Microplastics from Agricultural Soils to Freshwaters: Pathways, Loads, and Controls

Microplastics (MPs) have become ubiquitous contaminants in agricultural landscapes, where intensive farming practices introduce large quantities through plastic mulches, sewage sludge, wastewater irrigation, and other inputs. These soils act as both reservoirs and sources of MPs, with potential leakage into freshwater ecosystems. This review synthesizes recent advances in understanding the mechanisms governing MP transfer from farmland to aquatic systems, focusing on surface runoff, soil erosion, wind-driven transport, and vertical leaching. Field surveys and experimental studies consistently demonstrate that only a small percentage of the large MP inventory in soils is exported during typical rainfall events; however, extreme storms, erosion hotspots, and biosolid applications can generate significant fluxes at watershed and national scales. Modeled estimates suggest that annual MP exports from farmland runoff may rival other major emission pathways, underscoring the environmental relevance of diffuse agricultural sources. The mobilization of MPs is strongly modulated by particle characteristics (size, density, and shape), soil texture and structure, vegetation cover, rainfall intensity, irrigation practices, and biological activity, including earthworms and plant roots. The complex interplay of these factors highlights both the persistence of MPs in soils and their episodic release into rivers and lakes. Controversies remain regarding the relative importance of agriculture compared to urban and wastewater sources, as well as the long-term role of biodegradable plastics in reducing soil-to-water transfers. By integrating empirical data and modeling insights, this review clarifies the dual role of agricultural soils as sinks and diffuse sources of MPs, and identifies critical controls for mitigation strategies.