Open Access

Raphael Rehm

• MP contamination is a global issue affecting all ecosystems. While oceans receive significant attention, soils may contain more MP. Arable land serves as a major MP sink due to agricultural practices. This study investigates MP erosion from arable soils and its impact on inland water systems, particularly through surface runoff. This thesis analyzes, on the one hand, experimentally the erosion and transport behavior of MP during heavy rainfall events. Therefore, a specific focus is set to preferential MP transport and MP-soil interactions, potentially leading to a more conservative transport behavior. On the other hand, a model-based estimation provides information on how much MP enters the river network due to erosion from a mesoscale catchment (400 km²). In this context, the amounts of MP applied to cropland are specifically analyzed and runoff rates are calculated taking into account the spatial distribution in the landscape. The experimental part of the thesis is based on a seriesMP contamination is a global issue affecting all ecosystems. While oceans receive significant attention, soils may contain more MP. Arable land serves as a major MP sink due to agricultural practices. This study investigates MP erosion from arable soils and its impact on inland water systems, particularly through surface runoff. This thesis analyzes, on the one hand, experimentally the erosion and transport behavior of MP during heavy rainfall events. Therefore, a specific focus is set to preferential MP transport and MP-soil interactions, potentially leading to a more conservative transport behavior. On the other hand, a model-based estimation provides information on how much MP enters the river network due to erosion from a mesoscale catchment (400 km²). In this context, the amounts of MP applied to cropland are specifically analyzed and runoff rates are calculated taking into account the spatial distribution in the landscape. The experimental part of the thesis is based on a series of rainfall simulations on paired plots (4.5 m x 1.6 m) of silty loam and loamy sand located in Southern Germany. The simulations (rainfall intensity 60 mm h-1) were repeated 3 times within 1.5 years. An amount of 10 g m-2 of fine (MPf, size 53-100 μm) and 50 g m-2 of coarse (MPc, size 250-300 μm) high-density polyethylene as common polymer was added to the topsoil (<10 cm) of the plots. The experiments showed a selective behavior of MP within the process of soil erosion, leading to a higher enrichment ratio in the eroded sediment and indicating MP losses from the plow horizon. Increasing interaction with mineral soil particles or aggregates leads to a decreasing MP delivery over time, showing that MP and soil interactions play a crucial role in the MP erosion process. There was a higher MP enrichment on the loamy sand but a higher sediment delivery on the silty loam resulting in nearly equal MP deliveries from both soil types. This implies that even less erosive coarse-textured soils can exhibit a substantial potential for MP transport. If soil presents a sink for MP depends on the size of the MP particles. The rain simulations have shown that more coarse MP was laterally lost via soil erosion, while the fine MP showed higher topsoil loss rates via vertical transport below the plow layer. The results generally indicate that arable land mainly retains the applied MP. However, in erosion-prone landscapes, arable land can be a substantial MP source for other land uses and aquatic. In the modeling part of the thesis, a soil erosion model was adapted to account for MP erosion, transport and deposition following water and tillage erosion. This modeling tool estimated the MP redistribution within and the MP delivery from a mesoscale (~ 400 km²) catchment in Southern Germany. To do so, an extensive evaluation of the potential MP contamination of especially the arable land in the catchment since 1950 was performed. The modeling analysis showed that most eroded MP is deposited within the catchment. Especially, the grassland areas along the stream network act as the most important MP sinks. Also important to note is that especially tillage erosion leads to a substantial burial of MP below the plow layer. Almost 5% of applied MP since 1950 is moved below the plough layer, while less than 1% of the MP potentially accumulated in the arable soils is transported to the stream network. However, in terms of mass, this amount is comparable to the MP delivery from wastewater treatment plants (WWTPs) within the same river system. Through the modeling, it could be concluded that in rural regions like the study area, the MP delivery into the river system caused by soil erosion can exceed the MP input from WWTPs. However, as only small proportions of MP in soils are transported, it also means that soils are long-term MP sources. Based on the scenario ‘stop MP input in 2020’, in 100 years, MP delivery to the river network will have only decreased by 14% compared to 2020. In the 'business-as-usual' scenario, on the other hand, the MP input into the stream network would increase fourfold within the next century. This study offers insights into MP erosion and transport from agricultural soils to inland waters. Lower-density MP particles are preferentially eroded and transported, with erosion rates decreasing over time due to binding with soil aggregates. Soil erosion plays a crucial role in transferring MP to water bodies, often exceeding inputs from wastewater treatment plants. Targeted actions and changes in agricultural management are needed to reduce MP inputs. Tire wear is highlighted as a significant source of MP. While arable soils act as MP sinks, soil erosion makes them long-term sources for inland water systems.…