## 4 Impact of Plastic Particles on Soil Properties
### Microplastic Transport Pathways in Soil System  
Microplastics can move through soil systems via various transport pathways; their transport within the soil matrix is influenced by a variety of factors, including soil aggregation, the shape and size of the MPs, interactions with soil organisms, and environmental conditions [117; 118; 119], as detailed in studies listed in Table S6 (SI). Bioturbation plays a pivotal role in both initiating and facilitating the vertical and horizontal movement of MPs within soil systems [120; 121; 122; 123]. Soil organisms, notably earthworms, are instrumental in this process. By creating burrows and tunnels, earthworms provide channels that enable MPs to migrate vertically and horizontally, potentially reaching deeper soil layers [120; 121]. Other organisms, such as collembolans, mites, insects, plants, and small animals, also contribute to the transport of MPs. Their ability to ingest and excrete MPs facilitates their movement from shallower to deeper soil layers [121], especially in the case of low-density MPs. These particles can penetrate soil pores, and their movement is further amplified by bioturbation activities like plant root growth and earthworm burrowing [120]. Abiotic factors play a significant role in the vertical transport of MPs in soil, primarily through soil water infiltration and the formation of soil cracks [120]. The movement of soil water can cause MPs to migrate vertically into deeper layers, following paths like macropores and cracks, alongside plant roots and through earthworm burrows [121]. In contrast, the predominant factor in the horizontal transport of MPs, particularly in soil or sand, is wind erosion [117]. Wind-driven processes, such as saltation and suspension, are crucial for the horizontal dispersal of MPs in the atmosphere [117]. However, it is worth noting that soil organisms like collembolans, mites, and other insects also contribute to the horizontal movement of MPs within the soil matrix [120; 122; 124]. The transportation mechanisms of conventional and biodegradable MPs might differ. Conventional MPs, known for their longevity in the environment, can be transported through various means. Their movement is influenced by factors such as shape and size, soil porosity and permeability, chemical and physical interactions with soil, biofilm formation, environmental conditions including physico-chemical properties of the pore water such as ionic strength, pH, types of dissolved cations, and flow rate [82]. Biodegradable MPs may exhibit distinct behaviours due to their faster degradation (chemical aging and UV irradiation [122]) and stronger interactions with soil particles or microorganisms [123]. In particular, surface hydrophobicity of MP particles is a critical factor. The faster aging of MBPs leads to an abundance of oxygen-containing groups, resulting in the reduction of the zeta potential and increase in the hydrophilicity. This reduces their interaction with soil particles, thereby enhancing their mobility in soil profile [125], potentially leading to contamination of ground or surface waters [126]. Conversely, lower wettability makes MPs more susceptible to interactions with soil particles, reducing their mobility and enhancing their retention in the soil. This MP-induced hetero-aggregation largely depends on the characteristics of the MPs, the soil organic matter content, and biofilm formation [127]. As a result, hydrophobic (or less aged) particles are less prone to vertical movement by water or bioturbation [128] and are more likely to be transported horizontally by wind erosion [129]. Experimental studies comparing transport of biodegradable and nondegradable plastics are still rare. Recently, [PERSON] et al. reported a comparative study in which demonstrated that biodegradable PLA was easier to transport in soil than PVC [126]. The UV-aging of the microplastics increased their mobility.