Are we underestimating microplastic emissions from agricultural soils?

Agricultural management practices significantly influence the the emission of particulate matter into the atmosphere, which is a key component of air quality indicators. In particular, agricultural soils in drylands, which constitute ~40% of Earth's terrestrial surface, are highly vulnerable to emissions via accelerated wind erosion because of factors such as increased aridity, recurrent droughts, crop failures, lack of irrigation, and unsustainable soil management practices. These lands are often subjected to large-scale biosolid application, irrigation with reclaimed (grey) water, and plastic mulching to meet the growing demand for water and to reduce reliance on fossil fuel-intensive fertilizers. However, these practices could significantly increase microplastics in the topsoil. Wind can transport these microplastic particles beyond agricultural systems, potentially carrying adsorbed contaminants such as per- and polyfluoroalkyl substances (PFAS). To evaluate inhalation exposure risks, it is crucial to understand the extent of microplastic pollution and the mechanisms driving their resuspension from agricultural soils. To investigate microplastic emission potential, we used a combination of wind tunnel studies and laboratory experiments on biosolid-amended agricultural soils. Our findings reveal that inhalable microplastics are preferentially entrained and enriched through two primary mechanisms: (1) the accelerated emission of fine plastic particles under wind conditions that are lower than those required for initiating movement of background soil particles (direct suspension without saltation), and (2) the generation and resuspension of fine plastic particles resulting from the abrasion of larger plastic fragments or soil-plastic aggregates by sand grains (saltation-induced suspension). We developed a theoretical framework to explain this preferential transport, attributing it to the low density and reduced interparticle forces between microplastics and soil. Our findings suggest that current methods and models for fugitive dust emissions may underestimate the particulate matter emission potential of amended soils. This is due to limitations in detecting fine particles during sampling and the inadequate representation of plastic entrainment mechanisms (e.g., suspension without saltation) in existing dust emission models. To illustrate this, we demonstrated that over 85% of wind events above bare soil surface exceed the threshold velocity required to mobilize microplastics of a specific size, while only 20% of these events surpass the threshold velocity for background soil particles. Given that fine microplastics may adsorb contaminants from agricultural soils, their preferential entrainment by wind could lead to a concentration of these contaminants in airborne dust, posing potential environmental and health risks.