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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.

1. Microplastics in soils have become an important threat for terrestrial systems as they may potentially alter the geochemical/biophysical soil environment and can interact with drought. As microplastics may affect soil water content, this could exacerbate the well-known negative effects of drought on ecosystem functionality. Thus, functions including litter decomposition, soil aggregation or those related with nutrient cycling can be altered. Despite this potential interaction, we know relatively little about how microplastics, under different soil water conditions, affect ecosystem functions and multifunctionality. 2. To address this gap, we performed an experiment using grassland plant communities growing in microcosms. Microplastic fibres (absent, present) and soil water conditions (well-watered, drought) were applied in a fully factorial design. At harvest, we measured soil ecosystem functions related to nutrient cycling (beta-glucosaminidase, beta-D-cellobiosidase, phosphatase, beta-glucosidase enzymes), respiration, nutrient retention, pH, litter decomposition and soil aggregation (water stable aggregates). As terrestrial systems provide these functions simultaneously, we also assessed ecosystem multifunctionality, an index that encompasses the array of ecosystem functions measured here. 3. We found that the interaction between microplastic fibres and drought affected ecosystem functions and multifunctionality. Drought had negatively affected nutrient cycling by decreasing enzymatic activities by up to similar to 39%, while microplastics increased soil aggregation by similar to 18%, soil pH by similar to 4% and nutrient retention by up to similar to 70% by diminishing nutrient leaching. Microplastic fibres also impacted soil enzymes, respiration and ecosystem multifunctionality, but importantly, the direction of these effects depended on soil water status. That is, under well-watered conditions, these functions decreased with microplastic fibres by up to similar to 34% while under drought they had similar values irrespective of the microplastic presence, or tended to increase with microplastics. Litter decomposition had a contrary pattern increasing with microplastics by similar to 6% under well-watered conditions while decreasing to a similar percentage under drought. 4. Synthesis and applications. Single ecosystem functions can be positively or negatively affected by microplastics fibres depending on soil water status. However, our results suggest that microplastic fibres may cause negative effects on ecosystem soil multifunctionality of a similar magnitude as drought. Thus, strategies to counteract this new global change factor are necessary.