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Microplastics enter the soil in a variety of shapes and polymer types altering soil properties with known consequences for plant growth. However, the effects of a range of different microplastic shapes and types on seed germination are mostly unknown. Here, we established a glasshouse experiment that included 12 microplastic types representing different shapes (fibers, films, foams and fragments) and polymers, and mixed each of them with soil at a concentration of 0.4% (w/w). Fifty seeds of Daucus carota were sown and monitored for 49 days to evaluate different germination parameters. Our results showed that microplastic films and fibers decrease seed germination velocity as they may affect soil water status, likely interfering with different phases of seed germination: Seeds may imbibe toxic microplastic leachates, and be affected by a physical blockage; testa rupturing may be delayed as this also depends on water uptake. Microplastic toxic leachates may affect activity of enzymes key for seed germination, and delay embryo growth and radicle emergence. Microplastics, irrespective of their shape and polymer type, increase synchrony of seed germination, which might be linked with microplastics exerting a mild stress on seeds. The final percentage of germination was not affected by microplastics in soil, implying that microplastics did not affect seed viability. Our results showed that microplastics affect seed germination mainly as a function of their shape.

Microplastic pollution is a topic of increasing concern, especially since this issue was first addressed in soils. Results have so far been variable in terms of effects, suggesting that there is substantial context-dependency in microplastic effects in soil. To better define conditions that may affect microplastic-related impacts, we here examined effects as a function of microplastic shape and polymer type, and we tested if effects on soil properties and soil microbial activities change with incubation time. In our laboratory study, we evaluated twelve different secondary microplastics representing four microplastic shapes: fibers, films, foams and fragments; and eight polymer types: polyamide (PA), polycarbonate (PC), polyethylene (PE), polyester (PES), polyethylene terephthalate (PET), polypropylene (PP), polystyrene (PS), and polyurethane (PU). We mixed the microplastics with a sandy soil (0.4% w/w) and incubated at 25°C for 31 days. Then, we collected soil samples on the 3rd, 11th, and 31st day, and measured soil pH, respiration and four enzyme activities (soil enzymatic activities). Our results showed that microplastics could affect soil pH, respiration and enzymatic activities depending on microplastic shape and polymer type, effects that were altered with incubation time. Soil pH increased with foams and fragments and overall decreased in the first days of incubation and then increased. Soil respiration increased with PE foams and was affected by the incubation time, declining over time. Overall, acid phosphatase activity was not affected by shape or polymer type. β-D-glucosidase activity decreased with foams, cellobiosidase activity decreased with fibers, films and foams while N-acetyl-β-glucosaminidase activities decreased with fibers and fragments. Enzymatic activities fluctuated during the incubation time, except N-acetyl-β-glucosaminidase, which showed a declining trend with incubation time. Enzymatic activities were negatively correlated with soil pH and this relationship was less strong when microplastics were added to the soil. Our study adds to the evidence that research should embrace the complexity and diversity of microplastics, highlighting the role of microplastic shape and polymer type in influencing effects; additionally, we show that incubation time is also a parameter to consider, as effects are dynamic even in the short term.

Feedbacks between plants and soil microbial communities play an importantrole in vegetation dynamics, but the underlying mechanisms remain unresolved.Here we show that the diversity of putative pathogenic, mycorrhizal andsaprotrophic fungi are primary regulators of plant-soil feedbacks across a broadrange of temperate grassland plant species. We show that plant species withresource-acquisitive traits, such as high shoot nitrogen concentrations and thinroots, attract diverse communities of putative fungal pathogens and specialistsaprotrophs, and a lower diversity of mycorrhizal fungi, resulting in strong plantgrowth suppression on soil occupied by the same species. Moreover, soilproperties modulate feedbacks with fertile soils promoting antagonisticrelationships between soil fungi and plants. This study advances our capacity topredict plant-soil feedbacks and vegetation dynamics by revealing fundamentallinks between soil properties, plant resource acquisition strategies and thediversity of fungal guilds in soil.

Microplastics affect plants and soil biota and the processes they drive. However, the legacy effect of microplastics on plant–soil feedbacks is still unknown. To address this, we used soil conditioned from a previous experiment, where Daucus carota grew with 12 different microplastic types (conditioning phase). Here, we extracted soil inoculum from those 12 soils and grew during 4 weeks a native D. carota and a range-expanding plant species Calamagrostis epigejos in soils amended with this inoculum (feedback phase). At harvest, plant biomass and root morphological traits were measured. Films led to positive feedback on shoot mass (higher mass with inoculum from soil conditioned with microplastics than with inoculum from control soil). Films may decrease soil water content in the conditioning phase, potentially reducing the abundance of harmful soil biota, which, with films also promoting mutualist abundance, microbial activity and carbon mineralization, would positively affect plant growth in the feedback phase. Foams and fragments caused positive feedback on shoot mass likely via positive effects on soil aeration in the conditioning phase, which could have increased mutualistic biota and soil enzymatic activity, promoting plant growth. By contrast, fibers caused negative feedback on root mass as this microplastic may have increased soil water content in the conditioning phase, promoting the abundance of soil pathogens with negative consequences for root mass. Microplastics had a legacy effect on root traits: D. carota had thicker roots probably for promoting mycorrhizal associations, while C. epigejos had reduced root diameter probably for diminishing pathogenic infection. Microplastic legacy on soil can be positive or negative depending on the plant species identity and may affect plant biomass primarily via root traits. This legacy may contribute to the competitive success of range-expanding species via positive effects on root mass (foams) and on shoot mass (PET films). Overall, microplastics depending on their shape and polymer type, affect plant–soil feedbacks.

1. Drylands are among the most threatened ecosystems in the world because of the effects of global change and because they are not particularly resilient. Regeneration after disturbance is quite slow and secondary succession can be delayed to the point of nearly stop. This is exacerbated when the community is strongly dominated by a single species able to quickly respond after disturbance. One example of such monospecific dominance is the colonization of abandoned fields in SE Spain by an early successional and native tussock grass, Lygeum spartum, that seems to halt succession. 2. Here we tested the competitive ability of Lygeum against Salsola oppositifolia, a shrub species that can be found interspersed with Lygeum in mid‐successional stages, and assessed how plant–soil interactions mediate the outcome of plant– plant competition. To do so, we sowed seeds and grew plants of Lygeum and Salsola in either intra‐ or interspecific competition under controlled conditions using sterile field soils inoculated with either live (i.e., with micro‐organisms) or sterile soil extracts from the understories of either Lygeum or Salsola. Soil nutrient content, seed germination rate, and shoot mass growth were determined after 5 months, and soil bacterial communities were characterized by sequencing. 3. Lygeum soil micro‐organisms and soil properties, such as the high content of N and organic matter, enhanced seed germination rate of Lygeum individuals. By contrast, Salsola adults outperformed Lygeum when growing in interspecific competition. 4. Synthesis. The enhanced competitive ability of Lygeum, which was mediated by soil micro‐organisms, may lead to complete dominance of Lygeum in the plant community right after abandonment of agricultural fields. However, when the plant community is already developed, Lygeum would be unable to enforce such dominance. We conclude that positive plant–soil feedbacks combined with certain plant traits such as clonal growth support the strong resilience of Lygeum and allow for its dominance in extreme habitats., AEI, Grant/Award Number: CGL2017‐84515‐R; Junta de Andalucía Regional Government, Grant/Award Number: P09‐RNM‐4821