<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.全球变化与植物多样性丧失对陆地生态系统功能构成威胁。微塑料（microplastic）污染被视作一类新型环境胁迫因子，或可影响植物生物量生产。然而，目前对于微塑料是否会与干旱等其他全球变化因子产生交互作用、进而影响植物群落以及植物多样性与生物量生产之间的关系，相关认知仍较为匮乏，其具体作用机制亦尚未明确。 为阐明上述问题及其潜在机制，本研究开展了一项温室实验。我们沿植物多样性梯度构建植物群落，并在草地微宇宙（grassland microcosms）中设置了4种微塑料与干旱的组合处理情景。 研究结果显示，植物多样性对生物量生产的正向效应显著依赖于干旱条件，但未检测到其与微塑料存在显著交互作用。尽管如此，微塑料可通过抑制草本植物与豆科植物的地上生物量，削弱正向选择效应，进而弱化植物多样性与生物量生产间的正向关联。与之相对，干旱可通过诱导负互补效应，显著弱化该正向关联：干旱强烈降低了豆科植物的地上生物量，进而产生负的净多样性效应。 在所有植物多样性梯度下，微塑料与干旱均会降低群落生物量，但微塑料可通过提升豆科植物的地上生物量，缓解干旱对群落生物量的负面影响。 综合分析：本研究结果表明，微塑料与干旱均可调控植物多样性对生物量生产的正向效应。此外，本研究发现，植物多样性影响群落生产力的作用机制对微塑料与干旱的响应敏感性存在差异。本研究强调，在面临微塑料污染与干旱风险时，豆科植物对于保护和维持生物多样性及生态系统功能具有重要意义。