Soil redistribution and organic carbon accumulation under long‐term (29 years) upslope tillage systems

Few studies have demonstrated soil redistribution under upslope tillage (UT) rather than downslope tillage (DT) and its impact on soil organic carbon (SOC) redistribution in long‐term agricultural practices in hillslope landscapes. We selected two neighbouring sites from the Sichuan Basin, China, one under DT and the other under UT, to determine the pattern of soil and SOC redistribution under a long‐term UT practice. DT caused soil loss at upper slope positions and soil accumulation at lower slope positions. However, UT resulted in soil accumulation at upper slope positions and soil loss at lower slope positions. The total erosion rate decreased by 60.5% after 29 years of UT compared with DT. Having the same direction of soil movement by tillage and water exaggerated total soil loss, whereas having the two movements in the contrasting direction of soil for the two reduced it. SOC stocks at positions from summit to downslope were much larger (33.8%) and at toe‐slope positions were only slightly greater (4.5%) in the UT soils than comparable values for the DT site. The accumulation rate of SOC at the UT site increased by 0.26 Mg/ha/year compared with that at the DT site. It is suggested that soil movement by water and tillage erosion occurred in the same direction accelerates the depletion of SOC pools, whereas the opposite direction of soil movement for the two can increase SOC accumulation. Our results suggest that UT has significant impacts on soil redistribution processes and SOC accumulation on steeply sloping land.     

Tillage translocation and tillage erosion in the complex upland landscapes of southwestern Ontario, Canada

Tillage translocation and tillage erosion were measured throughout the topographically complex landscapes of two fields in the upland region of southwestern Ontario. Translocation of soil by tillage was measured by labelling plots of soil with chloride and measuring the tracer's forward displacement in response to single passes by four tillage implements (mouldboard plough, chisel plough, tandem disc and field cultivator). The change in translocation within the landscape was used to measure tillage erosion. All four implements were erosive. A relationship between tillage translocation and slope gradient was observed; however, the variability in translocation could not be explained by slope gradient alone. Slope curvature was responsible for some translocation through the planning action of tillage implements. Tillage depth and speed were subject to considerable discontinuous and inconsistent manipulation by the operator in response to changing topographic and soil conditions. Tillage speed decreased by as much as 60% during upslope tillage and increased by as much as 30% during downslope tillage, relative to that on level ground. Tillage depth decreased by as much as 20% and increased by as much as 30%, relative to that on level ground. This manipulation is typical for tillage in complex landscapes and was presumed largely responsible for the variability in the results. The manipulation of tillage depth and speed are affected by the tractor-implement match and the responsiveness of the tillage operator.     

Characterization of soil profiles in a landscape affected by long-term tillage

Soil movement by tillage redistributes soil within the profile and throughout the landscape, resulting in soil removal from convex slope positions and soil accumulation in concave slope positions. Previous investigations of the spatial variability in surface soil properties and crop yield in a glacial till landscape in west central Minnesota indicated that wheat (Triticum aestivum) yields were decreased in upper hillslope positions affected by high soil erosion loss. In the present study, soil cores were collected and characterized to indicate the effects of long-term intensive tillage on soil properties as a function of depth and tillage erosion. This study provides quantitative measures of the chemical and physical properties of soil profiles in a landscape subject to prolonged tillage erosion, and compares the properties of soil profiles in areas of differing rates of tillage erosion and an uncultivated hillslope. These comparisons emphasize the influence of soil translocation within the landscape by tillage on soil profile characteristics. Soil profiles in areas subject to soil loss by tillage erosion >20 Mg ha⁻¹ year⁻¹ were characterized by truncated profiles, a shallow depth to the C horizon (mean upper boundary 75 cm from the soil surface), a calcic subsoil and a tilled layer containing 19 g kg⁻¹ of inorganic carbon. In contrast, profiles in areas of soil accumulation by tillage >10 Mg ha⁻¹ year⁻¹ exhibited thick sola with low inorganic carbon content (mean 3 g kg⁻¹) and a large depth to the C horizon (usually >1.5 m below the soil surface). When compared to areas of soil accumulation, organic carbon, total nitrogen and Olsen-extractable phosphorus contents measured lower, whereas inorganic carbon content, pH and soil strength measured higher throughout the profile in eroded landscape positions because of the reduced soil organic matter content and the influence of calcic subsoil material. The mean surface soil organic carbon and total nitrogen contents in cultivated areas (regardless of erosion status) were less than half that measured in an uncultivated area, indicating that intensive tillage and cropping has significantly depleted the surface soil organic matter in this landscape. Prolonged intensive tillage and cropping at this site has effectively removed at least 20 cm of soil from the upper hillslope positions.     

The long-term effect of tillage on soil displacement of hilly areas used for growing wheat in Greece

Abstract. Tillage displaces large amounts of soil from upper slopes and deposits soil in lower landscape positions, greatly affecting productivity in these areas. The long-term effect of tillage on soil erosion was studied in four field sites growing mainly rainfed wheat. The soil loss from landscape positions with slopes, ranging from 3 to 28%, was estimated by: (a) comparing data of horizon thickness described at the same position at different times; and (b) using soil movement tracers added to the soil. Existing empirical relationships were used for estimating soil loss by tillage and runoff water, and loss in wheat biomass production. The experimental data showed soil losses of 0.4 to 1.4 cm yr^–1 depending on slope gradient, plough depth, and tillage direction. In two of the sites, soil depth has been reduced by 24–30 cm in a period of 63 years. The mean soil displacement of the plough layer (30 cm thick), measured by soil movement tracers, ranged from 31 to 95 cm yr^–1 depending mainly on slope gradient, corresponding to a rate of soil loss of 0.3 cm to 1.4 cm yr^–1. Soil eroded from the upper slopes was deposited on the lower slopes increasing soil thickness by 0.4 cm to 1.4 cm yr^–1. The application of empirical relationships, estimating soil loss by tillage and water runoff, showed that soil erosion at the field sites can be mainly attributed to tillage. The loss in wheat biomass production due to erosion was estimated at 26% on upper slopes for a period of 63 years, while a 14.5% increase in wheat production was estimated due to deposition of soil material in the lower landscape.     

Variation of chemical properties as affected by soil erosion on hillslopes and terraces

The variation in soil nutrients is crucial to the understanding of productivity of soil undergoing erosion overall, as the latter can result in a decline in soil quality and crop production in the whole landscape. Two toposequences (a long slope and terraced field series) were selected from hilly areas of the Sichuan Basin, China, to determine the effects of soil redistribution rates and topographic changes on P, K and CaCO₃ contents, and examine the contribution of water and tillage erosion to the variation and distribution pattern in these chemical properties within different landscapes. For the long slope, soil loss occurred at upper slope positions and soil accumulation was present at lower slope positions. However, terrace banks create a line of zero downslope transport of soil, and lead to abrupt changes in ¹³⁷Cs inventories over very short distances between the upper (or lower) part of the terrace and the lower (or upper) part of the neighbouring terrace. Extractable K concentrations are significantly related to ¹³⁷Cs inventories on both the long slope and terraced fields, which suggests that the distribution of extractable K is closely linked to soil redistribution. However, it is noticeable that no close relationship between extractable P concentrations and ¹³⁷Cs inventories was found on the terraced fields, while there was a highly significant correlation between the two variables on the long slope. The variation in extractable P by soil redistribution was enhanced on the long slope, but was concealed on the terraced fields due to the presence of CaCO₃. It is suggested that the variation in extractable P not only depends on soil redistribution in relation to fine soil particles, but is also influenced by other factors such as P‐fixation onto CaCO₃, the concentration of which itself is linked to soil erosion and redistribution. Therefore, extractable P dynamics with reference to soil erosion are relatively complex on carbonate‐rich soil and parent materials in areas such as those represented by the Sichuan Basin. Tillage erosion, the dominant soil redistribution process on terraced fields, was found to be a major contributor to the variation in soil chemical properties in the terraced field landscape, while water erosion plays an important role in the variation in soil chemical properties in the long slope landscape. In the case of carbonate‐rich soils or parent materials, however, tillage erosion did not create accumulations of extractable P in depressions, whereas water erosion results in extractable P losses at upper slope positions and accumulation at lower slope positions.     

Modeling spatial variation in productivity due to tillage and water erosion

The advent of precision farming practices has heightened interest in managing field variability to optimize profitability. The large variation in yields across many producer fields demonstrated by yield–monitor–equipped combines has generated concern about management-induced causes of spatial variation in soil productivity. Soil translocation from erosion processes may result in variation in soil properties across field landscape positions that produce long-term changes in soil productivity. The objective of this study was to examine the relationships between soil redistribution caused by tillage and water erosion and the resulting spatial variability of soil productivity in a soil catena in eastern South Dakota. An empirical model developed to estimate tillage erosion was used to evaluate changes expected in the soil profile over a 50-year period on a typical toposequence found in eastern South Dakota and western Minnesota. Changes in the soil profile due to water erosion over a 50-year period were evaluated using the WEPP hillslope model. The tillage erosion model and the WEPP hillslope model were run concurrently for a 50-year period to evaluate the combined effect of the two processes. The resulting changes in soil properties of the root zone were evaluated for changes in productivity using a productivity index model. Tillage erosion resulted in soil loss in the shoulder position, while soil loss from water erosion occurred primarily in the mid to lower backslope position. The decline in soil productivity was greater when both processes were combined compared to either process acting alone. Water erosion contributed to nearly all the decline in soil productivity in the backslope position when both tillage and water erosion processes were combined. The net effect of soil translocation from the combined effects of tillage and water erosion is an increase in spatial variability of crop yields and a likely decline in overall soil productivity.     

Fine-earth translocation by tillage in stony soils in the Guadalentin, south-east Spain: an investigation using caesium-134

Tillage erosion is increasingly recognised as an important soil erosion process on agricultural land. In view of its potential significance, there is a clear need to broaden the experimental database for the magnitude of tillage erosion to include a range of tillage implements and agricultural environments. The study discussed in this paper sought to address the need for such data by examining tillage erosion by a duckfoot chisel plough in stony soils on steep slopes in a semi-arid environment. Results of the investigation of coarse fraction (rock fragment) translocation by tillage in this environment have been presented elsewhere and the paper focuses on tillage translocation and erosion of the fine earth. Tillage translocation was measured at 10 sites, representing both upslope and downslope tillage by a duckfoot chisel plough on five different slopes, with tangents ranging from 0.02 to 0.41. A fine-earth tracer, comprising fine earth labelled with ¹³⁴Cs, was introduced into the plough layer before tillage. After a single pass of the plough, incremental samples of plough soil were excavated and sieved to separate the fine earth from the rock fragments. Translocation of the fine-earth tracer was established by analysing the ¹³⁴Cs content of the samples of fine earth. These data were used to establish translocation distances for each combination of slope and tillage direction. Translocation distances of the fine earth were not significantly different from translocation distances of the coarse fraction. For all sites, except uphill on the 0.41 slope, translocation distances were found to be linearly related to slope tangent. The soil flux due to tillage for each site was calculated using the translocation distance and the mass per unit area of the plough layer. For slopes with tangents <0.25, the relationship between soil flux and tangent was linear and the soil flux coefficient derived was 520–660 kg m⁻¹ per pass. This is much larger than the coefficients found in other studies and this high magnitude is attributed to the non-cohesive nature and high rock fragment content of the soil in this investigation. A second contrast with previous studies was found in non-linearity in the relationship between soil flux and tangent when steeper slopes were included. This was a product of variation in plough depth between the steepest slopes and the remainder of the study area. On the basis of the study it is suggested that an improved understanding of tillage erosion may be obtained by considering the dual processes of tillage detachment (mass per unit area of soil subject to tillage) and tillage displacement (equivalent to translocation distance per pass) in assessing, comparing and modelling tillage translocation. An improved model is proposed that recognises the complexity of soil redistribution by tillage, provides a framework for process-based investigation of the controls on tillage fluxes, and allows identification of potential self-limiting conditions for tillage erosion.     

Tillage and water erosion on different landscapes in the northern North American Great Plains evaluated using Cs technique and soil erosion models

Total soil erosion is the integrated result of all forms of soil erosion — wind, water and tillage. It has been recognized that in topographically complex landscapes, individual soil erosion processes and their interactions all contribute towards total soil erosion. In this study, two field sites, representing different landscapes in the northern region of the North American Great Plains, were examined. Water and tillage erosions were estimated using the established water and tillage erosion models and total soil erosion was estimated using the ¹³⁷Cs technique.We determined that the patterns of water and tillage erosion across the landscapes are mainly dependent on topographic features and they are fundamentally different within topographically complex landscapes. On the slope of undulating landscapes, tillage and water erosion both contribute considerably to total soil erosion. On the knoll of hummocky landscapes, tillage erosion dominates the pattern of total soil erosion. Tested against the Cs measurements, the patterns of total soil erosion cannot be well estimated by water or tillage erosion model alone unless one of the two erosion processes predominate over the other erosion processes. Combining water and tillage erosion models generally provides better estimations of total soil erosion than the component models on their own. Most soil properties and crop yield were found to be closely correlated with total soil erosion. For a given erosion process, the soil erosion patterns estimated using different models with reasonable parameter settings were similar to each other. However, it is necessary to choose an optimal model and to obtain accurate parameters for the purpose of accurate assessments of the erosion rates.     

Soil landscape evolution due to soil redistribution by tillage: a new conceptual model of soil catena evolution in agricultural landscapes

This paper focuses on analysing tillage as a mechanism for the transformation of soil spatial variability, soil morphology, superficial soil properties and development of soil–landscape relationships in agricultural lands. A new theoretical two-dimensional model of soil catena evolution due to soil redistribution by tillage is presented. Soil profile truncation occurs through loss of soil mass on convexities and in the upper areas of the cultivated hillslopes; while the opposite effect takes place in concavities and the lower areas of the field where the original soil profile becomes buried. At sectors of rectilinear morphology in the hillslope (backslope positions), a null balance of soil translocation takes place, independent of the slope gradient and of the rate of downslope soil translocation. As a result, in those backslope areas, a substitution of soil material in the surface horizon with material coming from upslope areas takes place. This substituted material can produce an inversion of soil horizons in the original soil profile and sometimes, the formation of “false truncated soil”. In the Skogstad agricultural field (Cyrus, MN) spatial patterns of soil properties (soil calcium carbonate content) in the surface soil horizons and soil morphology along several slope transects were analyzed. These spatial patterns are compared with those estimated for soil redistribution (areas of erosion and deposition) due to tillage using the Soil Redistribution by Tillage (SORET) model and water erosion using the models Water Erosion Prediction Project (WEPP) and Universal Soil Loss Equation (Usle2D). Results show that tillage was the predominant process of soil redistribution in the studied agricultural field. Finally, some practical implications of the proposed model of soil landscape modification by tillage are discussed. Nomographs to calculated the intensity of the expansion process of the eroded soil units by tillage are proposed for three different patterns of tillage.     

Use of Cs to estimate tillage- and water-induced soil redistribution rates on agricultural land under different use and management in central-south Chile

The purpose of this research was to evaluate the applicability of conventional ¹³⁷Cs sampling and a simplified approach, for estimating medium-term tillage- and water-induced soil erosion and sedimentation rates on agricultural land in Chile. For this purpose, four study sites under contrasting land use and management were selected in central-south Chile. First, a conventional ¹³⁷Cs approach, based on grid sampling was applied, adapting a mass balance conversion model incorporating soil movement by tillage to the site specific conditions of the study region. Secondly, using the same conversion model, the feasibility of estimating soil redistribution rates from measurements of ¹³⁷Cs inventories based on composite soil samples taken along contour lines was also tested at all four sites. The redistribution rates associated with tillage and water and the total rates estimated using both methods correlated strongly at all four sites. The conventional method provides more detailed information concerning the redistribution processes operating over the landscape. The simplified method is suitable for assessing soil loss and sediment accumulation in areas exhibiting simple topography and almost similar slopes along the contour lines. Under these conditions, this method permits faster estimation of soil redistribution rates, providing the possibility of estimating soil redistribution rates over larger areas in a shorter time. In order to optimise the costs and benefits of the methods, the sampling and inventory quantification strategy must be selected according to the resolution of the required information, and the scale and complexity of the landscape relief. Higher tillage- and water-induced erosion rates were observed in the annually ploughed cropland sites than in the semi-permanent grassland sites. Subsistence managed crop and grassland sites also show greater erosion effects than the commercially managed sites. The ¹³⁷Cs methods used permit discrimination between redistribution rates observed on agricultural land under different land use and management. The ¹³⁷Cs technique must be seen as an efficient method for estimating medium-term soil redistribution rates, and for planning soil conservation and sustainable agricultural production under the climatic conditions and the soil type of the region of Chile investigated.     

Assessment of tillage erosion by mouldboard plough in Tuscany (Italy)

This study was designed to characterise the soil translocation effect induced by mouldboard ploughing with an implement traditionally used in the Tuscany region (Central Italy). We discuss the results of a set of field experiments performed to measure soil displacement along slopes of varying gradient in different directions and at several depths of tillage. Using the Soil Erosion by Tillage (SETi) model, soil translocation patterns for different tillage scenarios were analysed, with special attention paid to the effects of the direction and depth of tillage on the extent and spatial pattern of soil movement. The lateral slope gradient S_P and tillage depth D were found to be the dominant controlling factors for total soil displacement. The effect of the slope gradient in a direction parallel to tillage S_T was much less pronounced. These findings reveal the importance of the asymmetric nature of the soil movement produced by mouldboard ploughing and the predominant effect of the lateral displacement d_P on the actual trajectory of soil motion. Results demonstrate that spatial patterns of soil redistribution due to mouldboard ploughing are highly variable and depend on the particular characteristics of the implement used. This dependence is so strong that maximum downslope soil translocation can occur during both, contour tillage or up–down tillage. For this particular mouldboard plough, maximum downslope soil transport took place at a tillage direction ca. 70° and not when tillage was conducted along the steepest slope direction (0°). These findings highlight the potential of the combined approach applied. The physically based SETi model can be properly calibrated using a relatively limited dataset from field experiments. Once calibrating, the SETi model can then be used to generate synthetic tillage translocation relationships, which can predict the intensity and spatial pattern of soil translocation over a much wider range of tillage scenarios than the particular experimental conditions, in terms of topography complexity (slope gradients and morphology) and the direction and depth of tillage. These synthetic relationships are useful tools for evaluating strategies designed to reduce tillage erosion.     

Soil organic carbon and fly-ash distributions in eroded phases of soils in Illinois and Russia

Soil organic carbon (SOC) dynamics are affected by tillage, soil erosion and depositional processes. The objectives of this paper are to evaluate soil organic carbon and fly-ash distribution methods for identifying eroded phases of soils in Illinois and Russia and quantifying the extent of soil loss from erosion. The effect of accelerated erosion on soils is recorded on National Cooperative Soil Survey maps as phases of soil series that reflect the percentage of the original A horizon materials remaining. Identification depends on knowledge of the original A horizon thicknesses, SOC and fly-ash contents at uncultivated and uneroded sites when determining erosion phases of soil at cultivated and eroded sites. However, locating uncultivated and uneroded comparison sites with similar landscape and slope characteristics can be difficult. The amount of A horizon materials within the plow layers (Ap horizons) or topsoils are often determined by soil colors which reflect the SOC contents. Soil erosion phases based on original A horizon materials remaining in the topsoils may underestimate the extent of soil losses from topsoils and subsoils, particularly where soils have been cultivated for hundreds of years and are severely eroded. The SOC contents and soil erosion phases can be affected by losses or gains of organic C-rich sediments from tillage translocation and erosion, by management input level differences, oxidation, or as a result of land use and landscape position variations. Fly-ash was found to be more stable and act as a better indicator of soil erosion phase than SOC content.     

Assessment of tillage translocation and tillage erosion by hoeing on the steep land in hilly areas of Sichuan, China

Most of the tillage erosion studies have focused on the effect of tractor-plough tillage on soil translocation and soil loss. Only recently, have a few studies contributed to the understanding of tillage erosion by manual tillage. Furthermore, little is known about the impact of tillage erosion in hilly areas of the humid sub-tropics. This study on tillage erosion by hoeing was conducted on a purple soil (Regosols) of the steep land, in Jianyang County, Sichuan Province, southwestern China (30°24′N and 104°35′E) using the physical tracer method.

The effects of hoeing tillage on soil translocation on hillslopes are quite evident. The tillage transport coefficients were 26–38 kg m⁻¹ per tillage pass and 121–175 kg m⁻¹ per tillage pass respectively for k₃- and k₄-values. Given that there was a typical downslope parcel length of 15 m and two times of tillage per year in this area, the tillage erosion rates on the 4–43% hillslopes reached 48–151 Mg ha⁻¹ per year. The downslope soil translocation is closely related to slope gradient. Lateral soil translocation by such tillage is also obvious though it is lower than downslope soil translocation. Strong downslope translocation accounts for thin soil layers and the exposure of parent materials/rocks at the ridge tops and on convexities in the hilly areas. Deterioration in soil quality and therefore reduction in plant productivity due to tillage-induced erosion would be evident at the ridge tops and convex shoulders.     

The effect of tillage direction on soil redistribution by mouldboard ploughing on complex slopes

Seven mouldboard ploughing experiments were conducted to systematically investigate the effect of different tillage directions on soil redistribution on hillslopes. The present study included tillage directions other than parallel to the gradient or along the contour, that is, in our experiments the slope gradient changed simultaneously in tillage and in turning direction. Using physical tracers we developed a model of the two-dimensional tracer displacement as a function of topography and tillage variables. The displacements in tillage and in turning direction were separately described as 2nd degree polynomials in both tillage and in turning directions. This model fully accounted for the directionality of tillage. Displacement in turning direction additionally depended on tillage depth, while that in tillage direction was affected by tillage speed and soil bulk density. We found a large effect of tillage directionality on soil redistribution, and tillage at 45° to the gradient turning soil upslope was the least erosive tillage direction. We obtained non-linear relationships between soil redistribution and profile curvature, instead of the linear relationships reported previously. Consequently tillage erosivity varied in tillage direction and a unique tillage transport coefficient could not be obtained for all tillage directions.     

东北黑土区典型坡面耕作侵蚀定量分析

东北黑土区水土流失主要集中在坡耕地,以往研究多关注水蚀而忽略了耕作侵蚀的存在。为印证并定量描述黑土耕作侵蚀,该文采用物理示踪法,测定了典型坡耕地耕作位移量及其分布格局。结果表明:铧式犁耕作后示踪剂沿耕作方向发生扩散,上坡耕作示踪剂集中分布在0~20 cm范围,而下坡耕作示踪剂集中分布在0~20和50~150 cm。一次耕作引起的耕作位移量为32.68~134.14 kg/m,耕作迁移系数234 kg/m。坡度是影响耕作位移的重要因素,二者呈显著的正相关关系,且对上坡耕作的影响大于下坡耕作。研究区耕作年侵蚀速率0.4~11.0 Mg/(hm2·a),凸起的坡背、坡肩处及坡度较大的位置侵蚀严重。虽然黑土区坡度较小,但由于耕作深度大,速度快,耕作侵蚀严重,应引起足够重视。     

Development and application of landform segmentation procedures

Landscape-scale approaches to research in soil science are explicitly focused on transfers of components within and between landscapes. Despite wide-spread recognition of the importance of these transfers, the application of landscape-scale approaches has been hindered by the lack of clear, reproducible research designs. Landform segmentation is used to divide natural and human-influenced landscapes into functionally distinct units. A specific type of landform segmentation, landform element classification, was used in a comparative mensurative design to compare the effects of cultivation on soil distribution and soil organic carbon (SOC) storage and in a manipulative design to determine the relationship between N₂O emissions and fertilizer rate in a hummocky till geomorphic surface in southern Saskatchewan. Significant transfers of SOC and surface soil from convex shoulder units to lower slope positions occurred over the past 90 years, resulting in a change in the type of soils that occupy these positions at two research sites. The observed pattern is consistent with a tillage translocation dominated surface. The dominant control on N₂O emissions in the landscape are spatial differences in water-filled pore space (WFPS) that are strongly controlled by water redistribution. Emissions from drier, shoulder landform element complexes are consistently low throughout the year, whereas a strong positive relationship between N fertilizer rate and N₂O emissions occur in the wettest, level depressional elements.     

Fly-ash distribution to assess erosion and deposition in an Illinois landscape

Fly ash, the particulate matter resulting from high temperature combustion of coal, was historically dispersed into the atmosphere and settled as fly-ash spheres on the surface soil from a variety of boilers, including those of steam locomotives and steam-powered farm machinery. In Central Illinois, fly ash provides a time marker extending back to 1850s, coinciding with the development of railroads and cultivation. Two railroads, the primary sources of fly ash, were constructed just south of the Cahokia study site in 1852 and 1854. The objectives of this study were to determine: (1) the distribution, depth of occurrence and the total amount of fly ash present in soil profiles on stable, cultivated and uncultivated summit sites with little or no soil erosion; (2) the effects of elevation, aspect, slope gradient, landscape position, distance from source, past vegetation and time on the amount and depth of fly ash; (3) the effects of erosion on sloping sideslopes; and (4) the amount of deposition of fly-ash rich sediment on footslopes and toeslopes. Total fly-ash content of soil was similar for stable, cultivated and uncultivated summits. Two mound sideslopes maintained a high amount of fly ash because of a lack of cultivation and erosion for the past 80 or more years. Erosion reduced the depth of occurrence and the amount of fly ash present on cultivated sideslopes. It appeared that fly-ash content was initially deposited uniformly within the local landscape even though there were slight variations in the aspect, elevation, slope gradient, and distance from the source. The erosion phases of the soils on all landscape positions were determined based on the amount of fly ash remaining in soil surface layers. Accelerated erosion of cultivated sideslopes resulted in the deposition of fly-ash rich sediment on the adjacent footslopes or toeslopes. The proposed fly-ash method provides a tool to assess the extent of soil translocation from a cultivated landscape and subsequent deposition.     

Ridge tillage and contour natural grass barrier strips reduce tillage erosion

Large amounts of soil are eroded annually from tilled, hilly upland soils in the humid tropics. Awareness has been increasing that much of this erosion may be due to tillage operations rather than water-induced soil movement. This field study estimated soil translocation and tillage erosion for four tillage systems on Oxisols with slope gradients of 16–22% at Claveria, Misamis Oriental, Philippines. Soil movement was estimated using ‘soil movement tracers' (SMT) which consisted of painted 12-mm hexagonal steel nuts. The SMT were buried in three replicate plots of the following tillage treatments: (1) contour moldboard plowing in the open field (MP-open); (2) contour ridge tillage in the open field (RT-open); (3) contour moldboard plowing plus contour natural grass barrier strips (MP-strip); and (4) contour natural grass barrier strips plus ridge tillage (RT-strip). Two hundred SMT were placed at the 5-cm depth at 5-cm spacings on 10 rows and 20 columns in two microplots within each plot. The microplots were oriented with the boundaries running downslope and along the contour of each 8-m-wide × 38-m-long (downslope) tillage plot. After tilling the land for four successive corn (Zea mays L.) crops (20 tillage operations), the SMT were manually excavated and their positions recorded. Recovery of SMT ranged from 82% to 85%. Displacement of SMT was directly related to slope length, percent slope, and tillage method. Mean displacement distance of SMT during the four corn growing seasons was 3.3 m for MP-open, 1.8 m for RT-open, 1.5 m for the RT-strip, and 2.2 m for MP-strip. Based on tillage operations associated with two corn crops per year, mean annual soil flux was estimated to be 241, 131, 158 and 112 kg m⁻¹ for MP-open, RT-open MP-strip, and RT-strip, respectively. Compared to the mean annual soil loss for MP-open of 63 Mg ha⁻¹, soil loss was reduced by 30%, 45%, and 53% for the MP-strip, RT-open, and RT-strip systems, respectively. Both ridge tillage and natural grass barrier strips reduced soil displacement, soil translocation flux, and tillage erosion rates.     

Use of caesium-137 data for validation of spatially distributed erosion models: the implications of tillage erosion

Validation of spatially distributed models using spatially distributed data represents a vital element in the development process; however, it is rarely undertaken. To a large extent, this reflects the problems associated with assembling erosion rate data, at appropriate temporal and spatial scales and with a suitable spatial resolution, for comparison with model results. The caesium-137 (¹³⁷Cs) technique would appear to offer considerable potential for meeting this need for data, at least for longer timescales. Nevertheless, initial attempts to use ¹³⁷Cs for model validation did not prove successful. This lack of success may be explained by the important role of tillage erosion in redistributing soil within agricultural fields and, therefore, contributing to the ¹³⁷Cs-derived soil redistribution rates. This paper examines the implications of tillage erosion for the use of ¹³⁷Cs in erosion model validation and presents an outline methodology for the use of ¹³⁷Cs in model validation. This methodology acknowledges and addresses the constraints imposed by the need to: (1) separate water and tillage erosion contributions to total soil redistribution as represented in ¹³⁷Cs derived rates; (2) account for lateral mixing of ¹³⁷Cs within fields as a result of tillage translocation; (3) simulate long-term water erosion rates using the model under evaluation if ¹³⁷Cs-derived water erosion rates are to be used in model validation. The methodology is dependent on accurate simulation of tillage erosion and tillage translocation. Therefore, as greater understanding of tillage erosion is obtained, the potential for the use of ¹³⁷Cs in water erosion model validation will increase. Caesium-137 measurements remain one of the few sources of spatially distributed erosion information and, therefore, their potential value should be exploited to the full.     

Quantifying tillage translocation and deposition rates due to moldboard plowing in the Palouse region of the Pacific Northwest, USA

Most of the erosion research in the Palouse region of eastern Washington State, USA has focused on quantifying the rates and patterns of water erosion for purposes of conservation planing. Tillage translocation, however, has largely been overlooked as a significant geomorphic process on Palouse hillslopes. Tillage translocation and tillage deposition together have resulted in severe soil degradation in many steep croplands of the Palouse region. Few controlled experiments have heretofore been conducted to model these important geomorphic processes on Palouse hillslopes. The overarching purpose of this investigation, therefore, was to model tillage translocation and deposition due to moldboard plowing in the Palouse region. Soil movement by moldboard plowing was measured using 480-steel flat washers. Washers were buried in silt loam soils on convex–convex shoulder, linear-convex backslope, and linear-concave footslope landform components, and then displaced from their original burial locations by a moldboard plow pulled by a wheel tractor traveling parallel to the contour at ca. 1.0 m s⁻¹. Displaced washers were located using a metal detector, and the distance and azimuth of the resultant displacement of each washer from its original burial location was measured using compass and tape. Resultant displacement distances were then resolved into their component vectors of displacement parallel and perpendicular to the contour. A linear regression equation was developed expressing mean soil displacement distance as a function of slope gradient. Tillage translocation and deposition were modeled as diffusion-type geomorphic processes, and their rates were described in terms of the diffusion constant (k). A multivariate statistical model was developed expressing mean soil displacement distance as a function of gravimetric moisture content, soil bulk density, slope gradient, and direction of furrow slice displacement. Analysis of variance (ANOVA) revealed a weak correlation between soil displacement and both bulk density and moisture content. Soil displacement was, however, significantly correlated with direction of furrow slice displacement. Tillage translocation rates were expressed in terms of the diffusion constant (k) and ranged from 105 to 113 kg m⁻¹ per tillage operation. Tillage deposition rates ranged from 54 to 148 kg m⁻¹ per tillage operation. With respect to tillage deposition, the diffusion constant calculated from volumetric measurements of tillage deposits equals ca. 150 kg/m. The rates of tillage translocation and deposition are not completely in balance; however, these rates do suggest that soil tillage is a significant geomorphic process on Palouse hillslopes and could account for the some of the variations in soil physical properties and crop yield potential at the hillslope and farm-field scale in the Palouse region.