Reply on RC1

Abstract. Soils are generally accepted as sinks for microplastic (MP), but at the same time might be a MP source for inland waters. However, little is known regarding the potential MP delivery from soils to aquatic systems via surface runoff and erosion. This study provides for the first time an estimate of the extent of soil erosion-induced MP delivery from an arable-dominated mesoscale catchment (390 km²) to its river network within a typical arable region of Southern Germany. To do this, a soil erosion model was used and combined with potential particular MP load on arable land from different sources (sewage sludge, compost, atmospheric deposition and tyre wear) since 1950. The modelling resulted in an annual mean MP flux into the stream network of 6.33°kg° in 2020, which was dominated by tyre wear (80 %). Overall, 0.11–0.17 % of the MP applied to arable soils between 1950 and 2020 was transported into the stream network. In terms of mass, this small proportion was in the same range as the MP inputs from wastewater treatment plants within the test catchment. More MP (0.5–1 % of input between 1950 and 2020) was deposited in the grassland areas along the stream network, and this could be an additional source of MP during flood events. Most (5 % of the MP applied between 1950 and 2020) of the MP translocated by tillage and water erosion was buried under the plough layer. Thus, the main part of the MP added to arable land remained in the topsoil and is available for long-term soil erosion. This can be illustrated based on a ‘stop MP input in 2020’ scenario, indicating that MP delivery to the stream network until 2100 would only be reduced by 14 %. Overall, arable land at risk of soil erosion represents a long-term MP sink, but also a long-term MP source for inland waters.