Understanding the role of water and tillage erosion from <sup>239+240</sup>Pu tracer measurements using inverse modelling

Abstract

Abstract. Soil redistribution on arable land is a major threat for a sustainable use of soil resources. The majority of soil redistribution studies focus on water erosion, while wind and tillage erosion also induce pronounced redistribution of soil materials. Tillage erosion especially is understudied, as it does not lead to visible off-site damages. The analysis of on-site/in-field soil redistribution is mostly based on tracer studies, where radionuclide tracers (e.g. 137Cs, 239+240Pu) from nuclear weapon tests are commonly used to derive the erosion history over the past 50–60 years. Tracer studies allow us to determine soil redistribution patterns but integrate all types of soil redistribution processes and hence do not allow us to unravel the contribution of individual erosion processes. The aim of this study is to understand the contribution of water and tillage erosion leading to soil patterns found in a small hummocky ground moraine kettle hole catchment under intensive agricultural use. Therefore, 239+240Pu-derived soil redistribution patterns were analysed using an inverse modelling approach accounting for water and tillage erosion processes. The results of this analysis clearly point out that tillage erosion is the dominant process of soil redistribution in the study catchment, which also affects the hydrological and sedimentological connectivity between arable land and the kettle hole. A topographic change up to 17 cm (53 yr)−1 in the eroded parts of the catchment is not able to explain the current soil profile truncation that exceeds the 239+240Pu-derived topographic change substantially. Hence, tillage erosion already started before the onset of intense mechanisation since the 1960s. In general, the study stresses the urgent need to consider tillage erosion as a major soil degradation process that can be the dominant soil redistribution process in sloped arable landscapes.

Tillage erosion as an important driver of in‐field biomass patterns in an intensively used hummocky landscape

“…4.2 ha) in the centre of our study area using 239 + 240 Pu and an inverse modeling analysis. The results showed that soil erosion by water is an order of magnitude lower compared to tillage erosion (Wilken et al, 2020) and, thus, support our findings that tillage erosion and the corresponding patterns in soil properties and plant growth conditions are dominant in this region. It is important to note that in our study, EVI is only a proxy for crop biomass.…”

“…We can safely assume that most of the erosion-related pattern of EVI was not caused by recent erosion (e.g., due to water losses by runoff) but related to long-term soil truncation and colluviation, which modify important soil properties influencing plant growth and crop biomass (e.g., rooting depth, bulk density, water and nutrient availability, etc.). Nevertheless, the modeled erosion patterns served well as proxy variables for long-term soil truncation or colluviation even though they were based on recent data of soil use valid for the last 60 years (Wilken et al, 2020). Most changes in erosion parameters like rain erosivity, cropping sequence, or tillage intensity, which might have happened, would not change the soil redistribution pattern but only the absolute amount.…”

“…The dramatic increase of sedimentation rates in kettle holes and at footslopes, which was dated on the second half of the 20th century, was related to increasing mechanization of tillage practices (Frielinghaus & Vahrson, 1998;Keller, Sandin, Colombi, Horn, & Or, 2019;Li et al, 2002;Van der Meij et al, 2019). Wilken et al (2020) assessed soil redistribution by tillage and water in a small, representative sub-catchment (ca. 4.2 ha) in the centre of our study area using 239 + 240 Pu and an inverse modeling analysis.…”

“…The tillage transport coefficient k til depends on the tillage implement, tillage speed, tillage depths, bulk density, texture, and soil moisture at time of tillage (Van Oost, . For our study, we used a constant k til value of 350 kg m À1 yr À1 , which was recently determined for this region (Wilken, Ketterer, Koszinski, Sommer, & Fiener, 2020). As k til only determines the intensity of the calculated erosion rates, the parameterization of k til is not sensitive to the spatial pattern of tillage translocation.…”

“…The parameterization of the water erosion module follows Wilken et al (2020) with a k tc value of 150 m, R factor of 450 MJ mm ha À1 hr À1 yr À1 , K factor of 0.027 Mg ha hr ha À1 MJ À1 mm À1 , and P factor of 1.0 (i.e., no erosion control practices). The C factor was calculated for a conventional small grain tillage crop rotation that is typical for the study region (winter rapeseed-winter wheat-winter barley, cultivated without cover crops; Wilken et al, 2018).…”