Microplastics transport in soils: A critical review

Microplastics (MPs) in terrestrial environments are an emerging contaminant of high concern to ecosystems and human health. However, our understanding of the MPs' fate, particularly their transport within soils, remains elusive. This knowledge gap arises from the multiplicity of coupled physical, chemical and biological processes and parameters affecting MPs transport, together with the scarcity of systematic studies that aim to isolate their individual effects. In this paper, we provide a critical review of the state-of-the-art in our understanding of MPs transport, highlight knowledge gaps and suggest future research to bridge them. We classify the governing factors into four main categories: (i) MPs properties; (ii) soil physicochemical properties; (iii) hydrological conditions; and (iv) biological activity. Our analysis reveals that lack of clear trends in the dependence between MP transport and individual key parameters—often leading to contradictory findings—could be explained by the interference (“co-effects”) with other parameters and processes. • Microplastic mobility in soils is affected by multiple, often coupled, factors. • Factors include microplastic & soil properties, hydrological conditions, and organisms. • Multiplicity of control parameters and mobility metrics complicates comparison across studies. • Interference among factors can explain lack of clear trends and contradictory findings.