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

15 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 20 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 25 the most vulnerable regions of the World. https://doi.org/10.5194/soil-2020-95 Preprint. Discussion started: 28 December 2020 c © Author(s) 2020. CC BY 4.0 License.


Introduction
Soil erosion is considered to be the major threat to global soil resources and substantially contributes to crop yield reduction (Amundson et al., 2015;Montanarella et al., 2016;Govers et al., 2017), which challenges food security in regions facing 30 population growth beyond sustainable limits in the 21st century. In particular, the White Nile-Congo rift (NiCo) region faces a strong impact of soil erosion (Lewis and Nyamulinda, 1996;FAO and ITPS, 2015;Montanarella et al., 2016) due to steep terrain, high rainfall erosivity and low soil cover conditions throughout the cultivation period (Lewis and Nyamulinda, 1996).
The region is also predicted to undergo substantial climate change, which might further increase soil erosion (Borrelli et al., 2020). The loss of soil resources and yield decline in the NiCo region goes hand in hand with a rapid population growth 35 (population of Rwanda, Uganda and Democratic Republic of Congo 2020: 150 millions -predicted 2100: 430 millions; WPR, 2020), which drives rising food demands that are expected to triple for entire Sub-Saharan Africa between 2010 and 2050 (van Ittersum et al., 2016). The associated pressure on land resources leads to various problems that will have a dramatic ecological and social impact (food insecurity, political unrest, migration) in the NiCo region (Chamberlin et al., 2014;FAO and ITPS, 2015). Under current practices, an increasing demand in food is typically compensated through deforestation to assess new 40 non-degraded soils, which are often located in areas with steep slopes (Govers et al., 2017). This causes a loss of highly valuable forest ecosystem services (e.g. carbon storage, biodiversity, imbalance of the hydrological cycle) and the onset of soil erosion (Nyssen et al., 2004). In steep cropland sites of the NiCo region, it is frequently observed that the entire deep tropical saprolite body is removed and the bedrock is exposed at the surface, which means a quasi-permanent loss of cropland and potential reforestation areas on decadal to centennial timescales (Evans et al., 2020). A pressing need persists to predict future 45 dynamics and timescales of cropland degradation in order to understand the pace of a rising food shortage and to develop adapted agricultural management strategies. Smart intensification of existing cropland soils due to adapted agricultural practices in suitable locations and the conservation/restoration of soils prone to erosion (e.g. reforestation or grassland use) has been discussed as a key management strategy to combat degradation (Govers et al., 2017). A cornerstone to develop a smart intensification plan is detailed information on soil degradation dynamics of specific regions under specific conditions 50 (e.g. land use, topography, soil type, rainfall characteristics). Soil erosion plot experiments were carried out in tropical Africa (Lewis and Nyamulinda, 1996;Xiong et al., 2019) to understand the rates of soil loss. However, plot experiments are limited to soil erosion processes, while soil redistribution dynamics (interaction of erosion and sedimentation) remain unexplained, but are important to understand soil degradation rates on a landscape-scale. However, catchment monitoring that provides insight of internal soil redistribution dynamics necessitates a sufficiently long time series (years to decades) to integrate a 55 statistically representative variety of erosive rainfall events impacting different land cover conditions .
Particularly in regions of limited infrastructure, long-term catchment monitoring projects are challenging and typically rare.
This problem can be overcome by fallout radionuclides from nuclear weapon tests (i.e. 137 Cs, 239+240 Pu) as soil redistribution tracers (Meusburger et al., 2016;Alewell et al., 2017;Evrard et al., 2020), which have the major advantage to provide insight on spatial patterns of both soil erosion and sedimentation processes integrated over the period since 1963-1964 Treaty that caused a rapid decrease of atmospheric bomb tests; Wallbrink and Murray, 1993;Evrard et al., 2020). The most widely used fallout radioisotope in soil redistribution studies is 137 Cs (e.g. Porto and Walling, 2012;Chartin et al., 2013;Evrard et al., 2020), which has a rather short half-life of about 30 yrs. Hence, decay has already led to a pronounced reduction (73% in 2020) of the activity until today . In tropical regions, this is a critical limitation of using 137 Cs for soil redistribution analysis due to a much lower fallout compared to the mid latitudes of the northern hemisphere (Hardy et al., 65 1973;Evrard et al., 2020). Furthermore, extreme soil erosion rates in the tropics (Lewis and Nyamulinda, 1996;Angima et al., 2003;Nyesheja et al., 2019;Xiong et al., 2019) additionally depleted 137 Cs inventories. Over the past decade, the fallout radionuclides 239 Pu and 240 Pu have been discussed and tested as an alternative radioisotopic tracer to 137 Cs for soil redistribution studies. The major advantage of both isotopes is the long half-life ( 239 Pu = 24110 yrs, 240 Pu = 6561 yrs) without relevant decay.
Furthermore, 239 Pu and 240 Pu show a very limited plant uptake (Akleyev et al., 2000) and preferentially form associations with 70 soil iron oxides (Ryan et al., 1998;Lal et al., 2013), which potentially makes the isotopes very suitable tracers for tropical environments dominated by Ferralsols. Hence, the 239+240 Pu activity in tropical soils might be high enough to successfully carry out soil redistribution studies and provide an important insight on soil redistribution dynamics in tropical Africa. Few fallout radionuclide based soil redistribution studies have been carried out in the Tropics (Evrard et al., 2020) where, to our best knowledge, non was located in the wet Tropics of Africa (Af, Am climate; Kottek et al., 2006). 75 In our study we follow two major aims: (i) Testing the suitability of 239 Pu and 240 Pu as a soil redistribution tracers in the wet Tropics of Africa, and (ii) exemplarily analysing the soil redistribution dynamics following conversion from forest to cropland in the East African NiCo region.

Study sites and sampling design 80
The NiCo region is located in the headwater catchments of the White Nile (Lake Eduard) and the Congo River (Lake Kivu) that are part of the East African Rift Valley system (Fig. 1). The region faces rapid population growth creating substantial pressure on land resources and initiating forest to cropland conversion. Soil degradation by water erosion is a recognised problem in the region (Lewis and Nyamulinda, 1996;Montanarella et al., 2016) indicated via frequent soil erosion events resulting in ephemeral rills and gullys as well as permanent deep gully systems. The region is characterised by steep terrain 85 ( Fig. 1) and tropical climate with a mean annual air temperature between 16.7 and 19.3°C and an annual rainfall sum ranging between 1400 and 1900 mm. The seasonal rainfall distribution is subdivided in two rain and two corresponding dry seasons (Fick and Hijmans, 2017). The rainfall erosivity in the region is high due to frequently occurring storm events of large rainfall amounts linked to high rainfall intensities during the rain seasons (on average 20 erosive rainfall events per rain season; events exceeding 10 mm h -1 of rainfall per 30 min interval). Soils in the region are deeply weathered Ferralsols (> 6 m; WRB, 2006;90 Doetterl et al., 2021) developed from three geochemically distinct parent materials (DR Congo: mafic magmatic rocks; Uganda: felsic magmatic rocks; Rwanda: sedimentary rock of mixed geochemical composition). Soils throughout the study https://doi.org/10.5194/soil-2020-95 Preprint. Discussion started: 28 December 2020 c Author(s) 2020. CC BY 4.0 License. area are typically classified as clay loam while at the Ugandan forest and cropland study sites a lower clay and higher sand content is found .
The forest sites in the study area are primary tropical mountain forests (for detailed information see Doetterl et al., 2021). 95 Farming is documented since the 1950 th for the site in DR Congo, while conversion to cropland at the Ugandan site took place during the 1970 th (personal communication with local villagers). The cropland sites represent the typical smallholder farming found in the region, which is based on small fields with non-mechanised tillage practices. Due to the small fields (mean field size = 450 m 2 ) and an individual and dynamic field management, soil cover conditions are very patchy and can alternate between bare soil and fully grown vegetation cover in direct proximity (Fig. 1). 100 In 2018, a soil sampling campaign was carried out in three pristine forests and two cropland sites in the NiCo region in order to collect soil samples for a soil redistribution assessment based on the fallout radionuclides 239 Pu and 240 Pu. As part of this campaign, a total of 347 samples were taken. Soil sampling was carried out using a manual closed tube soil corer (VSI soil core sampler, Vienna-Scientific, Austria) with a diameter of 6.8 cm and a length of 120 cm. 105 Sampling sites in forests were located within the Kahuzi Biega (DR Congo), Kibale (Uganda) and Nyungwe Forest (Rwanda) National Parks (Fig. 1). There, the sampling scheme is aligned to a toposequence approach that covers three different landscape positions situated along a catena: the plateau, slope (up to 30°) and foot-slope. Sampling was carried out at 19 locations within each catena and covers different soil layers (L-horizon, O-horizon and mineral soil; see Tab. 1). Discrete sampling at plateau locations took place in order to understand variation of radionuclide inventories at sites. To average out the typical variability 110 of fallout radionuclide inventories, composite samples were taken at the slope and foot-slope positions. In addition to mineral soil layers, two organic layers at two different levels of decomposition (L and O horizon) were collected over an area of 20 cm x 20 cm at each sampling site. About 40% of the total forest samples were taken at plateau positions (high proportion due to non-composite sampling), while 30% of the samples were taken at slope and foot-slope locations, respectively. At foot-slope locations soils were additionally sampled from 60 cm to 120 cm soil depth to cover colluvial sites with 239+240 Pu activity in the 115 subsoil.
Sampling sites in cropland were placed along two catenae within DR Congo and in Uganda, covering 51 individual locations at each study site (Tab. 1). The majority (~50%) of sampling sites are distributed along slope positions (12-13° steepness in both cropland sites) while 25% of sites are located at foot-slope and plateau sites. To understand the depth distribution of 239+240 Pu and the variability of radionuclide inventories under stable geomorphic conditions, three depth increments of 20 cm 120 thickness were taken to a total soil depth of 60 cm at plateau sites. In DR Congo, a cropland site (converted to grassland approximately in 2005) located about 8 km apart from the study slope was sampled, while in Uganda, flat plateau sites under arable use in direct proximity to the study slope were sampled. At slope locations, a single soil increment down to 60 cm was taken. At the foot-slope locations, to cover potential sedimentation, an additional increment from 60 to 100 cm was sampled to assure full cover of the radionuclide inventory. 125

239+240 Pu measurements
An assessment of fallout radionuclides 239+240 Pu inventories was used to estimate effective soil redistribution since the 1960s along the investigated geomorphic transects. Plutonium isotopes measurements were conducted following Calitri et al. (2019) and Ketterer et al. (2004). The chemical preparation consisted of the following sequence: 1. Soil material was milled and subsequently dry-ashed for at least eight hours at 600°C to remove organic matter. An aliquot 130 of the dry-ashed material of up to 50 grams was weighed into a 250 mL polypropylene bottle. Samples were tested for excessive reaction of carbonates by addition of 5 mL of 8 M HNO3.
3. 125 mL of 8 M aqueous HNO 3 were added, with caution being exercised to add acid slowly when carbonates were present. 135 4. The sample vessels were capped and heated at 80°C overnight (~ 16 hours) with occasional mixing. 5. Following heating, the sample leach solutions were recovered by filtration with 0.45 micron cellulose nitrate membranes.
6. The plutonium was converted to the +4 oxidation state via addition of 0.5 g FeSO 4 *7H 2 O dissolved in 2 mL water, followed by 2 grams of NaNO 2 dissolved in 5 mL water. Thereafter, the solutions were heated uncapped in a 90 o C convection oven for 1.5 hours to release evolved NO 2 (g) and allow for conversion of the Pu into Pu(IV). 140 7. 300 milligrams of Pu-selective resin TEVA (EIChrom, Lisle, IL, USA) was added to the sample solution; the mixtures were agitated over a 4-hour timeframe to allow for the resin to uptake the Pu(IV).
8. The TEVA resin was collected on a 20 mL polyethylene column equipped with a glass wool plug; the pass-through solution was drained and discarded. The columns were rinsed with the following sequence: i) 50 mL of 2 M aqueous HNO 3 ; ii) 20 mL of 9 M aqueous HCl; and iii) 10 mL of 2 M aqueous HNO 3 . The rinse sequence removes matrix elements, uranium and 145 thorium.
9. Plutonium was eluted from the columns using the following sequence: i) 0.5 mL water; ii) 0.5 mL of 0.05 M aqueous ammonium oxalate; and iii) 0.5 mL water, all of which were collected together for analysis directly after elution.
In the preparations, quality control samples were included to assess the results; these consisted of blanks (35 g powdered sandstone devoid of detectable Pu). Blanks consisting of 35 g sandstone spiked with small quantities (50-100 milligrams) of 150 IAEA 384 (Fangataufu Sediment) were also prepared. Eleven measurements of IAEA 384 resulted in an average of 102 Bq kg -1 239+240 Pu, and a standard deviation of 10 Bq kg -1 ; these results compare well to the reference value of 107 Bq kg -1 239+240 Pu.
The blanks were used to determine a detection limit of 0.01 Bq kg -1 239+240 Pu.
Sample Pu fractions were measured with a Thermo X Series II quadrupole ICP-MS (Bremen, Germany) equipped with an APEX HF high-efficiency sample introduction system (ESI Scientific, Omaha, NE, USA). The APEX is equipped with a self-155 aspirating concentric fluorinated ethylene-propylene nebulizer operating at an uptake rate of ~ 0.15 mL per minute. The instrument is located at Northern Arizona University; the laboratory is licensed with the State of Arizona for handling 242 Pu spike solutions. The intensities of 235 U, 239 Pu, 240 Pu and 242 Pu were recorded using a peak-jump algorithm (10 ms dwell time, https://doi.org/10.5194/soil-2020-95 Preprint. Discussion started: 28 December 2020 c Author(s) 2020. CC BY 4.0 License. 1000 sweeps/integration, three integrations per sample). The 235 U isotope was measured as a proxy for 238 U, the latter whose intensity occasionally exceeded the linear range of the ICPMS's pulse-counting detector; this was done in order to assess the 160 potential interference of 238 U 1 H + on 239 Pu. As many samples exhibited relatively high levels of 238 U 1 H + , generating a potential "false-positive" detection of 239 Pu, it was determined to be advantageous to measure Pu using the 240 Pu isotope, which is unaffected by uranium hydride species. The measured atom ratio 240 Pu/ 242 Pu was converted into a mass of 240 Pu detected, using the known mass of 242 Pu; the 240 Pu activity was calculated, and converted into the corresponding 239+240 Pu activity, based on the known 240 Pu/ 239 Pu atom and activity ratios in stratospheric fallout Pu (Kelley et al., 1999). The method resulted in a 165 detection limit of 0.01 Bq kg -1 239+240 Pu, which equates to a soil inventory of ca. 5-8 Bq m -2 239+240 Pu for a bulk density of ~0.8 to ~1.3 Mg m -3 .

Cropland soil redistribution calculation
To derive topographic change and corresponding soil redistribution using 239+240 Pu inventories, a mass balance model (Zhang et al., 2019) was applied integrating soil erosion and sedimentation over the period 1964 to 2018. To account for the different 170 nature of soil erosion and sedimentation processes, the processes were individually implemented (R-Core-Team, 2019) as follows below.
Due to topsoil loss by soil erosion, former subsoil with negligible low 239+240 Pu activity gets increasingly incorporated into the plough layer. This exponential decay of the 239+240 Pu inventory is mainly controlled by the soil erosion magnitude and frequency and plough depth. These process are addressed by the mass balance model that simulates the 239+240 Pu inventory 175 reduction on a year by year basis over the simulation period: where is the simulation year, is the 239+240 Pu inventory at the specific simulation year in Bq m -2 , −1963 is the annually updated 239+240 Pu inventory in Bq m -2 , is the average plough depth in m, is soil erosion of simulation year in m. Based on these results, an individual logarithmic function between and for each study site was fitted that can be used to derive 180 the amount of soil loss in cm (55 yrs.) -1 .
Sedimentation is represented as a linear increase of the inventory: where is the soil redistribution rate in m (55 yrs) -1 and is the 239+240 Pu activity difference (reference vs. local activity) in m x m (55 yrs) -1 and Bq kg -1 x Bq kg -1 (55 yrs) -1 over the simulation period. 185 Finally, the 239+240 Pu activity (Bq m -2 ) was converted from topographic change (in m) into soil redistribution rates in Mg ha - where is the bulk density in kg m -3 . is measured for each mineral and O-horizon sample, while for the L layer was taken 190 from literature 80 kg m -3 (Wilcke et al., 2002).

Cropland scenario assessment using a mass balance model
The mass balance soil mixing model was used to assess different scenario assumptions and their sensitivity. First, different 239+240 Pu reference inventories were determined in two ways: (i) the mean 239+240 Pu inventory of all forest sites of a specific region (Ref for ; i.e. mean inventory of Kahuzi Biega forest for the DR Congo cropland sites and Kibale forest for the Ugandan 195 cropland sites) and (ii) the mean 239+240 Pu inventory of the cropland plateau sites of the specific region (Ref plt ). Second, the sensitivity of the ploughing and corresponding mixing depth is assessed using a 20±5 cm ploughing depth deviation. Third, to address potential interannual variability of water erosion, a scenario with five extreme years producing the same total soil erosion as a 55 years continuous soil erosion rate was compared against the results of the first scenario.  (Fig. 2). However, the contribution of the O horizons to the 239+240 Pu inventory of the soil profile is small (mean: 1.2%), because of low bulk density (approx. 0.2 Mg m -3 ) and O horizon thickness (mean 5 cm). 239+240 Pu activities of the 0 to 60 cm depth mineral soil layers fall rarely below the detection limit (7 of 55 samples), while the 239+240 Pu activities of samples within the detectable range are at least three times higher than the detection limit (Fig. 3). In contrast, almost no activity is detected in the subsoil layer from 60 to 120 cm. 210

Results
Comparing the 239+240 Pu activities at different topographic positions does not result in a consistent 239+240 Pu activity to topography relation (Fig. 2). While the foot-slopes in Rwanda show the highest 239+240 Pu activities, the opposite is the case for foot-slopes in Uganda and DR Congo. At the plateau sites in Uganda and Rwanda, a lower 239+240 Pu activity compared to the slope sites is found. The mean inventories found at the slope and foot-slope within each forest fall within the range of one standard deviation ± mean 239+240 Pu activity of the region specific plateau sites (Fig. 2). The only exception is the foot-slope in 215 the Ugandan forest that falls in range by two standard deviations of the plateau mean. At both cropland study sites, a distinctively lower 239+240 Pu activity relative to the forest sites is found. The lowest activity of 239+240 Pu is found at slope positions in DR Congo where 50% (n = 16) of sampling sites fall below the detection limit. The 220 measurable slope samples show a mean and standard deviation of 0.019±0.006 Bq kg -1 . A pronounced increase of the 239+240 Pu activities can be observed at foot-slope positions with activity also detectable in the sampled 60-100 cm subsoil layer (Fig. 2).
Hence, the 239+240 Pu activity at the DR Congo cropland site follows a topography related spatial pattern from low activities at slope to elevated activities at foot-slope positions.
In comparison to the cropland study site in DR Congo, the activities at the Ugandan cropland site are much higher (mean 225 239+240 Pu activity at slope sites DR Congo: 0.012 Bq kg -1 , Uganda: 0.046 Bq kg -1 ) and do rarely (3 of 44 samples) fall below the detection limit. Variability of 239+240 Pu activities at Ugandan site is extremely high and shows for slope positions a coefficient of variation of 76%. In contrast to DR Congo cropland, lower 239+240 Pu activities are found at Ugandan foot-slope sites compared to slope positions (Fig. 2). Furthermore, the Ugandan foot-slope positions showed almost no 239+240 Pu activity in the subsoil layer of 60-100 cm soil depth (Fig. 2). Congo show a sharp reduction of the 239+240 Pu activity in subsoil (Fig. 2), while at the Ugandan cropland sites, soil layers down to 40 cm show significant 239+240 Pu activity.

Cropland soil erosion and sedimentation
An important piece of information that is provided by the erosion module of the mass balance model is the minimum quantity 240 of soil loss that is required to cause a reduction of the 239+240 Pu inventory that falls below the detection limit after the model integration period of 55 yrs. The difference between the Ref for and the Ref plt 239+240 Pu baseline reference leads to substantial differences in modelled erosion. The model indicates that at the DR Congo cropland sites, soil loss of at least 37 cm (55 yrs.) -1 is necessary before the 239+240 Pu activity falls below detection limit using Ref for and in contrast 10 cm (55 yrs) -1 using Ref plt .
At the Ugandan cropland sites, a 239+240 Pu inventory reduction to reduce activity below detection limit is found for 43 cm soil 245 loss (55 yrs.) -1 when applying Ref for and 32 cm (55 yrs) -1 when applying Ref plt , respectively (Fig. 4).
Also, a pronounced sensitivity of the mass balance model on the tillage depth parameter is found. A deviation from an assumed 20 cm plough depth of ±5 cm causes a change of the required soil loss until the detection limit is reached of about ±24%.
Testing the concentrated scenario (only 5 extreme erosion years within 55 years simulation period), showed that detection limit is reached after 19% less total soil loss. Hence, the sensitivity of the ±5 cm plough depth exceeds the impact of the erosion 250 year frequency, even for this extreme scenario assumption in the NiCo ecosystem (approx. 20 erosive rainfall events per rain season). scenarios, sedimentation at sloping positions is much weaker in DR Congo as compared to Ugandan cropland (Fig. 5). The foot-slope sites in DR Congo show distinctively higher 239+240 Pu inventories compared to the slope sites (Fig. 5). However, the mean inventory of foot-slope positions is still lower than the Ref for 239+240 Pu inventory (28 vs. 32 Bq m -2 ), which would be interpreted as an indicator for weak soil erosion considering Ref for for soil redistribution calculation. In contrast, if Ref plt is used in the calculation, the same foot-slope positions would be interpreted as sites that received substantial sedimentation 260 exceeding 40 cm (55 yrs) -1 (Fig. 5). The foot-slope sites in DR Congo show a pronounced 239+240 Pu activity in many subsoil samples (60-100 cm), while no such subsoil 239+240 Pu activity is found at foot-slope sites of the Ugandan study site (Fig. 2).

Applicability of 239+240 Pu as soil erosion tracer in Tropical Africa
Within this study, a 239+240 Pu based soil redistribution analysis at three forest and two cropland sites in the NiCo region was carried out. It is shown that for the majority of samples the topsoil 239+240 Pu activity is high enough to be successfully measured and provide insight on soil redistribution in Tropical Africa over the past decades. To our knowledge, this is the first 239+240 Pu 270 based soil redistribution study in Tropical Africa.
The 239+240 Pu inventory findings in this study are much higher than expected based on the global fallout estimates reported by Kelley et al. (1999) and Hardy et al. (1973) (4.8 Bq m -2 and 11.1 Bq m -2 for 10°N and 10°S). For the latitudinal classification of Hardy et al. (1973) only two measurements between 10°N and 10°S were located in Africa (Muguga, Kenya & Luanda, Angola; Hardy et al., 1973;Kelley et al., 1999). Both stations receive a substantially lower annual precipitation (960 and 430 275 mm for Kenya and Angola, respectively) than the NiCo region (>1400 mm yr -1 ; Fick and Hijmans, 2017) and show contrasting 239+240 Pu inventories of 19.2 Bq m -2 in Kenya and 3.4 Bq m -2 in Angola. Hence, it is not surprising to find higher baseline 239+240 Pu inventories within the NiCo region than in Kenya or Angola. The three pristine forests show mean 239+240 Pu inventories between 33 and 48 Bq m -2 (DR Congo: 32.7±7.7 Bq m -2 ; Uganda: 42.91±15.5 Bq m -2 ; Rwanda: 48.4±18.2 Bq m -2 ), which is sufficiently high for soil redistribution studies. 280 However, half of the slope sites (14 of 28) at the cropland site in DR Congo fall below the detection limit (0.01 Bq kg -1 ; ~5 Bq m -2 ). This is partly caused by the sampling design of this study, which is based on large and deep single soil increments that cover the soil depth from 0 to 60 cm and 60 to 100 cm at the slope and foot-slope positions. A straightforward way to increase the 239+240 Pu activity in the sample is the reduction of the sample increment depth for a corresponding increase of the topsoil https://doi.org/10.5194/soil-2020-95 Preprint. Discussion started: 28 December 2020 c Author(s) 2020. CC BY 4.0 License.
proportion that has a higher 239+240 Pu activity (see 20 cm increments taken at cropland plateau sites Fig. 2). However, a 285 reduction of the sampling increments necessarily requires an additional subsoil analysis in highly degraded soil systems, particularly in regions with complex soil redistribution patterns to cover the full 239+240 Pu inventory.  (Fig. 2). The variation of forest 239+240 Pu inventories due to bioturbation and fallout infiltration patterns (e.g. caused by through fall or stem flow patterns) exceeds a potential soil redistribution impact, which is illustrated by the standard deviation of the plateau sites that covers the variability of the slope and foot-slope composite samples (Fig. 2). Additional 295 evidence that soil redistribution processes in the studied forest systems are small is that no major differences between chemical and physical soil properties are found along geomorphic gradients (Reichenbach et al., 2021). This finding is in line with global erosion plot studies from tropical forest plots (mean erosion 0.2 Mg ha -1 yr -1 , 39 plots with 116 plot years; Xiong et al., 2019).
Observations on sediment delivery monitoring in the NiCo region show that the amount of sediment delivery from pristine forests is typically less than 1 Mg ha -1 yr -1 (personal communication with Simon Baumgartner, UCL runoff monitoring 300 FORSEDCO project). Furthermore, Drake et al. (2019) exemplarily showed in the NiCo region that particular matter export within pristine tropical forest catchments are dominated by organic matter export with little to no mineral sediment being transported. In contrast, partly deforested catchments with agricultural use show substantial carbon delivery by organo-mineral complexes that indicates detachment and transport of the mineral soil layers, which is again in line with the soil erosion results of this study (Drake et al., 2019). Hence, the forest sites are assumed to represent almost the entire 239+240 Pu inventory of the 305 global fallout. The basic assumption behind the reference sites is that the full inventory is preserved as no soil redistribution has taken place and the 239+240 Pu inventories of both Ref for and Ref plt are supposed to be similar. However, the mean cropland plateau 239+240 Pu inventory in Uganda is about half (24.4±7.6 Bq m -2 ) and in DR Congo only a quarter (8.0±1.0 Bq m -2 ) of the mean inventories found in the nearby (<30 km) pristine forest sites (Fig. 3), which cannot be explained by local rainfall and corresponding fallout patterns. Subsoil below 60 cm depth at cropland foot-slope positions show 239+240 Pu activity, which is a 310 clear proxy for substantial sedimentation (Fig. 2). However, the 239+240 Pu inventory of these locations are not exceeding the forest reference inventory Ref for and would therefore be interpreted as weak soil erosion applying the mass balance model. This is unexpected and can point at a variety of different processes at play not investigated by this study. For example, the measured 239+240 Pu activity at the foot-slope positions may underestimate the 239+240 Pu inventory due to the limited sampling depths of 100 cm. However, an indication for this process would be an increasing 239+240 Pu activity in subsoil, which is not 315 reflected in the data (Fig. 2). Another potential explanation is that 239+240 Pu inventories are reduced due to plant uptake and subsequent plant harvest. However, a substantial plant uptake by crops, like observed for the fallout radionuclide 137 Cs (White and Broadley, 2000;Zhu and Smolders, 2000), is unlikely as no elevated 239+240 Pu activity in harvested crops was reported in other studies (Akleyev et al., 2000). Another potential pathway of soil and 239+240 Pu leaving the cropland plateau sites is harvest erosion associated to commonly cultivated root crops (i.e. cassava, sweet potato, groundnuts). In temperate regions, harvest 320 erosion rates up to 12 Mg ha -1 yr -1 have been reported for different crop types (potato: 2.5 -6 Mg ha -1 yr -1 ; Auerswald and Schmidt, 1986;Belotserkovsky and Larinovo, 1988;Ruysschaert et al., 2007) (sugarbeet: 5 -8 Mg ha -1 yr -1 ; Auerswald et al., 2006) (chicory: 8.1 -11.8 Mg ha -1 yr -1 ; Poesen et al., 2001). With cassava and sweet potato being the main food crops within the NiCo region (cassava has a higher proportion on the less fertile soils in the DR Congo, while more sweet potato is cultivated in Uganda), this is a likely source of reduction of 239+240 Pu inventories. To illustrate the potential effect of harvest erosion, a 325 simple example shows that 5.5 Mg ha -1 yr -1 of sediment delivery would roughly cause a 20% reduction of the baseline reference over 55 years (assuming a 20 cm plough depth and 1.35 Mg m -2 bulk density). Harvest erosion can be assumed as a process that has a limited spatial distribution as long as the land use and crop yields are not causing pronounced spatial patterns.
Therefore, in systems where harvest erosion is a relevant driver of 239+240 Pu export, Ref plt would be the valid reference for soil redistribution estimations. However, an accurate estimation on the contribution of 239+240 Pu loss due to harvest erosion since 330 the 1960s is impossible as limited information is available on soil harvesting loss of cassava and potato by hand cultivation.
Therefore, both Ref for and Ref plt are taken into account within this study to cover the range from a fully preserved to a depleted 239+240 Pu reference inventory in the study sites.

Soil redistribution in cropland of the NiCo region 335
Both cropland sites show indications of soil redistribution. Particularly the cropland study site in DR Congo shows evidence for (i) soil loss due to a high number of slope samples falling below the detection limit (50%) and (ii) sedimentation as evidenced by a clear 239+240 Pu fingerprint in the subsoil of the foot-slope samples. Compared to the Ugandan cropland, the DR Congo cropland shows a much stronger difference between 239+240 Pu inventories of slope positions and Ref for (Fig. 3). We relate this discrepancy to the varying length since DR Congo and Uganda cropland has been converted from tropical forest. 340 Forest to cropland conversion at the Ugandan study site took place during the 1970's. Hence, the area was under arable use for about 40 years compared to 55 years (since the test ban treaty) at the DR Congo study site. Therefore, the Ugandan cropland was exposed to soil erosion for a roughly 27% shorter time compared to DR Congo cropland. However, the relative 239+240 Pu inventory reduction at slope sites in Uganda is about 29% compared to Ref for , while in DR Congo the relative reduction is 83%. The much stronger relative 239+240 Pu reduction in DR Congo cannot be just explained by the shorter cropland use of the 345 Ugandan cropland site. In direct comparison between the two sites, no major difference regarding slope steepness (12°-13° in both study sites) and rainfall erosivity (Fenta et al., 2017) were observed. However, the crop rotation between the Ugandan and DR Congo study site differ substantially, with Uganda being dominated by sweet potato and maize, while DR Congo cropland is dominated by cassava that may cause different soil erosion conditions.
The determined mean soil redistribution rates at cropland slopes found in this study ( observed globally (Boardman and Poesen, 2006;Borrelli et al., 2017) the high erosion Ref for simulations are in good agreement with plot monitoring results within the region (mean soil loss of 68.2 Mg ha -1 yr -1 ; Lewis and Nyamulinda, 1996). The range of observed values at slopes spans from net sedimentation to heavy soil loss in direct proximity to each other. This high 355 variation on short spatial distances might be an effect of smallholder farming structures, which mitigate soil loss rates in the NiCo region due to decreasing hydrologic connectivity (Nunes et al., 2018;Baartman et al., 2020) along slopes due to a high degree of "patchiness" and a large number of field boundaries (mean field size 450 m²; Fig. 5). Any conversion of this smallholder farming structure into large scale farming structures, as known from regions with mechanised agriculture, will have devastating effects on soil degradation rates in the region. 360

Conclusions
This study demonstrates the usability of fallout radionuclides 239 Pu and 240 Pu as a tool to assess soil degradation processes in Tropical Africa. Interpreting 239+240 Pu activity and inventories in soils and organic layers, we assessed soil redistribution rates along three pristine forests catena and in two cropland catchments in the White Nile-Congo rift region. 239+240 Pu inventories in forest did not follow a topography related distribution, indicative for little to no soil erosion. In contrast, cropland sites show 365 signs for substantial soil erosion and sedimentation that exceeds 40 cm over a period of 55 years. However, the selection of an appropriate reference is critical due to a potential 239+240 Pu inventory reduction by harvest erosion in root crop dominated cropland systems. Very high intra-slope variability of the 239+240 Pu inventories in cropland was found (coefficient of variation up to 67%) with sites of pronounced sedimentation in close distance to highly eroded sites, potentially a result of soil cover dynamics due to smallholder farming structures with small fields and individual management. Keeping smallholder farming 370 structures active is essential to mitigate soil degradation in the region, also under current agricultural intensification efforts.
Particularly in regions with limited infrastructure and challenging monitoring conditions, 239+240 Pu based soil redistribution analysis can shed light on the pace of soil degradation, which remains a major challenge for future food security in Tropical Africa.

Data availability 375
The data will be made available within the framework of the TropSOC database publication: Doetterl, S., Asifiwe, R.K.,  Tables   Table 1: Numbers of samples taken at three forest and two cropland study sites (DR Congo, Uganda, Rwanda). Please note that data on cropland does not include study sites in Rwanda (L = L horizon; O = O horizon; M = mineral layer 1: 0-60 cm and 2: 60-120 in forest and 60-100 cm in cropland). While the O horizon depth is measured individually for each sample, the L horizon depth is assumed to be 1 cm as it was not possible to be accurately measured. 515