Vertical transport of microplastic in agricultural soil in controlled irrigation plot experiments

Microplastic pollution in agricultural sites has gained increased attention in recent years. Many studies focused on the impact of plastic residues on soil functions, such as soil physiochemical properties, fertility, and biodiversity. However, research on the transport behavior of microplastics (MPs) in agricultural soil remains rare. Therefore, it is important to understand the transport mechanism of MPs in the natural environment. Plot experiments (1x1 m) were conducted in an agricultural site (silty loam) near Prague to investigate the size-dependent movement of MPs under both artificial irrigation (first campaign) and natural rainfall (second campaign). Before irrigation, fluorescent PE microspheres with four different size ranges (53-63 µm, 125-150 µm, 250-300 µm, 425-500 µm) were mixed with soil and then uniformly distributed on the plot surface (upper 1 cm). Deuterium as a conservative tracer was added and well mixed with water for the rainfall simulation. The rainfall simulation with an intensity of 60 mm/h was applied. Results from the first campaign show that the maximum migration depth of MPs was up to 4-6 cm, which is consistent with the results from the tracer experiment. Moreover, larger particles were mostly found on the top layer up to 2 cm, with small MPs at 53-63 µm transported down to 6 cm. These results indicate that the infiltration of water could enhance the movement of MPs in the soil profile, with smaller MPs having higher mobility under rainfall simulation. This finding provides insight into the mobility of MPs in agricultural soils, and it could be applied for control and risk assessment to estimate the potential of MPs leaching into aquifer systems.