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Microplastics are ubiquitous in the environment. While it is well documented that microplastics can be harmful to aquatic and terrestrial life, little is understood about how microplastics affect soil environments. Understanding the processes enhancing or constraining microplastic transport through the environment, including within soils, is essential in order to minimize the negative impacts of microplastic pollution. Soil erosion is thought to be a primary process responsible for translocating microplastics from soils to aquatic environments. This study aims to investigate the processes controlling microplastic movement in response to rainfall and overland flow in laboratory rainfall simulations. Using fluorescent photography, we compared the real- time movement of a natural soil particle, sand, with two types of microplastics of differing densities (linear low-density polyethylene and acrylic) in two size fractions (250-355 μm and 500-600 μm). We quantified the rate and number of microplastic particles transported by overland flow and splash erosion, along with the depth to which the particles migrate into the soil profile. The results from this study represent some of the first empirical data that seek to quantify microplastic surface transport processes. Ultimately, additional research in this area will be required to more accurately estimate the extent to which microplastics are exported from soils and the factors that control this export.

In modern agriculture, plastic material is intentionally used for different purposes. In 2020, it was estimated that about 7.1 105 t of plastic was used in European agriculture, whereas most plastic was applied in form of plastic mulch films to improve growing conditions, e.g., via temperature regulation or reduction of evaporation. Most of this plastic films are made from Light Density Polyethene (LDPE) with different physical (e.g., thickness between 15 and 200 µm) and chemical properties (e.g., different types and amounts of additives for UV stabilization). Plasticulture improves productivity but a growing number of studies indicate soil contamination with macro-, meso- and microplastic particles originating from plastic mulching films. The aim of this study is to compare the changes in plastic film properties and stability as basis for the fragmentation into meso- to microplastic following different environmental stressors applied to plastic films used for different purposes. A series of lab experiments was set-up to mimic natural UV radiation as well as mechanical stress. Overall, six different agricultural films were tested (two black LDPE mulch films, thickness 20 µm; two transparent LDPE films of small tunnels, thickness 180 µm; and two black-white LDPE asparagus mulch films, thickness 100 and 150 µm). In a first step, different UV light exposures time were used (Q-SUN Xe-1-SE xenon test chamber, TUV (300-400 nm)) to simulate LDPE aging as exposed to sunlight at the soil surface, in second step mechanical stress was applied during an abrasion test (20 g of standard soil were mixed with the degraded samples at 4 rpm for 61 days). Both treatments and their combinations were then analyzed regarding changes in plastic properties. Therefore, we used a 3d laser scanner confocal microscope (LSM) to analyze changes in plastic surfaces, a Fourier-transform infrared-attenuated total reflectance spectrometer (FTIR-ATR) to determine changes in the chemical compounds, an optical contact angle (OCA) to determine changes in hydrophobicity and finally tested changes in mechanical stability with a universal nanomechanical tester (UNAT). First results indicate that UV alone can affect the stability of the films, which is increased by mechanical stress. The FTIR-ATR spectra, especially from the thin black film, presented variation at the carbonyl band (1800-1600 cm-1) after the degradation test; LSM results for the degradation test didn’t show any significant change, while preliminary results from the mechanical stress present changes on its surface; for OCA preliminary results the thin black film showed a variation from 85.7o to 77.2o after UV degradation. Overall, the tests indicate the importance of a combination of UV radiation and mechanical stress for LDPE film degradation, which especially in case of the thin, black mulch film leads to a change in plastic properties paving the way for plastic fragmentation within months of environmental exposure, while the thicker foils are less affected within such timeframe. Hence, thin plastic mulching foils might improve agricultural productivity but on the cost of increasing soil plastic contamination.

The use of plastic in agriculture (plasticulture) started together with the rise of plastic in global markets, around 1950. Nowadays, plastic plays a key role in agriculture as an inexpensive, lightweight and resistant material. In European vegetable production, around 80% of plastic use is attributed to plastic films for crop covers. While meeting the needs of both producers and consumers (e.g., higher yields, early harvest, availability of out-of-season products), serious concerns have been raised about end-of-life management of plastic films. Different management practices, crop and film types are potentially controlling the amount of macro- and microplastic residues that remain in soil. While part of the scientific studies is improving our understanding of the environmental risks of plasticulture, a tool to monitor and quantify its extent at large scale (e.g., regions up to countries) is still lacking. The presence of publicly and freely available satellite data with increasing temporal and spatial resolution, coupled with increasing computing capabilities, qualify satellite remote sensing as a potential data source for large scale plasticulture monitoring. The challenge of detecting plastic covered crops by satellite data stimulated a growing number of papers. However, most of the literature focuses on local plasticulture hotspots, while a few authors recently tried to identify either greenhouses or plastic mulched farmlands (PMF) at larger scale. To the best of our knowledge, no attempts have been made for large scale plasticulture mapping in regions where different types of plastic covers are present (PMF, tunnels and greenhouses).In this research study we aim at identifying PMF, tunnels and greenhouses across Germany, one of the most active countries in the EU agricultural plastic film market. Google Earth Engine cloud computing capabilities were combined with Sentinel-1 and Sentinel-2 data coming from a whole year of acquisitions. In this context, time series analyses were supplied with a novel multi-image classification-based index (called Plastic Detection Frequency, PDF). The index was created by looping a random forest classifier over a yearly image collection to address the seasonality of temporary plastic covers (e.g., mulch foils, tunnels), which might not be caught by analysing time series with traditional methods alone. The results were evaluated in two German sub-regions (Cologne-Bonn region and Rhine-Valley south of Mainz), by combining ground truth sampling on Google Earth images and regional crop type databases. The overall accuracy assessed in the evaluation regions reaches 90% when plasticulture is distinguished between PMF, tunnels and greenhouses, while it exceeds 95% when the total area of plasticulture is mapped. Based on the feature rank, the PDF resulted to be the most important feature in the classification process. Moreover, the PDF showed higher values for crops typically associated with use of plastic films, which points at a strong relationship between the PDF and the presence of plastic covers. This study demonstrates the potential of an operational workflow for large scale plasticulture mapping and monitoring by synergising freely available optical and SAR satellite data.

Soil organic carbon (SOC) dynamics depend on soil properties derived from the geoclimatic conditions under which soils develop and are in many cases modified by land conversion. However, SOC stabilization and the responses of SOC to land use change are not well constrained in deeply weathered tropical soils, which are dominated by less reactive minerals than those in temperate regions. Along a gradient of geochemically distinct soil parent materials, we investigated differences in SOC stocks and SOC ( delta C-14) turnover time across soil profile depth between montane tropical forest and cropland situated on flat, non-erosive plateau landforms. We show that SOC stocks and soil delta C-14 patterns do not differ significantly with land use, but that differences in SOC can be explained by the physicochemical properties of soils. More specifically, labile organo-mineral associations in combination with exchangeable base cations were identified as the dominating controls over soil C stocks and turnover. We argue that due to their long weathering history, the investigated tropical soils do not provide enough reactive minerals for the stabilization of C input in either high input (tropical forest) or low-input (cropland) systems. Since these soils exceeded their maximum potential for the mineral related stabilization of SOC, potential positive effects of reforestation on tropical SOC storage are most likely limited to minor differences in topsoil without major impacts on subsoil C stocks. Hence, in deeply weathered soils, increasing C inputs may lead to the accumulation of a larger readily available SOC pool, but does not contribute to long-term SOC stabilization., Global Change Biology, 29 (9), ISSN:1354-1013, ISSN:1365-2486

Suspended sediment is an integral part of riverine transport and functioning that has been strongly altered during the Anthropocene due to the overwhelming human pressure on soils, sediments, and the water cycle. Understanding the controls of changing suspended sediment in rivers is therefore vital for effective management strategies. Here we present results from a trend analysis of suspended sediments covering 62 monitoring stations along the German waterways (catchment sizes range between 2000 and 160 000 km2) with more than 440 000 water samples taken between 1990 and 2010. Based on daily monitoring of suspended sediment concentration (SSC), we found significant declines in mean annual SSC and annual suspended sediment loads (SSLs) at 49 of 62 monitoring stations totaling −0.92 mg L−1 yr−1. At some stations decreases during the 20 years represent up to 50 % of the long-term average SSC. Significant decreases in SSC are associated with declining SSL despite an increase in sheet and rill erosion by almost 150 % derived from modeling due to changes in land use and management as well as rainfall erosivity. The contemporary suspended sediment loads of the Rhine at the German–Dutch border are approaching the natural base level of ∼1 Mt yr−1, which was reached by the Rhine during the mid-Holocene when the suspended sediment load was adjusted to the Holocene climatic conditions and before the onset of increased loads due to human-induced land use changes in the Rhine catchment. At this point we can only speculate regarding potential reasons for a decline in sediment transport in larger rivers despite erosion increase. We argue that increased sediment retention in upstream headwaters is presumably the major reason for declining SSC in the large river channels studied.

Tropical Africa is globally one of the most sensitive regions for soil erosion and is characterised by an important yield gap. Rapid population growth is expected to triple food demands in Sub-Saharan Africa by 2050. These rising food demands need to be met by cropland that is highly prone to soil erosion. In particular, the White Nile-Congo ridge region between the DR Congo and Uganda is a hotspot for issues relating to food security linked to massive soil degradation due to steep terrain, highly erosive rainfall and low soil cover conditions. Despite its importance, most soil erosion studies in the region are based on plot or large-scale modelling studies. Both approaches lack information on inter-field connectivity processes, which are especially important in smallholder farming structures with an average field size of 0.1 ha. To address this, a UAV-based monitoring campaign was carried out over different cropland sites (two sites in the DR Congo and two in Uganda) at high spatial and temporal resolution. The campaign covered more than 500 individual fields which were monitored twice per month (for two years) using UAV-based aerial photography to get insights into event-based rill erosion processes and the role of landscape connectivity. The aerial photography data was classified according to its field condition: (i) vegetation covered, (ii) bare soil without signs of rill erosion, (iii) field with rill erosion. The results highlight the role of land cover patchiness due to smallholder farming structures on inter-field connectivity with rill erosion often discontinuing downslope across field boundaries. Therefore, rill development is highly localised and affects individual fields. We further conclude that rill erosion in the White Nile-Congo ridge region is not an episodic process but takes place regularly during the rainy season as a result of high frequency erosive rainfall (on average 20 erosive rainfall events per rain season) falling on bare soil in fields that are left fallow for individual cultivation periods. Rill erosion dynamics in the study area are complex and controlled by processes that are challenging to be represented in large scale predictions on soil degradation.

Soil erosion rates on arable land frequently exceed the pace at which new soil is formed. This imbalance leads to soil thinning (i.e. truncation), whereby subsoil horizons and their underlying parent material become progressively closer to the land surface. As soil erosion is a selective process and subsurface horizons often have contrasting properties to the original topsoil, truncation-induced changes to soil properties might affect erosion rates and runoff formation through a soil erosion feedback system. However, the potential interactions between soil erosion and soil truncation are poorly understood due to a lack of empirical data and the neglection of long-term erodibility dynamics in erosion simulation models. Here, we present a novel model-based exploration of the soil erosion feedback system over a period of 500 years using measured soil properties from a diversified database of 265 agricultural soil profiles in the UK. For this, we adapted the Modified Morgan–Morgan–Finney model (MMMF) to perform a modelling experiment in which topography, climate, land cover, and crop management parameters were held constant throughout the simulation period. As selective soil erosion processes removed topsoil layers, the model gradually mixed subsurface soil horizons into a 0.2 m plough layer and updated soil properties using mass-balance mixing models. Further, we estimated the uncertainty in model simulations with a forward error assessment. We found that modelled erosion rates in 99 % of the soil profiles were sensitive to truncation-induced changes in soil properties. The soil losses in all except one of the truncation-sensitive profiles displayed a decelerating trend, which depicted an exponential decay in erosion rates over the simulation period. This was largely explained by decreasing silt contents in the soil surface due to selective removal of this more erodible particle size fraction and the presence of clayey or sandy substrata. Moreover, the soil profiles displayed an increased residual stone cover, which armoured the land surface and reduced soil detachment. Contrastingly, the soils with siltier subsurface horizons continuously replenished the plough layer with readily erodible material, which prevented the decline of soil loss rates over time. Although our results are limited by the edaphoclimatic conditions represented in our data, as by our modelling assumptions, we have demonstrated how modelled soil losses can be sensitive to erosion-induced changes in soil properties. These findings are likely to affect how we calculate soil lifespans and make long-term projections of land degradation.

Suspended sediment is an integral part of riverine transport and functioning that has been strongly altered during the Anthropocene due to the overwhelming human pressure on soils, sediments and the water cycle. Understanding the controls of changing suspended sediment in rivers is therefore vital for effective management strategies. Here we present results from a trend analysis of suspended sediments covering 62 monitoring stations along the German waterways with more than 440 000 water samples taken between 1990 and 2010. Based on daily monitoring of suspended sediment concentration (SSC), we found significant declines of mean annual SSC and annual suspended sediment loads at 49 of 62 monitoring stations between 1990 and 2010. On average SSC declines by -0.92 mg l-1yr-1. At some stations decreases during the 20 years represent up to 50 % of the long-term average SSC. Significant decreases of SSC are associated with declining SSL loads. The contemporary suspended sediment loads of the Rhine at the German-Dutch border approaches the natural base level of ~1 Mt yr-1, which was achieved by the Rhine during the mid-Holocene when the suspended sediment load was adjusted to the Holocene climatic conditions and before the onset of increased loads due to human induced land use changes in the Rhine catchment. At this point we can only speculate regarding potential reasons for a decline in sediment supply to larger rivers. We argue that changes in soil erosion within the catchments and/or the sediment connectivity in upstream headwaters, e.g. due to the construction of small rainwater retention basins, are the major reason for declining SSC in the studied river channels.

Este artículo está sujeto a una licencia CC BY 4.0, Understanding the impact of land use and land cover change on surface energy and water budgets is increasingly important in the context of climate change research. Eddy covariance (EC) methods are the gold standard for high temporal resolution measurements of water and energy fluxes, but cannot resolve spatial heterogeneity and are limited in scope to the tower footprint (few hundred meter range). Satellite remote sensing methods have excellent coverage, but lack spatial and temporal resolution. Long-range unmanned aerial systems (UAS) can complement these other methods with high spatial resolution over larger areas. Here we use UAS thermography and multispectral data as inputs to two variants of the Two Source Energy Balance Model to accurately map surface energy and water fluxes over a nutrient manipulation experiment in a managed semi-natural oak savanna from peak growing season to senescence. We use energy flux measurements from 6 EC stations to evaluate the performance of our method and achieve good accuracy (RMSD ≈ 60 W m− 2 for latent heat flux). We use the best performing latent heat estimates to produce very high-resolution evapotranspiration (ET) maps, and investigate the drivers of ET change over the transition to the senescence period. We find that nitrogen and nitrogen plus phosphorus treatments lead to significant increases in ET (P < 0.001) for both trees (4 and 6%, respectively) and grass (12 and 9%, respectively) compared to the control. These results highlight that the high sensitivity and spatial and temporal resolution of a UAS system allows the precise estimation of relative water and energy fluxes over heterogeneous vegetation cover., This research was supported by the DAAD/BMBF program Make Our Planet Great Again – German Research Initiative Project MONSOON (grant number 57429870).

The net primary productivity (NPP) of tropical forests is an important component of the global terrestrial carbon (C) cycle. The lack of field-based data, however, limits our mechanistic understanding of the drivers of NPP and C allocation. In consequence, the role of local edaphic factors for forest growth and C dynamics is unclear and introduces substantial uncertainty in estimating ecosystem C stock accrual. Here, we present data from field measurements on standing biomass as well as leaf, wood, and root production collected along topographic and geochemical gradients in old-growth African tropical mountain forests in the East African Rift System. We show that forests converge towards nutrient uptake more strongly when soil properties and parent material geochemistry indicate fertility constraints due to low amounts of rock-derived nutrients. In contrast, topography did not constrain the variability in C allocation and NPP fluxes. In consequence, aboveground:belowground biomass ratios and total NPP can differ greatly between geochemical regions for similar old-growth tropical forest types. Furthermore, soil organic carbon (SOC) stocks were not related to NPP C allocation and plant C input seemingly exceeding the maximum potential of these soils to stabilize C. We conclude that even after many millennia of weathering and the presence of deeply developed soils, tropical above and belowground C allocation, as well as soil C stocks, vary substantially due to the geochemical properties which soils inherit from parent material.

Tillage erosion is known to be a major soil degradation process that is mainly associated with increasingly mechanised agriculture since the early 1950s. However, especially soil truncation on convex hilltops and slope shoulders can be already identified on historical aerial photos of our study region in Northeast Germany from the 1950s.The aim of the study is to better understand the effect of mechanised and especially long-term non-mechanised soil redistribution processes on soil organic carbon (SOC) dynamics over the past 1000 years since the beginning of widespread soil cultivation in our study region and their contribution to the question of soil being a carbon (C) sink or source.Therefore, a modified version of the spatially explicit soil redistribution and C turnover model SPEROS-C was applied on a large-scale catchment (approx. 200 km²) to simulate lateral soil and SOC redistribution, SOC turnover and erosion-induced vertical mixing within the profile (spatial and vertical resolution 5 m x 5 m and 0.1 m soil depth increments, respectively). The uncertainty of the modelling approach was estimated by varying the input variables according to different realisations of the development of agricultural management over the past 1000 years. The results were validated with an erosion classification derived from Sentinel-2 data and UAV based estimation of topsoil SOC. The lowest SOC stocks were found on hilltops, which points at tillage erosion as the major driver of soil degradation.Our results show that the beginning influence of tillage erosion on catchment wide vertical SOC fluxes can be traced back to around 500 years ago. This clearly indicates that non-mechanised tillage erosion from the early stage of cultivation affected the SOC patterns in the study area and hence impacts todays C cycling.

There is a growing interest to understand the sources of sediments in river channels as basis for potential mitigation aiming to reduce soil erosion and sediment delivery in larger catchments. Within the last decades, sediment fingerprinting has been established as a powerful tool to unravel the sources of sediments in larger catchments. However, most sediment fingerprinting techniques are based on time-consuming and costly chemical analysis of sediment samples from river channels and sub- catchments. Recent studies have shown the potential of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) as a rapid, cost-effective, and nondestructive tracers for sediment fingerprinting. The aim of this study is to analyses the sensitivity of DRIFTS based sediment fingerprinting against particle size of sediment tracers and to determine the potential of using multi-size approaches. We used mid-infrared spectroscopy (MIRS; 4000-600 cm-1) to analyze four size fractions (125-250, 63-125, 63-38, and -1 intervals to reduce the continues wavelength data to a defined number of spectral bands (n = 104) that is practicable and realistic for a statistical analysis of differences. A one-way ANOVA was used to evaluate the presence of significant contrasts between the content of individual MIRS spectra in the different size fractions. The results showed that MIRS spectra were present and distributed across all size fractions. The results of one-way ANOVA indicated that in sub-basin both, MIRS spectra form spatial sediment sources and target sediment samples, were significantly affected by the particle size fractions. Thus, this confirmed that it was appropriate to identify the dominant particle size fraction in the sediment samples and to confirm that MIRS spectra were present across that fraction rather than a sub-fraction.

Deforestation followed by fertilizer intensive agriculture is widely recognized as a significant contributor to anthropogenic greenhouse gas emissions (GHG), particularly carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). However, empirical studies focusing on soil GHG flux dynamics from deforestation hotspots in the tropics are still limited creating major uncertainties for constraining global GHG budgets. In this study, we investigated how deforestation for fertilizer intensive sugarcane cultivation in Uganda affects soil-borne GHGs. Therefore, soil GHG fluxes were measured in a primary forest and in a completely randomized experiment premised in the neighboring sugarcane fields with different fertilizer regimes, representing both smallholder and industrial-scale sugarcane farm management. Despite the use of different fertilization rates (low, standard, and high) as treatments for the sugarcane CRD experiment, neither auxiliary controls nor soil GHG fluxes significantly differed among the CRD treatments. Soil respiration was higher in the sugarcane than in the forest, which we attribute to the increased autotrophic respiration from the sugarcane’s fine root biomass and the likely exposure of the sugarcane’s larger soil organic carbon stocks to microbial decomposition through ploughing operations. The forest soils were a stronger net sink of CH4 than the sugarcane soils despite forest soils having both higher bulk densities and larger water-filled pore space (WFPS), and we suspect that this was due to alteration of the methanotroph abundance upon the conversion. Soil N2O emissions were smaller in the sugarcane than in the forest, which was surprising, but most likely resulted from the excess N being lost either through leaching or uptake by the sugarcane crop. Only seasonal variability in WFPS, among the auxiliary controls, affected CH4 uptake at both sites and soil CO2 effluxes in the sugarcane. Noteworthy, soil N2O fluxes from both sites were unaltered by the seasonality-mediated changes in auxiliary controls. All the findings put together suggest that forest conversion for sugarcane cultivation alters soil GHG fluxes by increasing soil CO2 emissions and reducing both soil CH4 sink strength and soil N2O emissions., EGUsphere

Soil erosion rates frequently exceed the pace at which new soil is formed. This imbalance can lead to soil thinning (i.e., truncation) whereby subsoil horizons, and the underlying parent material, emerge progressively closer to the land surface. These subsurface horizons may have contrasting physical, chemical, and biological properties from those of the original topsoil. Hence, soil thinning can induce changes in topsoil erodibility – a fact that has been largely overlooked in erosion modelling research and could affect long-term projections of soil erosion rates. Here we present a model-based exploration of the potential feedbacks between water erosion and soil thinning, using measured data from 265 agricultural soil profiles in the United Kingdom. We simulated annual erosion rates on these soil profiles with the Modified Morgan-Morgan-Finey model, assuming time-constant land cover, topographic, and rainfall parameters. As the original topsoil was successively removed, our model gradually mixed the subsurface horizons into a 20 cm ploughing layer. We applied this modelling framework on a yearly time-step over a 500-year period, or until the ploughing layer reached the bottom of the lowermost soil horizon. Soil texture, stone cover, and soil organic carbon content for the ploughing layer were recalculated for each time-step through a mass-balance model. Soil bulk density and soil moisture content at field capacity were estimated for each time-step by pedo-transfer functions developed from our own dataset. In addition, we employed a Monte Carlo simulation with 100 iterations per year to provide a forward error assessment of the modelled soil losses. We found that simulated erosion rates on 42 % of the soil profiles were sensitive to truncation-induced changes in soil properties during the analysed period. Among the profiles sensitive to soil thinning, 68 % displayed a negative trend in modelled erosion rates. This was largely explained by decreasing silt contents on the surface soil due to selective removal of this more erodible particle size fraction and the presence of clayey or sandy substrata. Moreover, an increased residual stone cover shielded the surface soils from detachment by raindrop impact and surface runoff. The soil profiles with a positive trend in erosion rates were characterised by the presence of siltier subsoil horizons, which increased topsoil erodibility as they were mixed into the ploughing layer. Overall, our results demonstrated how modelled erosion rates could be sensitive to truncation-induced changes in soil properties, which in turn may accelerate or slow down soil thinning. These feedbacks are likely to affect how we calculate soil lifespans and make long-term projections of land degradation.

To gain a better understanding of the global application of soil erosion prediction models, we comprehensively reviewed relevant peer-reviewed research literature on soil-erosion modelling 1994-2017. Our aim was to identify (i) processes and models most frequently addressed in the literature, (ii) regions within which models are primarily applied, (iii) what regions remain unaddressed and why, and (iv) how frequently studies are conducted to validate/evaluate model outcomes relative to measured data. To perform this task, we merged the knowledge of a group of 66 soil-erosion scientists from 67 research institutions and 25 countries. The resulting database ‘Global Applications of Soil Erosion Modelling Tracker (GASEMT)’ includes 3,030 individual modelling records from 126 counties encompassing all continents (except Antarctica). Out of 8,471 articles identified as potentially relevant, we reviewed 1,697 articles and transferred relevant information from each into the database. For each record reported in the GASEMT database, 42 attributes were evaluated. The GASEMT database provides insights into the state-of-the-art of soil- erosion models and model applications worldwide. The database is also intended to support the upcoming country-based United Nations global soil-erosion assessment. This database may help inform soil erosion research priorities in that it builds a foundation for future targeted in-depth analyses. GASEMT is an open-source database that anyone can use to develop research, rectify errors, and expand.

Soils used for crop production cover 15.5 million km2 and almost all have been tilled at some point in their history. However, how the changes in soil depth and soil properties associated with tillage affect crop yields is unclear. Here, we show that tillage on slopes thins soils and reduces wheat and maize yields. At the landscape scale, tillage erosion gradually reduces crop yields as the duration and intensity of tillage increase. Over the next fifty to 100 years, the overall yields are likely to further decline as modern mechanized agriculture accelerates the process of tillage erosion compared to centuries of non-mechanized tillage. Arresting this downward trend will require more widespread adoption of no-tillage practices and avoidance of downslope cultivation. The downward pressure on landscape-scale yields due to tillage erosion is expected to be amplified by climate change-induced increases in dry spells during crop growth.

The lack of field-based data in the tropics limits our mechanistic understanding of the drivers of net primary productivity (NPP) and allocation. Specifically, the role of local edaphic factors - such as soil parent material and topography controlling soil fertility as well as water and nutrient fluxes - remains unclear and introduces substantial uncertainty in understanding net ecosystem productivity and carbon (C) stocks. Using a combination of vegetation growth monitoring and soil geochemical properties, we found that soil fertility parameters reflecting the local parent material are the main drivers of NPP and C allocation patterns in tropical montane forests, resulting in significant differences in below- to aboveground biomass components across geochemical (soil) regions. Topography did not constrain the variability in C allocation and NPP. Soil organic C stocks showed no relation to C input in tropical forests. Instead, plant C input seemingly exceeded the maximum potential of these soils to stabilize C. We conclude that, even after many millennia of weathering and the presence of deeply developed soils, above- and belowground C allocation in tropical forests, as well as soil C stocks, vary substantially due to the geochemical properties that soils inherit from parent material., New Phytologist, 236 (5), ISSN:0028-646X, ISSN:1469-8137

The accelerated sediment supply from agricultural soils to riverine and lacustrine environments leads to negative off-site consequences. In particular, the sediment connectivity from agricultural land to surface waters is strongly affected by landscape patchiness and the linear structures that separate field parcels (e.g. roads, tracks, hedges, and grass buffer strips). Understanding the interactions between these structures and sediment transfer is therefore crucial for minimising off-site erosion impacts. Although soil erosion models can be used to understand lateral sediment transport patterns, model-based connectivity assessments are hindered by the uncertainty in model structures and input data. Specifically, the representation of linear landscape features in numerical soil redistribution models is often compromised by the spatial resolution of the input data and the quality of the process descriptions. Here we adapted the Water and Tillage Erosion Model and Sediment Delivery Model (WaTEM/SEDEM) using high-resolution spatial data (2 m × 2 m) to analyse the sediment connectivity in a very patchy mesoscale catchment (73 km2) of the Swiss Plateau. We used a global sensitivity analysis to explore model structural assumptions about how linear landscape features (dis)connect the sediment cascade, which allowed us to investigate the uncertainty in the model structure. Furthermore, we compared model simulations of hillslope sediment yields from five subcatchments to tributary sediment loads, which were calculated with long-term water discharge and suspended sediment measurements. The sensitivity analysis revealed that the assumptions about how the road network (dis)connects the sediment transfer from field blocks to water courses had a much higher impact on modelled sediment yields than the uncertainty in model parameters. Moreover, model simulations showed a higher agreement with tributary sediment loads when the road network was assumed to directly connect sediments from hillslopes to water courses. Our results ultimately illustrate how a high-density road network combined with an effective drainage system increases sediment connectivity from hillslopes to surface waters in agricultural landscapes. This further highlights the importance of considering linear landscape features and model structural uncertainty in soil erosion and sediment connectivity research.

The accelerated sediment supply from agricultural soils to riverine and lacustrine environments leads to negative off-site consequences. In particular, the sediment connectivity from agricultural land to surface waters is strongly affected by landscape patchiness and the linear structures that separate field parcels (e.g. roads, tracks, hedges, and grass-buffer-strips). Understanding the feedbacks between these structures and sediment transfer is therefore crucial for minimising off-site erosion impacts. Although soil erosion models can be used to understand lateral sediment transport patterns, model-based connectivity assessments are hindered by the uncertainty in model structures and input data. In particular, the representation of linear landscape features in numerical soil redistribution models is often compromised by the spatial resolution of the input data and the quality of the process descriptions. Here we adapted the WaTEM/SEDEM model using high resolution spatial data (2 m × 2 m) to analyse the sediment connectivity in a very patchy mesoscale catchment (73 km2) of the Swiss Plateau. Specifically, we used a global sensitivity analysis to explore model structural assumptions about how linear landscape features (dis)connect the sediment cascade. Furthermore, we compared model simulations of hillslope sediment yields from five sub-catchments to tributary sediment loads, which were calculated with long-term water discharge and suspended sediment measurements. Our results showed that roads were the main regulators of sediment connectivity in the catchment. In particular, the sensitivity analysis revealed that the assumptions about how the road network (dis)connects the sediment transfer from field-blocks to water courses had a much higher impact on modelled sediment yields than the uncertainty in model parameters. Moreover, model simulations showed a higher agreement with tributary sediment loads when the road network was assumed to directly connect sediments from hillslopes to water courses. Our results ultimately illustrate how a high-density road network combined with an effective drainage system increase sediment connectivity from hillslopes to surface waters in this representative catchment of the Swiss Plateau. This further highlights the importance of considering linear structures in soil erosion and sediment connectivity models.

Tillage erosion causes substantial soil redistribution that can exceed water erosion especially in hummocky landscapes under highly mechanized large field agriculture. Consequently, truncated soil profiles can be found on hill shoulders and top slopes, whereas colluvial material is accumulated at footslopes, in depressions, and along downslope field borders. We tested the hypothesis that soil erosion substantially affects in-field patterns of the enhanced vegetation index (EVI) of different crop types on landscape scale. The interrelation between the EVI (RAPIDEYE satellite data; 5 m spatial resolution) as a proxy for crop biomass and modeled total soil erosion (tillage and water erosion modeled using SPEROS-C) was analyzed for the Quillow catchment (size: 196 km2) in Northeast Germany in a wet versus normal year for four crop types (winter wheat, maize, winter rapeseed, winter barley). Our findings clearly indicate that eroded areas had the lowest EVI values, while the highest EVI values were found in depositional areas. The differences in the EVI between erosional and depositional sites are more pronounced in the analyzed normal year. The net effect of total erosion on the EVI compared to areas without pronounced erosion or deposition ranged from −10.2% for maize in the normal year to +3.7% for winter barley in the wet year. Tillage erosion has been identified as an important driver of soil degradation affecting in-field crop biomass patterns in a hummocky ground moraine landscape. While soil erosion estimates are to be made, more attention should be given toward tillage erosion., Land Degradation & Development, 32 (10), ISSN:1085-3278, ISSN:1099-145X

Changing soil use from cropland to grassland influences organic carbon storage in a highly complex way. This includes the root/shoot allocation, the root depth distribution, the incorporation of shoot biomass and lateral organic carbon fluxes, by erosion and removal of harvested carbon, and finally the aeration by tillage. An experiment was designed allowing resampling a number of soils 18 yr after conversion to grassland (either pasture or meadow or set-aside) only 20 cm apart from the original sampling to exclude site variation. Before conversion to grassland the cropland was prone to erosion, with a mean lateral carbon flux during 20 yr prior to conversion of 13 t ha−1. Harvest had removed another 29 t ha−1 of carbon at eroding sites. Colluvial carbon inputs had been up to 18 t ha−1 while harvest had removed 38 t ha−1 at colluvial sites. The carbon fluxes by erosion were negligible during the 18 yr period after conversion. After conversion the carbon losses by harvest also ceased at set-aside grassland and pastures while the net losses on meadows were 45 t ha−1. Conversion to grassland significantly changed depth functions of carbon, stones, bulk density and porosity. Despite the large changes in carbon fluxes, carbon stocks did only change significantly within 18 yr under poorly drained, gleyic soils. Well-aerated soils did not show a significant increase in SOC stocks. This was even true for heavily eroded soils, where conversion from cropland to grassland (without erosion) should foster dynamic replacement of SOC. The widespread drainage of wet grassland soils prior to conversion to cropland thus can cause a large release of carbon, while an influence of tillage by either increasing aeration or erosion could not be detected in this study. Therefore, fostering carbon sequestration by conversion of cropland to grassland requires restoring former draining conditions.

Active floodplains can putatively store large amounts of organic carbon (SOC) in subsoils originating from catchment erosion processes with subsequent floodplain deposition. Our study focussed on the assessment of SOC pools associated with alluvial floodplain soils that are affected by human-induced changes in floodplain deposition and in situ SOC mineralisation due to land use change and drainage. We evaluated depth-dependent SOC contents based on 23 soil cores down to 3 m and 10 drillings down to 7 m in a floodplain area of the lower Cosumnes River. An estimate of 266 Mg C ha-1 or about 59% of the entire SOC stored within the 7 m profiles was found in the upper 2 m. Most profiles (n = 25) contained discrete buried A horizons at depths of approximately 0.8 m. These profiles had up to 130% higher SOC stocks. The mean δ13C of all deep soil profiles clearly indicated that arable land use has already altered the stable isotopic signature in the first meter of the profile. Radiocarbon dating showed that the 14C age in the buried horizon was younger than in overlaying soils indicating a substantial sedimentation phase for the overlaying soils. An additional analysis of total mercury contents in the soil profiles indicated that this sedimentation was associated with upstream hydraulic gold mining after the 1850s. In summary, deep alluvial soils in floodplains store large amounts of SOC not yet accounted for in global carbon models. Historic data give evidence that large amounts of sediment were transported into the floodplains of most rivers of the Central Valley and deposited over organically rich topsoil, which promoted the stabilization of SOC, and needs to be considered to improve our understanding of the human-induced interference with C cycling.

Generating high resolution spatial information on the movement of sediment in response to soil erosion remains a major research challenge. In this paper we present a new tracing method that utilises LED (light emitting diode) light to induce fluorescence in a sand-sized tracer, which is then detected using a complementary metal oxide semiconductor (CMOS) sensor in a commercial digital camera, at mm-resolution without the need for removal of soil material. First, we detail two complementary, but independent, methods for quantifying the concentration of tracer from images: particle counting and an intensity based method. We show that both methods can produce highly resolved estimates of particle concentrations under laboratory conditions. Secondly, we demonstrate the power of the method for collecting spatial information on soil redistribution by tillage, with mm precision, over an approximately 50 m hillslope and vertically down the soil profile. Our work demonstrates the potential to collect quantitative time-resolved data about soil movement without disturbing the soil surface which is being studied, and with it the possibility to parameterise or evaluate dynamic distributed soil erosion models or to undertake fundamental research focussed on particle movement that has been impossible to conduct previously.

The exchange of the climate-relevant greenhouse gases (GHGs) at the soil-atmospheric interface is regulated by both abiotic and biotic controls. However, evidence on nutrient limitations of soil GHG fluxes from African tropical forest ecosystems is still rare. Therefore, an ecosystem-scale nutrient manipulation experiment (NME) consisting of nitrogen (N), phosphorus (P), N + P, and control treatments was set up in a tropical forest in northwestern Uganda. Soil carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes were measured monthly using static vented chambers for 14 months. A root trenching treatment was also done in all the experimental plots in order to disentangle the contribution of root and microbial respiration to total soil CO2 effluxes. In parallel to soil GHG flux measurements, soil temperature, soil moisture, and mineral N were determined. Lifting the N limitation on the soil nitrifiers and denitrifiers through N fertilization significantly increased N2O fluxes in N, and N + P addition plots in the transitory phase (0-28 days after N fertilization, p < 0.01). However, sustained N fertilization did not significantly affect background (measured more than 28 days after fertilization) N2O fluxes. Alleviation of the P limitation on soil methanotrophs through P fertilization marginally and significantly increased CH4 consumption in the transitory (p = 0.052) and background (p = 0.010) phases, respectively. Simultaneous addition of N and P (N + P) significantly affected transitory soil CO2 effluxes (p = 0.010), suggesting a possible co-limitation of N and P on soil respiration. Microbial CO2 effluxes were significantly larger than root CO2 effluxes (p < 0.001) across all treatment plots so was the contribution of microbial respiration to the total soil CO2 effluxes (about 70 %, p < 0.001). Despite the fact that soil respiration was affected through N + P fertilization, neither heterotrophic nor autotrophic respiration significantly differed in either the N + P or the other treatments. Overall, the study findings suggest that the contribution of tropical forests to the global soil GHG budget could be altered by changes in N and P availability in these biomes., EGUsphere

Irrigation is vital for humans, critical in the management of land and water resources, and functions as an important environmental forcing. While irrigation systems are largely framed by environmental conditions, actual irrigation activities are determined by human decision-making. The types of irrigation systems can vary in technology and management and in turn influence water resources and the hydrological cycle differently. To date we lack an understanding of the drivers of establishing different irrigation systems in the Sudanian Savanna ecoregion of West Africa, and how land use decisions are made at the farm level. Insights on decision-making in water management contribute to the understanding of anthropogenic impacts on human-water systems and can help to identify strategies to adapt to ongoing and future risks. This paper aims to identify the heterogeneity of irrigation systems in our study region, and their drivers, as well as provide a qualitative assessment of their sustainability. Here we systematically analyze literature published between 1980 and 2020 which provide evidence of irrigation activities in the study area, we selected 188 publications documenting more than 100 cases of irrigation. For each study we identify each type of irrigation system, as defined by the water source, lifting and distribution type. We then clustered each case into ‘driver types’ based on external conditions, required resources, irrigation season, proximate causes and utilization. To assess the sustainability we identified environmental, governance, social and economic variables which we then coded on a 5-point Likert-scale. Our results show that irrigation systems in the West African Sudan Savanna are highly diversified in terms of techniques and organization, but that drivers are relatively similar. The decision making and management within an irrigation system is strongly influenced by its organizational structure. The type of technology depends on the available capital, and again impacts the efficiency, longevity and environmental impact of irrigation systems. We observed that economic factors, the access to water sources and markets, as well as restricted female technology adoption constrain the potential of existing irrigation schemes. The main causes to irrigate are to provide food security and raise more income in order to adapt to climate change and to increasing demands due to population growth. We detected that the cases are largely located in or in the vicinity of formal irrigation schemes. There is an increase in irrigation schemes in the study region over time, however, we found little information on why irrigation is not more widely adapted compared to similar ecoregions such as southern India, despite the growing demand. In respect of India, irrigation in West Africa is relatively new. We conclude that further research on organizational structures, and financial support can contribute to a sustainable expansion of irrigation in the West African Sudan Savanna.

Tropical forest soils are a vital component of the global carbon (C) cycle and their response to environmental change will determine future atmospheric carbon dioxides (CO2). For example, increasing biomass productivity in tropical forests suggests a potential sink for C. However, its storage and stability are driven by factors acting from small to large scale. For tropical Africa, these factors are not well known and documented. Predicting tropical soil C dynamics ultimately depends on our understanding and the ability to determine the primary environmental controls on soil organic carbon content and respiration.Here, using samples collected along strong geochemical gradients in the East African Rift Valley, we demonstrate how soil chemistry and soil fertility, derived from the geochemical composition of soil parent material, can drive soil respiration even in deeply weathered soils. To address the drivers of soil respiration, we incubated soils from three regions with contrasting geochemistry (mafic, felsic, and mixed sedimentary). For three soil depths, we measured the potential maximum heterotrophic respiration as well as the radiocarbon isotopic signature (Δ14C) of the bulk soil and respired CO2 under stable environmental conditions. We found that soil microbial communities were able to mineralize C from fossil as well as other poor quality C sources under laboratory conditions representative of tropical topsoils. Despite similarities in terms of climate, vegetation, and the size of soil C stocks, soil respiration showed distinct patterns with soil depth and parent material geochemistry. Our study shows that soil fertility conditions are the main determinant of C stability in tropical forest soils. Further, in the presence of organic carbon sources of poor quality or the presence of strong mineral-related C stabilization, microorganisms tend to discriminate against these sources in favor of more accessible forms of soil organic matter as energy sources, resulting in a slower rate of C cycling. Our results demonstrate that even in deeply weathered tropical soils, parent material has a long-lasting effect on soil chemistry that can influence and control microbial activity, the size of subsoil C stocks, and the turnover of C in soil. Soil parent material and its lasting control on soil chemistry need to be taken into account to understand and predict C stabilization and rates of C cycling in tropical forest soils.

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 CO2.

Agricultural soils play a key role as sink of microplastic (MP) coming from different sources, especially via the application of sewage sludge, compost, plastic mulch films, and tire ware. However, the effectiveness of this sink might be substantially reduced in areas subjected to water erosion. The aim of this study is to determine the transport behavior of MP during water erosion events on arable land. More specifically it is analyzed if MP is preferentially transported or behaves more conservative as attached to soil minerals and/or encapsulated in soil aggregates. A series of rainfall simulations were performed over 1.5 years on two plots at two test sites representing different intensively used soils (silty loam and loamy sand) in Southern Germany. The plots (4.5 m x 1.6 m) were spiked with microplastic (high density polyethylene) consisting of two different size fractions, fine MP (MPf, 53-100 μm) and coarse MP (MPc, 250-300 μm) incorporated into the topsoil (< 10 cm). The results clearly underline the selective nature of MP erosion leading to an enrichment ratio of MP in the eroded sediments of the loamy sand plot of 3.82 to 7.86, compared to an enrichment ratio from the silty loam plots of 1.41 to 5.29. Interestingly, there was no significant difference in enrichment ratios between MPf and MPc. Over time, an increasing connection between MP and soil particles could be observed. During the first rainfall simulation only 12% (MPc) and 34-49% (MPf) of the eroded plastic particles were connected to mineral particles or soil aggregates, while during the last simulation 1.5 years later about 31-47% (MPc) and 57-67% (MPf) of the eroded particles were bond to the soil matrix. Overall, our results indicate a strong dependency of the erosion transport behavior of MP depending on soil characteristics and time since application, while surprisingly we found little effect of MP size.

Sediment connectivity is highly influenced by landscape patchiness. In particular, linear features such as roads, ditches, and terraces, modify landscape patterns and affect sediment transport from hillslopes to surface waters. Connectivity patterns are commonly assessed by spatially-distributed models, which rely on semi-qualitative indices or numerical simulations of soil erosion and sediment transport. However, model-based connectivity assessments are hindered by the uncertainty in model structure and parameter estimation. Moreover, representing linear landscape features is often limited by the spatial resolution of the model input data. Here we demonstrate how a global sensitivity analysis of the WaTEM/SEDEM model can be used to improve our understanding of sediment connectivity in patchy agricultural catchments of the Swiss Plateau. Specifically, we explored model structural connectivity assumptions regarding road drainage and the presence of edge-of-field buffer strips, as well as the uncertainty in the input data, by means of a Monte Carlo simulation and a high resolution 2 m x 2 m DEM. Our results showed that roads are the main regulators of sediment connectivity in ameliorated Swiss landscapes. That is, our sensitivity analysis revealed that assumptions about how the road network (dis)connects sediment transport from cropland to water courses had a much higher impact on modelled sediment loads than the uncertainty in model parameters. These results illustrate how a high-density road network combined with an effective drainage system increases sediment connectivity from arable land to surface waters in Switzerland. Additionally, our approach underlines the usefulness of sensitivity and uncertainty analysis for identifying relevant processes in model-based sediment connectivity assessments.

With the development of low-cost, lightweight, integrated thermal infrared-multispectral cameras, unmanned aerial systems (UAS) have recently become a flexible complement to eddy covariance (EC) station methods for mapping surface energy fluxes of vegetated areas. These sensors facilitate the measurement of several site characteristics in one flight (e.g., radiometric temperature, vegetation indices, vegetation structure), which can be used alongside in-situ meteorology data to provide spatially-distributed estimates of energy fluxes at very high resolution. Here we test one such system (MicaSense Altum) integrated into an off-the-shelf long-range vertical take-off and landing (VTOL) unmanned aerial vehicle, and apply and evaluate our method by comparing flux estimates with EC-derived data, with specific and novel focus on heterogeneous vegetation communities at three different sites in Germany. Firstly, we present an empirical method for calibrating airborne radiometric temperature in standard units (K) using the Altum multispectral and thermal infrared instrument. Then we provide detailed methods using the two-source energy balance model (TSEB) for mapping net radiation (Rn), sensible (H), latent (LE) and ground (G) heat fluxes at

Heterotrophic soil respiration is an important component of the global terrestrial carbon (C) cycle, driven by environmental factors acting from local to continental scales. For tropical Africa, these factors and their interactions remain largely unknown. Here, using samples collected along strong topographic and geochemical gradients in the East African Rift Valley, we study how soil chemistry and soil fertility, derived from the geochemical composition of soil parent material, can drive soil respiration even after many millennia of weathering and soil development. To address the drivers of soil respiration, we incubated soils from three regions with contrasting geochemistry (mafic, felsic, and mixed sedimentary) sampled along slope gradients. For three soil depths, we measured the potential maximum heterotrophic respiration under stable environmental conditions as well as the radiocarbon content (Δ14C) of the bulk soil and respired CO2. We found that soil microbial communities were able to mineralize C from fossil as well as other poor quality C sources under laboratory conditions representative of tropical topsoils. Furthermore, despite similarities in terms of climate, vegetation, and the size of soil C stocks, soil respiration showed distinct patterns with soil depth and parent material geochemistry. The topographic origin of our samples was not a main determinant of the observed respiration rates and Δ14C. In situ, however, soil hydrological conditions likely influence soil C stability by inhibiting decomposition in valley subsoils. Our study shows that soil fertility conditions are the main determinant of C stability in tropical forest soils. Further, in the presence of organic carbon sources of poor quality or the presence of strong mineral related C stabilization, microorganisms tend to discriminate against these sources in favor of more accessible forms of soil organic matter as energy sources, resulting in a slower rate of C cycling. Our results demonstrate that even in deeply weathered tropical soils, parent material has a long-lasting effect on soil chemistry that can influence and control microbial activity, the size of subsoil C stocks, and the turnover of C in soil. Soil parent material and its lasting control on soil chemistry need to be taken into account to understand and predict C stabilization and rates of C cycling in tropical forest soils., Soil Discussions

16 Pags.- 12 Figs.- 8 Tabls., Soil erosion can present a major threat to agriculture due to loss of soil, nutrients, and organic carbon. Therefore, soil erosion modelling is one of the steps used to plan suitable soil protection measures and detect erosion hotspots. A bibliometric analysis of this topic can reveal research patterns and soil erosion modelling characteristics that can help identify steps needed to enhance the research conducted in this field. Therefore, a detailed bibliometric analysis, including investigation of collaboration networks and citation patterns, should be conducted. The updated version of the Global Applications of Soil Erosion Modelling Tracker (GASEMT) database contains information about citation characteristics and publication type. Here, we investigated the impact of the number of authors, the publication type and the selected journal on the number of citations. Generalized boosted regression tree (BRT) modelling was used to evaluate the most relevant variables related to soil erosion modelling. Additionally, bibliometric networks were analysed and visualized. This study revealed that the selection of the soil erosion model has the largest impact on the number of publication citations, followed by the modelling scale and the publication's CiteScore. Some of the other GASEMT database attributes such as model calibration and validation have negligible influence on the number of citations according to the BRT model. Although it is true that studies that conduct calibration, on average, received around 30% more citations, than studies where calibration was not performed. Moreover, the bibliographic coupling and citation networks show a clear continental pattern, although the co-authorship network does not show the same characteristics. Therefore, soil erosion modellers should conduct even more comprehensive review of past studies and focus not just on the research conducted in the same country or continent. Moreover, when evaluating soil erosion models, an additional focus should be given to field measurements, model calibration, performance assessment and uncertainty of modelling results. The results of this study indicate that these GASEMT database attributes had smaller impact on the number of citations, according to the BRT model, than anticipated, which could suggest that these attributes should be given additional attention by the soil erosion modelling community. This study provides a kind of bibliographic benchmark for soil erosion modelling research papers as modellers can estimate the influence of their paper., Nejc Bezak and Matjaž Mikoš would like to acknowledge the support of the Slovenian Research Agency through grant P2-0180. Diana Vieira is funded by national funds (OE), through FCT – Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen - DL57/2016 (CDL-CTTRI-97-ARH/2018 - REF.191-97-ARH/2018), and acknowledges CESAM financial support of through (UIDP/50017/2020+UIDB/50017/2020). Jae E. Yang and Pasquale Borrelli are funded by the EcoSSSoil Project, Korea Environmental Industry & Technology Institute (KEITI), Korea (Grant No. 2019002820004). Walter Chen is funded by the Ministry of Science and Technology (Taiwan) Research Project (Grant Number MOST 109-2121-M-027-001).

Stabilization of organic carbon in soils (SOC) depends on several soil properties, including the soil weathering stage and the mineralogy of parent material. As such, tropical SOC stabilization mechanisms likely differ from those in temperate soils due to contrasting soil development. To better understand these mechanisms, we investigated SOC dynamics at three soil depths under pristine tropical african mountain forest along a geochemical gradient from mafic to felsic and a topographic gradient covering plateau, slope and valley positions. To do so we conducted a series of soil C fractionation experiments in combination with an analysis of the geochemical composition of soil and a sequential extraction of pedogenic oxides. Relationships between our target and predicting variables were investigated using a combination of regression analyses and dimension reduction. Here, we show that reactive secondary mineral phases drive SOC properties and stabilization mechanisms together with, and sometimes more strongly than, other mechanisms such as aggregation or C stabilization by clay content. Key mineral stabilization mechanisms for SOC were strongly related to soil geochemistry, differing across the study regions. These findings were independent of topography in the absence of detectable erosion processes. Instead, fluvial dynamics and changed hydrological conditions had a secondary control on SOC dynamics in valley positions, leading to higher SOC stocks there than at the non-valley positions. We also detected fossil organic carbon (FOC) at several sites, constituting up to 52.0 ± 13.2 % of total SOC stock in the C depleted subsoil. Interestingly, total SOC stocks for these soils did not exceed those of sites without FOC. Additionally, FOC decreased strongly towards more shallow soil depths, indicating decomposability of FOC by microbial communities under more fertile conditions. Regression analysis showed that variables affiliated with soil weathering, parent material geochemistry and soil fertility, together with soil depth, explained up to 75 % of the variability of SOC stocks and Δ14C. Furthermore the same variables explain 44 % of the variability in the relative abundance of C associated with microaggregates versus free silt and clay associated C fractions However, geochemical variables gained or retained importance for explaining SOC target variables when controlling for soil depth. We conclude that despite long-lasting weathering, geochemical properties of soil parent material leave a footprint in tropical soils that affects SOC stocks and mineral related C stabilization mechanisms. While identified stabilization mechanisms and controls are similar to less weathered soils in other climate zones, their relative importance is markedly different in the investigated tropical soils., Soil Discussions

40 Pags.- 10 Figs.- 2 Tabls.- Suppl. Informat. The definitive version is available at: https://www.sciencedirect.com/science/journal/00489697, To gain a better understanding of the global application of soil erosion prediction models, we comprehensively reviewed relevant peer-reviewed research literature on soil-erosion modelling published between 1994 and 2017. We aimed to identify (i) the processes and models most frequently addressed in the literature, (ii) the regions within which models are primarily applied, (iii) the regions which remain unaddressed and why, and (iv) how frequently studies are conducted to validate/evaluate model outcomes relative to measured data. To perform this task, we combined the collective knowledge of 67 soil-erosion scientists from 25 countries. The resulting database, named ‘Global Applications of Soil Erosion Modelling Tracker (GASEMT)’, includes 3030 individual modelling records from 126 countries, encompassing all continents (except Antarctica). Out of the 8471 articles identified as potentially relevant, we reviewed 1697 appropriate articles and systematically evaluated and transferred 42 relevant attributes into the database. This GASEMT database provides comprehensive insights into the state-of-the-art of soil- erosion models and model applications worldwide. This database intends to support the upcoming country-based United Nations global soil-erosion assessment in addition to helping to inform soil erosion research priorities by building a foundation for future targeted, in-depth analyses. GASEMT is an open-source database available to the entire user-community to develop research, rectify errors, and make future expansions., Jae E. Yang and Pasquale Borrelli are funded by the EcoSSSoil Project, Korea Environmental Industry & Technology Institute (KEITI), Korea (Grant No. 2019002820004). Diana Vieira is funded by national funds (OE), through FCT – Fundação para a Ciência e a Tecnologia, I.P., in the scope of the framework contract foreseen - DL57/2016 (CDL-CTTRI-97-ARH/2018 - REF.191-97-ARH/2018), and acknowledges CESAM financial support of through (UIDP/50017/2020+UIDB/50017/2020). Walter Chen is funded by the Ministry of Science and Technology (Taiwan) Research Project (Grant Number MOST 109-2121-M-027-001). Nejc Bezak and Matjaž Mikoš would like to acknowledge the support of the Slovenian Research Agency through grant P2-0180.