Dynamics of microplastics in soils amended with digestate: interactions with pesticides and antibiotics - Project 3Impact

The agricultural application of digestate (D) derived by the anaerobic treatment of organic waste can contribute to the microplastics (MPs) accumulation in soils. MPs represent an emerging environmental concern because of their high persistence and their documented effects on terrestrial and aquatic ecosystems, as well as their capacity to interact with co-occurring contaminants such as pesticides and antibiotics, with consequences for ecosystem functioning. A deeper investigation of the interplay among MPs, digestate, and xenobiotics is therefore essential to clarify their dynamics in soils, as well as to assess potential long-term environmental impacts.

The 3IMPACT(*) project evaluated soil health, as well as sorption and persistence of xenobiotic compounds (seven antibiotics and one herbicide), under the concurrent presence of D and a mixture of MPs (namely, polyethylene, polystyrene, polypropylene, and polylactic acid), each at real case dose. Laboratory experiments were conducted on two Italian soils (S) treated with MPs (S+MPs), and soils amended with D contaminated with MPs (S+D+MPs) to study xenobiotics mobility and persistence, microbial composition and functionality, and organic matter dynamics.

The results showed that the incorporation of MPs into D-amended soils impaired organic matter turnover, reduced microbial respiration, and suppressed key enzymatic activities, including dehydrogenase, β-glucosidase, urease, and fluorescein diacetate hydrolysis. MPs prolonged the half-life of herbicide Foramsulfuron, whereas D reduced it by 10% in both soils. The herbicide adsorption increased in the presence of MPs and D, following the order (S) < (S+MPs) < (S+D+MPs). Soil dehydrogenase, β-glucosidase, and urease were stimulated by Foramsulfuron, MPs, and D, likely due to the addition of carbon and energy sources for soil microorganisms. Effects varied depending on soil type, exposure time, and interactions among treatments. MPs also prolonged the half lives of Gamithromicin, Tiamulin and Tilmicosin, while had no significant effect on other studied antibiotics. Adsorption coefficients of Gamithromicin, Lincomicin, Marbofloxacin, Oxytetracyclin, Tiamulin increased in soils amended with D but decreased for Florfenicol and Tilmicosin. MPs had no significant effects on adsorption coefficients.

Summarising, MPs contamination in soils amended with digestate exhibited significant alteration of microbiota activity. As a trend, the effect of digestate on xenobiotics adsorption within soils was higher than the effect of MPs, likely due to the additional digestate surface available for adsorption. Additionally, we hypothesized that the increased persistence of xenobiotics in soils observed in the presence of MPs can be associated to changes in microbiota composition, which is currently under study.

 

(*) 3IMPACT project (2022M24ASJ) is within the PRIN 2022 call (D.D. 104/2022 MUR) funded by the European Union - Next Generation EU, Mission 4, Component 2, CUP J53D23010170006.