Effects of soil microplastic heterogeneity on plant growth vary with species and microplastic types

Microplastics are heterogeneously distributed in soils. However, it is unknown whether soil microplastic heterogeneity affects plant growth and root foraging responses and whether such effects vary with plant species and microplastic types. We grew each of seven herbaceous species (Platycodon grandiflorus, Trifolium repens, Portulaca oleracea, Medicago sativa, Taraxacum mongolicum, Perilla frutescenst, and Paspalum notatum) in heterogeneous soil (patches without microplastics and patches with 0.2 % microplastics) and homogeneous soil (patches with 0.1 % microplastics). Three microplastic types were tested: polypropylene (PP), polyacrylonitrile (PAN), and polyester (PET). P. frutescens showed no response to soil microplastic heterogeneity. For P. grandiflora, microplastic heterogeneity tended to decrease its biomass (total, shoot and root) when the microplastic was PAN and also shoot biomass when it was PET, but had no effect when it was PP. For T. repens, microplastic heterogeneity promoted biomass when PAN was used, decreased total and root biomass when PET was used, but showed no effect when PP was used. Microplastic heterogeneity increased biomass of P. oleracea and decreased that of M. sativa when PET was used, but had no effect when PP or PAN was used. For T. mongolicum, microplastic heterogeneity reduced biomass when the microplastic was PAN, tended to increase total and root biomass when it was PP, but showed no effect when it was PET. For P. notatum, microplastic heterogeneity increased biomass when the microplastic was PP, decreased it when PET was used, but had no effect when PAN was used. However, biomass of none of the seven species showed root foraging responses at the patch level. Therefore, soil microplastic heterogeneity can influence plant growth, but such effects depend on species and microplastic types and are not associated with root foraging. Our findings highlight the roles of soil microplastic heterogeneity, which may influence species interactions and community structure and productivity.