Microplastics (MPs) frequently co-occur with pesticides and veterinary medicines in agricultural soils. However, their interactive effects on soil microbiota remain largely unknown. Therefore, we investigated the effects of three MP types (LDPE-, PBAT-, and starch-based), applied at two concentrations (0.01% and 0.1%), either alone or in combination with the fungicide pyraclostrobin and/or the anthelminthic albendazole (ABZ), on soil microbial functioning. Nitrate levels, nitrification rates, ammonia-oxidizing microorganisms, and denitrifying bacteria served as indicators of perturbations on soil N cycling in soils from France, the Netherlands, and Greece. Microbial responses were soil-dependent, with the Greek soil being the most affected. In contrast, plastic type- and dose-dependent effects were sporadic and limited in the French and Dutch soil. In the Greek soil, all MP types increased the abundance of ammonia-oxidizing bacteria and nitrification rates, accompanied by a compensatory decline in ammonia-oxidizing archaea and commamox bacteria. These effects were reversed by the co-application of MPs with ABZ. On the other hand, denitrifying bacteria remained unaffected in all soils. Our results are alarming, considering the perturbation of nitrification imposed by MPs and other soil pollutants, which could enhance greenhouse gas emissions or adversely affect soil fertility and agricultural production.
