<b>Microplastic and drought influence the positive effect of plant diversity on plant biomass production</b>

Global change and the loss of plant diversity threaten terrestrial ecosystem functionality. Microplastic pollution is considered a novel environmental stressor potentially affecting plant biomass production. However, it is poorly understood whether and how microplastic interacts with other global change factors, such as drought, to affect plant communities and the relationships between plant diversity and biomass production. To unravel the above question and any underlying mechanisms, we conducted a glasshouse experiment. We assembled plant communities along a gradient of plant diversity and then subjected them to four microplastic and drought scenarios in grassland microcosms. Our results showed that the positive effect of plant diversity on biomass production strongly depended on drought, whereas no significant interaction with microplastic was detected. Nevertheless, microplastic tended to decrease this positive relationship by reducing positive selection effect through suppressed shoot biomass of grasses and legumes. By contrast, drought significantly weakened this positive relationship by inducing a negative complementarity effect that led to a negative net diversity effect through strongly reduced shoot biomass of legumes. Both microplastic and drought decreased community biomass across all plant diversity levels, but microplastic could alleviate the negative effect of drought on community biomass by enhancing the shoot biomass of legumes. Synthesis. Our findings reveal that microplastic and drought influence the positive effects of plant diversity on plant biomass production. Moreover, our study suggests that the mechanisms by which plant diversity affects productivity are differently sensitive to microplastic and drought. We highlight the importance of legumes in protecting and maintaining biodiversity and ecosystem functions in the face of microplastic pollution and drought risks.