Detection of microplastics in soil by non-invasive imaging - Analyzing microplastic effects on soil properties and root-soil interaction

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Soils around the world are increasingly polluted with anthropogenic contaminants such as microplastics. The continuous input of microplastics changes the conditions in the soil habitat and affects soil biota. However, our understanding of how microplastics alters the structure and functioning of soil is still very limited. It is unclear how microplastics affect processes in the rhizosphere and what risks this poses for plants. Various analytic methods have become available to study different aspects of microplastics in soil. However, all of these methods involve sampling or processing steps that destroy the integrity of the sample. Thus, essential information about the relationship of the distribution of microplastics in the sample and soil microstructure and hydraulics is inevitably lost. Recently, however, we have developed a non-invasive approach that can detect microplastics in sandy soil. Complementary neutron and X-ray tomography allows for detection of microplastic particles in the dry soil and simultaneous mapping of the 3D structure of soil matrix and pore space. In this project, the method will be tested, optimized, and then applied to develop a better mechanistic understanding of how microplastics of different sizes and shapes affect soil microstructure and properties. We will investigate the incorporation of microplastic fibers in soil aggregates and explore their possible effects. We will also study whether the presence of microplastics modify the conditions in the rhizosphere relevant for root growth and water uptake and track water movements in soil in the presence of different sizes, shapes of microplastics. To begin with, the resolution of the combined tomography will be optimized for detection of fine structures such as microplastic fibers and film fragments. The consideration of shape descriptors and the integration of machine learning will support the segmentation of microplastic particles by distinguishing them from soil organic matter. For a natural sandy soil, the influence of microplastic fibers on the formation and stability of soil aggregates is analyzed in an aggregation experiment using high-resolution dual-mode tomography. The next step is to study the rhizosphere of young maize and lupine plants growing in sandy soil containing microplastics of various shapes to determine local changes in root structure, soil matrix structure and water movement. Finally, we will use high-speed neutron tomography to capture the dynamic water infiltration patterns in soil columns in 3D, with and without root systems. Evaluation of the shape and propagation speed of infiltration fronts will provide new insights into whether and how soil wettability is affected by embedded microplastic particles. Implementation of the non-invasive analytical approach will provide unique insights into soil microstructure and hydraulics modified by microplastics and provide a better mechanistic understanding of their shape-mediated effects on sandy soil.