Assessing soil redistribution of forest and cropland sites in wet tropical Africa using ^239+240Pufallout radionuclides

Due to the rapidly growing population in tropical Africa, a substantial rise in food demand is predicted in upcoming decades, which will result in higher pressure on soil resources. However, there is limited knowledge on soil redistribution dynamics following land conversion into arable land in tropical Africa that is partly caused by infrastructure limitations for long-term landscape-scale monitoring. In this study, fallout radionuclides 239+240Pu are used to assess soil redistribution along topographic gradients at two cropland sites and at three nearby pristine forest sites located in the DR Congo, Uganda and Rwanda. In the study area, a 239+240Pu baseline inventory is found that is higher than typically expected for tropical regions (mean forest inventory 41 Bq m−2). Pristine forests show no indication of soil redistribution based on 239+240Pu along topographical gradients. In contrast, soil erosion and sedimentation on cropland reached up to 37 cm (81 Mg ha−1 yr−1) and 40 cm (87 Mg ha−1 yr−1) within the last 55 years, respectively. Cropland sites show high intra-slope variability with locations showing severe soil erosion located in direct proximity to sedimentation sites. This study shows the applicability of a valuable method to assess tropical soil redistribution and provides insight into soil degradation rates and patterns in one of the most socio-economically and ecologically vulnerable regions of the world.

Modulators of soil organic carbon (SOC) stocks and dynamics in an intensively used hummocky landscape in North-East Germany

<p>The young moraine landscape of North-East Germany is highly prone to tillage-dominated soil erosion processes due to highly mechanised farming in a rolling topography. The corresponding soil redistribution pattern highly influences crop biomass production and soil organic carbon (SOC) dynamics. The aims of the study are to understand the effect of soil redistribution processes on SOC dynamics like dynamic replacement and efficient SOC burial. Therefore, an updated version of the spatially explicit soil redistribution and carbon turnover model SPEROS-C was applied for a large-scale (200 km²) simulation of lateral soil and SOC redistribution and vertical SOC turnover (spatial and vertical resolution 5 m x 5 m and 1 m soil depth, respectively). A sensitivity analysis was applied to identify the dominant modulators of SOC in the modelling approach (carbon input by roots, manure, and residues, decomposition of SOC, etc.). Uncertainties in model structure, process parameterisation, and input data are analysed with the GLUE approach (Generalized Likelihood Uncertainty Estimation). This approach is also used to estimate regional model parameters (e.g. SOC turnover rates, crop-specific root length density distribution, C input by aboveground biomass, manure, residues, etc.) to allow landscape-scale estimations of soil redistribution and accompanied C balance and hence, if this leads to a sink or source of CO<sub>2</sub>.</p>

Disentangling the effect of past and present agroforestry practices in modifying landscapes of Mediterranean mountains

<p>Soil erosion and sediment delivery to rivers are important drivers for land degradation and environmental change in mountain agroecosystems.  These factors are especially severe in areas affected by intermittent heavy rainfalls after dry periods, and human practices such as deforestation or clearcutting practices. Many Mediterranean mountain environments underwent conversion of rangelands into croplands during the previous centuries increasing the risk of erosion. After land abandonment the process was gradually reversed during the middle of the 20<sup>th</sup><sub></sub>century, allowing the recovery of natural land cover and reduced soil erosion rates. To further control the high erosion rates, several afforestation programs introduced pine forests at the headwater of most Mediterranean mountain catchments transforming the landscape by terracing and reducing the sediment connectivity. However, nowadays, forests’ different management could lead to high erosion rates and subsequent landscape modifications. To understand the possible effect of these practices together with the current agricultural management, we have combined the strength of empirical data and spatially distributed modelling. Surface soil samples from different land uses were collected in a representative catchment at the foot of Santo Domingo range at the border of the central Ebro River valley. The study catchment was mostly cultivated at the beginning of the 19<sub>th</sub> century but changed to rangeland and pine afforestation in the last 50 years. The remaining croplands are predominated by rainfed agriculture that leaves soils mostly unprotected from June to January when erosive storms occur. The main land uses are croplands, pine afforestation, scrubland and Mediterranean forest.</p><p>In this study, we propose an ensemble technique composed of <sup>137</sup>Cs derived soil redistribution rates as specific point values and as a calibration tool for the widely used WaTEM/SEDEM sediment delivery model. Thus, by the use of ground truth and modelled data we aim to i) apply the WaTEM/SEDEM model at the catchment scale and calibrate it with <sup>137</sup>Cs derived soil redistribution rates for finding an optimal set of input parameters; ii) examine the effect of clearcutting and agricultural practices on the total erosion; iii) compare the modelled results with recently calculated sediment apportionments by using the sediment fingerprinting technique.</p><p>Our findings highlight the use of spatially distributed models combined with <sup>137</sup>Cs derived rates as a powerful tool to understand the driving factors of soil erosion in the last decades and to delineate the hotspot areas that could suffer high erosion if subjected to certain management practices.</p>

Assessing soil erosion of forest and cropland sites in wet tropical Africa using ^239+240Pu fallout radionuclides

Due to the rapidly growing population in tropical Africa, a substantial rise in food demand is predicted in upcoming decades, which will result in higher pressure on soil resources. However, there is limited knowledge on soil redistribution dynamics following land conversion to arable land in tropical Africa that is partly caused by challenging local conditions for long-term landscape scale monitoring. In this study, fallout radionuclides 239+240Pu are used to assess soil redistribution along topographic gradients at two cropland sites and at three nearby pristine forest sites located in the DR Congo, Uganda and Rwanda. In the study area, a relatively high 239+240Pu baseline inventory is found (mean forest inventory 41 Bq m−2). Pristine forests show no indication for soil redistribution based on 239+240Pu along topographical gradients. In contrast, soil erosion and sedimentation on cropland reached up to 37 and 40 cm within the last 55 years, respectively. Cropland sites show high intra-slope variability with locations showing severe soil erosion located in direct proximity to sedimentation sites. This study shows the applicability of a valuable method to assess tropical soil redistribution and provides insight on soil degradation rates and patterns in one of the most vulnerable regions of the World.

Understanding the role of water and tillage erosion from ^239+240Pu tracer measurements using inverse modelling

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.