence and absence of soil biota. We also assessed whether antibiotic effects on plant performance are stronger under drought stress. Results: Antibiotic presence did not affect plant biomass, but high tetracycline concentrations reduced positive soil microbial-community effects on biomass allocation to roots. Furthermore, high amoxicillin concentrations enhanced the positive soil-community effect on chlorophyll A fluorescence kinetics (a performance index), while limiting the soil biotic drought stress-alleviating effect on leaf chlorophyll concentration. Nevertheless, most antibiotic effects were antibiotic type- and dose-dependent, differed among species and depended on soil-community presence. Conclusion: Our study shows that antibiotics at realistic environmental concentrations can alter plant-soil interactions in plant-species-specific ways and therefore may eventually affect natural plant community composition.

Positive plant diversity–productivity relationships have partly been attributed to decreasing abundances of belowground specialist plant antagonists with increasing plant diversity, resulting in less negative plant–soil feedbacks (PSFs) on establishing conspecific plant individuals. Yet, generalist plant antagonists also contribute to negative PSFs among conspecific and heterospecific plants. Possibly, contributions of generalist plant antagonists to PSF outcomes may become more important in high‐diversity communities due to diversity‐driven specialist plant–antagonist dilution, but this has remained untested. We set up a three‐phase pot experiment to examine how plant diversity affects the differences between conspecific and heterospecific PSFs. In the first (‘pre‐conditioning') phase, we conditioned soils with communities of one, two, four or eight species. By mixing soils within each diversity level, we created soil mixtures with legacies of all eight species, which only differed in the diversity level at which the plants conditioned the soils. We then established monocultures of all eight species in each mixture (‘conditioning phase') and finally examined feedback responses of all individual species (‘feedback phase'). We tested the hypothesis that compared to heterospecific PSFs, conspecific PSFs are most negative in soil mixtures with a legacy of the lowest plant diversity, but become less negative in soil mixtures with legacies of increasing plant diversity. Diversity legacy and pre‐conditioning‐phase conspecific biomass positively or negatively affected plant performance in the feedback phase, depending on species identity. Plants experienced more negative conspecific than heterospecific feedback, but this effect did not depend on soil legacies established in the pre‐conditioning phase. Our study shows that diverse plant communities leave persistent soil legacies that differ from combined legacies of lower‐diversity communities, indicating that plant–soil interactions of individual plants are affected by the community context. However, our study does not provide evidence that community diversity predictably affects differences between conspecific and heterospecific PSF interactions.

Background and aims: Microplastics affect plant growth and change abiotic and biotic soil properties, such as soil structure and soil-community composition. However, how microplastics affect plant-soil interactions, such as plant-soil feedbacks (PSFs), is still poorly understood. Here, we tested how artificial particles affect heterospecific PSFs, depending on an intact or depleted soil community. Methods: We conducted a two-phase-greenhouse experiment using Centaurea jacea to condition soil containing an intact or initially depleted (by sterilization) soil community in the first phase. Subsequently, we grew individuals of Crepis biennis and Eragrostis minor in all combinations of soil conditioning (presence or absence of C. jacea in the first phase), soil-community status, and different material treatments including no added particles, glass particles, or three microplastics individually and mixed. Effects of soil community, material treatment and their interaction on PSFs were assessed based on plant biomass and root-morphology traits. Results: Particles in general, microplastics and glass, strengthened PSFs based on plant biomass. PSFs tended to be negative with the intact but positive with the initially depleted soil community. Overall, particle-addition effects on PSFs were stronger in the initially depleted community, indicating interactive effects of artificial particles in the soil and soil biota. Interactive particle and soil-community effects generally depended on material type and concentration. Conclusion: Our findings indicate that artificial particles can affect heterospecific PSFs and that these effects are likely to be partly mediated by the initial soil community. Further, they highlight the need for studies assessing potential ecological implications of microplastics on plant-soil interactions.

Terrestrial plant communities often become invaded by alien species, which may benefit from high growth rates, strong phenotypic plasticity and reduced negative impacts from local soil communities. At the same time, terrestrial communities are increasingly more often exposed to periods of drought. However, how drought affects the competition between alien and native plants directly, and indirectly, through changing impacts of soil communities on plant performance, remains poorly understood. Here, we performed a greenhouse pot experiment in which we examined biomass responses of five native and five naturalized alien species (all occurring in mesic grasslands) to drought and benign soil moisture conditions, while growing in interspecific, intraspecific or absence of competition, in the presence or absence of native soil biota. We expected that alien plant species are less negatively affected by soil biota, but more negatively affected by drought than native species, and that drought indirectly weakens soil‐community‐driven competitive benefits of alien plant species over native ones. On average, soil‐community effects on plant biomass were positive, but native performance was less positively affected by soil communities than alien performance, suggesting reduced impacts of soil‐borne enemies on alien plants. Drought more negatively affected alien‐ than native plant performance. Drought impacts on plant biomass did not depend on soil community presence, but in the presence of soil biota, plants overall invested more in root biomass when exposed to drought. The effects of competition were subtle and species‐specific. To better understand the observed positive soil‐community effects on plant performance in our study, we examined mycorrhizal root colonization of plants grown in absence of competition. Among‐species variation in mycorrhizal colonization explained plant performance differences between soils with and without live soil communities, indicating a key role for arbuscular mycorrhizal fungi as driver of plant performance. However, mycorrhizal colonization did not differ between alien and native plants and was unaffected by drought. Overall, our study suggests that drought may weaken alien plant invasions through stronger direct negative impacts on alien than on native plant performance, but that drought does not affect soil‐biota‐driven differences in plant performance between alien and native plants. Read the free Plain Language Summary for this article on the Journal blog.

Editorial: Drivers of host-microbiome interactions in the rhizosphere

Soil microbes can profoundly affect species coexistence and alien plant invasion via plant–soil feedbacks (PSFs). Theoretically, plants can modify soil biotic communities (e.g. fungal and bacterial communities) sequentially via rhizo‐inputs in the growing season and via litter decomposition in the non‐growing season, and in this way affect the performance of plant individuals that establish in the same soil in the following growing season. However, how plants affect soil microbes via sequential rhizosphere and litter inputs and how these soil microbial legacies feedback to subsequent plants remains unclear. Here, we first explored the sequential effects of living individuals and their leaf litter on soil fungal communities, using 42 common alien and native species in China, and then estimated their PSF effects on three common test species. Our results show that living plants growing in field soil developed distinct rhizosphere fungal communities within one season. After the removal of conditioning plants, the fungal communities in these conditioned soils changed more dramatically in the absence than in the presence of leaf litter, suggesting that leaf litter inputs prolonged the species‐specific legacies that conspecific living individuals exerted in the soil. Moreover, leaf litter induced general negative effects on all test species in the second growing season. Conditioning plant origin (native vs. alien) and life cycle (annual vs. perennial), respectively, had a slight or no impact on fungal community composition, and conditioning plant origin did not affect PSF responses of the test species. Synthesis: These findings show that leaf litter prolongs soil fungal legacies of conspecific living individuals, which can lead to carryover effects on heterospecific plant growth in the next season. Our findings highlight the importance of leaf litter for plant–soil interactions and thereby stress the potential importance of plant litter as a driver of plant community dynamics.

Aboveground, large and higher trophic‐level organisms often respond more strongly to environmental changes than small and lower trophic‐level organisms. However, whether this trophic or size‐dependent sensitivity also applies to the most abundant animals, microscopic soil‐borne nematodes, remains largely unknown. Here, we sampled an altitudinal transect across the Tibetan Plateau and applied a community‐weighted mean (CWM) approach to test how differences in climatic and edaphic properties affect nematode CWM biomass at the level of community, trophic group and taxon mean biomass within trophic groups. We found that climatic and edaphic properties, particularly soil water‐related properties, positively affected nematode CWM biomass, with no overall impact of altitude on nematode CWM biomass. Higher trophic‐level omnivorous and predatory nematodes responded more strongly to climatic and edaphic properties, particularly to temperature, soil pH, and soil water content than lower trophic‐level bacterivorous and fungivorous nematodes. However, these differences were likely not (only) driven by size, as we did not observe significant interactions between climatic and edaphic properties and mean biomasses within trophic groups. Together, our research implies a stronger, size‐independent trophic sensitivity of higher trophic‐level nematodes compared with lower trophic‐level ones. Therefore, our findings provide new insights into the mechanisms underlying nematode body size structure in alpine grasslands and highlight that traits independent of size need to be found to explain increased sensitivity of higher trophic‐level nematodes to climatic and edaphic properties, which might affect soil functioning.