Tracing the temporal evolution of soil redistribution rates in an agricultural landscape using ^239+240Pu and ¹⁰Be

Abstract

Two principal groups of processes shape mass fluxes from and into a soil: vertical profile development and lateral soil redistribution. Periods having predominantly progressive soil forming processes (soil profile development) alternate with periods having predominantly regressive processes (erosion). As a result, short‐term soil redistribution – years to decades – can differ substantially from long‐term soil redistribution; i.e. centuries to millennia. However, the quantification of these processes is difficult and consequently their rates are poorly understood. To assess the competing roles of erosion and deposition we determined short‐ and long‐term soil redistribution rates in a formerly glaciated area of the Uckermark, northeast Germany. We compared short‐term erosion or accumulation rates using plutonium‐239 and ‐240 (239+240Pu) and long‐term rates using both in situ and meteoric cosmogenic beryllium‐10 (10Be). Three characteristic process domains have been analysed in detail: a flat landscape position having no erosion/deposition, an erosion‐dominated mid‐slope, and a deposition‐dominated lower‐slope site. We show that the short‐term mass erosion and accumulation rates are about one order of magnitude higher than long‐term redistribution rates. Both, in situ and meteoric 10Be provide comparable results. Depth functions, and therefore not only an average value of the topsoil, give the most meaningful rates. The long‐term soil redistribution rates were in the range of −2.1 t ha‐1 yr‐1 (erosion) and +0.26 t ha‐1 yr‐1 (accumulation) whereas the short‐term erosion rates indicated strong erosion of up to 25 t ha‐1 yr‐1 and accumulation of 7.6 t ha‐1 yr‐1. Our multi‐isotope method identifies periods of erosion and deposition, confirming the ‘time‐split approach’ of distinct different phases (progressive/regressive) in soil evolution. With such an approach, temporally‐changing processes can be disentangled, which allows the identification of both the dimensions of and the increase in soil erosion due to human influence. © 2019 John Wiley & Sons, Ltd.

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

“…2.5 Implementation and inverse modelling 2.5.1 Soil redistribution based on 239+240 Pu measurements Spatially distributed topsoil and subsoil bulk density and tillage depth information for each individual sampling location was applied to the proportional conversion approach. The reference inventory of undisturbed sites follows the value determined by Calitri et al (2019), who found a 239+240 Pu inventory of 43 ± 3 Bq m 2 based on four sites that did not show a profile morphological or geochemical indication of soil redistribution at a location 8.5 km from the study area. To address the uncertainty inherent to the reference measurements, a reference range from 40 to 46 Bq m 2 was accounted for in the simulations.…”

“…The fallout radionuclides 239+240 Pu were used to estimate effective soil redistribution since the 1960s. Plutonium isotope measurements were conducted following Calitri et al (2019) based on the procedure of Ketterer et al (2004). Before the mass spectrometry analysis, 10 g of milled fine earth was dry-ashed for at least 8 h at 600 • C to remove organic matter.…”

“…The quantification of water erosion requires a sufficiently long monitoring time (typically decades) to cover a statistically representative variation of rainfall events occurring in different land cover conditions (Fiener et al, 2019). Therefore, thousands of plot experiments, driven by either natural or artificial rainfall simulations, were carried out in different environments and different land cover conditions (Cerdan et al, 2010;Auerswald et al, 2014). Furthermore, a large number of small catchment studies were performed to quantify both erosion and depositional processes (for an overview, see Fiener et al, 2019).…”

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

“…1k yr. before present (Van der Meij et al, 2017;Kappler et al, 2018). In turn, tillage erosion is typically assumed to be a process that is linked to recent developments of increasing mechanical forces that are applied to soils over the past century (Sommer et al, 2008;Calitri et al, 2019). Within this study, a maximum topographical change by hilltop erosion up to 17 cm (53 yr) -1 was determined.…”

Rapid decrease of soil erosion rates with soil formation and vegetation development in periglacial areas

“…To date, most studies using FRN as a tracer for soil erosion have focused on agricultural soils (Schimmack et al, 2002; Calitri et al, 2019) or pastures (Tims et al, 2010; Lal et al, 2013; Alewell et al, 2013; Meusburger et al, 2018). In order to assess whether soil erosion rates are still sustainable, they need to be compared to the rates of soil production (Meusburger and Alewell, 2014; Alewell et al, 2015).…”