Climate warming masks the negative effect of microplastics on plant-soil health in a silt loam soil

Many anthropogenic pressures are being exerted on terrestrial ecosystems globally, perhaps the most pressing of which include microplastics (MPs; <5 mm in size) pollution and climate change, both of which may have unpredictable consequences on soil ecosystem functioning. We therefore hypothesized that a dual pressure (MPs and warming) on plant-soil functioning would be more severe than either stress alone. Thus, we studied the interactive effects of MPs and warming on soil quality and ecosystem multifunctionality. Maize (Zea mays L.) was grown for 6 weeks under ambient and warming (+5 °C) conditions in the absence (control) or presence (5 % loading) of either polyethylene (PE), polyvinylchloride (PVC), or biodegradable polyhydroxyalkanoate (PHA). We found that PHA stimulated microbial biomass and enzyme activity due to the additional C resources, thus changing soil quality and ecosystem multifunctionality under ambient temperature. However, the accelerated microbial growth in PHA-treated soils also promoted N immobilization and plant–microbe nutrient competition, consequently decreasing plant health index by 65 % relative to the Control. As PVC and PE are chemically more stable than PHA, they had limited effect on soil quality and plant health under ambient temperature in the short term (6 weeks). Most of the negative impacts of MPs only occurred under ambient temperature, with few effects evident under warming conditions. This suggested that the effect of heat stress (evidenced by stunted growth and chlorophyll content) was noticeably more acute than the effect of MPs. In conclusion, we showed that MPs do affect plant health, soil quality, and ecosystem multifunctionality but these effects on plant-soil health were not exacerbated by the effects of a warmer climate.