Mechanisms of microplastic accumulation in the root zones of agricultural soils

Root zones of agricultural soils are particularly vulnerable to microplastic accumulation due to high rates of inputs from agriculture-specific sources, including fertilizers and plastic mulch films. As microplastics degrade very slowly, any removal of microplastics from the root zone within agriculturally relevant timescales of several months or years likely requires mechanical means, such as hydrologically driven leaching and erosion. However, the potential for microplastic leaching appears to be low. Plastic mulch, a major contributor to microplastics in agricultural soils, is used to help retain soil moisture. Plastic mulch is thus often used in dryland agriculture, where leaching may be minimal due to similar mean water inputs (precipitation, irrigation) and outputs (evapotranspiration) at the soil surface. To gain new insight into this critical issue on agricultural soil sustainability, we conducted several studies on microplastic leaching and erosion in agricultural soils.

Firstly, we performed a column experiment with microplastics added to disturbed and undisturbed soil columns, subject to multiple irrigation-drying cycles. Results show that microplastics were highly immobile both in the soil matrices and fractures. The overall subsurface transport behavior of microplastics appeared to be primarily diffusive, meaning that remediating microplastic polluted soils by forced leaching may not be feasible.

Secondly, our pore-scale/fracture-scale hydrodynamic modelling study suggests that the trajectories of microplastics in soils are highly sensitive to fluctuations in water fluxes, spatial heterogeneities in soil properties, and the physical properties of microplastic particles (which change over time due to weathering), because most microplastics have near-neutral density in water. Collectively, the chaotic trajectories of numerous microplastics may partly explain the primarily diffusive transport observed in the column experiment. Furthermore, the model shows that particles with near-neutral density are especially likely to be trapped in low-flow parts of the soil where particles are unlikely to be remobilized by fluid flow. This may explain the low mobility of microplastics in our column experiment despite the large mean pore-water velocities (20 cm·h^-1) during irrigation.

Thirdly, from another ongoing experimental study, we find that because of the minimal downwards leaching of microplastics, microplastics are susceptible to overland transport during runoff-erosion events. Preliminary results suggest that the subsurface and overland transport mobility and behavior of microplastic film debris from agricultural mulch are highly distinct from that of microplastic fragments from other sources such as fertilizer.

Therefore, microplastics are prone to accumulate in the shallow layers of agricultural soils, whether at the source location, or off-site due to overland transport. Nevertheless, this transport and accumulation is very sensitive to microplastic type and physical properties.