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.     

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.     

Soil displacement and tillage erosion during secondary tillage operations: the case of rotary harrow and seeding equipment

This study reports the results of a series of experiments that were set up on agricultural land in central Belgium to investigate soil translocation and erosivity resulting from a secondary tillage operation using an implement sequence of a rotary harrow and seeder. Aluminium cubes were used as tracers of soil movement. Results show that soil displacement resulting from tillage with such an implement sequence is far from insignificant. This is mainly related to the relatively shallow tillage depth as well as to the loose initial soil condition of such secondary tillage operations. The calculated value for the tillage transport coefficient k (123 kg m⁻¹ per tillage operation) is comparable with k-values from implements that are considered to be more erosive, like mouldboard and chisel implements. In conclusion, this study shows that tillage erosion not only results from relatively aggressive tillage operations such as mouldboard and chisel passes, but that secondary operations contribute significantly to soil displacement and tillage erosion.     

Reassessment of tillage erosion rates by manual tillage on steep slopes in northern Thailand

Changing land-use practices in northern Thailand have increased tillage intensity. This study re-assesses the rate of tillage erosion by manual hoeing on steep slopes (17–82%) in northern Thailand. Previously collected soil translocation data during an on-farm tillage erosion experiment and additionally collected data during an on-farm tillage erosion survey have been analysed whereby a new calculation method (i.e. trapezoid tillage step) has been used. A comparison with previously collected data indicates that the trapezoid tillage step method and the tracer method are the most reliable methods to assess downslope translocation by manual tillage. Based on newly acquired understanding of the processes involved, soil fluxes by tillage erosion are quantified by linear functions for different slope gradient classes rather than one single diffusion-type equation for the whole slope range. For slope gradients smaller than 3%, soil fluxes are close to zero as farmers do not have a preferred tillage direction. For slope gradients between 3% and 70%, soil is tilled only in the downslope direction and soil fluxes range between 16 and 67 kg m⁻¹ tillage pass⁻¹. On slopes with gradients in excess of 70%, the angle of repose for soil clods is often exceeded resulting in a sliding down of the complete tilled top layer. These data are used to assess the soil flux for complete cropping cycles for the most dominant cropping systems in the highlands of northern Thailand: i.e. upland rice, maize, (soy) beans, cabbage and ginger. The on-site effects of tillage erosion will be very pronounced if parcels are short with respect to their slope length, cultivated for upland rice or cabbage, or when weed pressure is high. Tillage erosion results in a tillage step with low soil fertility and low infiltration capacity. Solutions to reduce tillage erosion intensity depend on the degree that tillage intensity can be reduced. This might happen by an improved weed management or by changing landuse to perrenial cropping. Other strategies are concentrating nutrients on the truncated hillslope sections and retaining soil on the field by vegetative buffers.     

Tillage erosion within potato production systems in Atlantic Canada: I. Measurement of tillage translocation by implements used in seedbed preparation

In Canada, there is growing acceptance that tillage erosion is a serious form of soil degradation and a threat to the sustainability of agriculture across the country. To date, the potential risk for tillage erosion within potato production systems has not been investigated. The objective of this study was to generate tillage translocation values for primary and secondary tillage implements common to seedbed preparation within conventionally and conservation tilled potato production systems in Atlantic Canada. Tillage translocation was measured for each implement by labelling a plot of soil with a tracer. The tracer redistribution along the path of tillage was used to generate a summation curve to calculate mean soil movement in the direction of tillage. The results show that each primary and secondary tillage operation moved vast quantities of soil and is potentially erosive. Maximum displacement distances were considerably larger in this project than those reported in previous studies looking at tillage erosion by primary and/or secondary tillage implements. All four tillage implements tested moved soil at least 3 m, with the greatest translocated distances (5.6 m) observed for the chisel plough (CP) and vibrashank (VS). The mass of translocated soil (T_M) was greatest for the CP, followed by the mouldboard plough (MP), VS and offset disc (OD). In addition, compared to travelling downslope, the upslope speed of tillage was reduced by 38%, 21%, 32% and 12% for the MP, CP, OD and VS, respectively, while the depth of tillage was reduced by 6%, 5%, 35% and 2%, respectively. It is apparent that conservation tillage implements (the CP is generally promoted to reduce water erosion in Atlantic Canada) and secondary tillage implements (OD and VS) can move as much soil as conventional tillage implements such as the mouldboard plough, and must be considered when developing plans to reduce soil erosion within potato fields in Atlantic Canada.     

Modelling tillage translocation using step, linear-plateau and exponential functions

The distance over which soil is displaced and mixed during tillage has important implications for the understanding the dynamics of soil variability within complex soil-landscapes. In two preceding studies of tillage translocation, tillage was observed to displace soil over a length of approximately 1 m following single passes of four tillage implements (chisel plough, mouldboard plough, tandem disc and field cultivator), and over a length of approximately 2 m per sequence of conventional tillage (one pass of mouldboard plough, two passes of tandem disc and one pass of field cultivator). Using data from these studies step, linear-plateau and exponential functions were assessed for their ability to estimate the magnitude of translocation and the redistribution pattern of soil within the till-layer, and to predict the redistribution pattern of soil within the till-layer. On average, step, linear-plateau and exponential models estimated 100.0%, 100.2% and 102.5% of the magnitude of translocation and 76%, 88% and 93% of the soil redistribution pattern, respectively. Based on these results, it was concluded that linear-plateau and exponential functions are suitable models of tillage translocation. The exponential model was superior to the step and linear-plateau models, and an improvement over the existing diffusion model.     

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.     

Terrain attributes, landform segmentation, and soil redistribution

The ¹³⁷Cs technique has greatly expanded our knowledge of the topography–soil redistribution relationship. For the technique to be useful in upscaling of process models and regional-scale conservation planning, we must be able to show that a consistent relationship exists between ¹³⁷Cs-derived soil redistribution rates and terrain attributes in a given region. In this paper, the association between ¹³⁷Cs-derived soil redistribution rates and quantitatively defined landform elements was examined at nine hummocky terrain sites in southern Saskatchewan, Canada. Shoulder (SH) elements with convex plan curvatures had the highest mean soil loss rates of 33 t ha⁻¹ yr⁻¹, followed closely by other SH and backslope (BS) elements. The erosional behavior of level elements (i.e. those with gradients less than 3°) was highly dependent on the specific dispersal area (SDA) of the element—elements with high dispersal areas were dominantly erosional (mean soil loss of 14 t ha⁻¹ yr⁻¹), whereas level elements with low dispersal areas were depositional (mean soil gain of 15 t ha⁻¹ yr⁻¹). Doubly concave footslope (FS) elements had mean soil gain of 10 t ha⁻¹ yr⁻¹. The dispersion of values across the nine sites was much greater for the depositional units than the erosional units, indicating a complex relationship between deposition and terrain attributes in the depositional units. The results clearly indicate that regional-scale patterns of soil redistribution can be developed using the ¹³⁷Cs technique.     

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.     

Tillage erosion,water erosion and soil quality on cultivated terraces near Xifeng in the Loess Plateau,China

This study sought to contribute to the understanding of soil redistribution by tillage on terraces and the extent and causes of within-field variation in soil properties by examining the spatial distributions of soil redistribution rates, derived using caesium-137, and of total nitrogen and total phosphorus concentrations, within a ribbon and a shoulder terrace in a yuan area of the Loess Plateau of China. Additional water erosion rate data were obtained for nine other terraces. Water erosion rates on the ribbon terraces were low (<1 kg m⁻² yr⁻¹), unless slope tangents exceeded 0·1. However, despite the use of animal traction, high rates of tillage erosion were observed (mean 5·5 kg m⁻² yr⁻¹). Soil nitrogen concentrations were related to rates of soil redistribution by tillage on the ribbon terrace examined in detail. In general, higher rates of water erosion (0·5–2·9 kg m⁻² yr⁻¹) and lower rates of tillage erosion (mean 1·4 kg m⁻² yr⁻¹) were evident on the longer shoulder terraces. On the shoulder terrace examined in detail, soil phosphorus concentrations were related to net rates of soil redistribution. A statistically significant regression relationship between water erosion rates and the USLE length and slope factor was used in conjunction with the simulation of tillage erosion rates to evaluate a range of terrace designs. It is suggested that off-site impacts of erosion could be further reduced by ensuring that the slope tangents are kept below 0·06 and lengths below 30 m, especially on the shoulder terraces. Tillage erosion and the systematic redistribution of soil nutrients could be reduced by modification of the contour-cultivation technique to turn soil in opposing directions in alternate years. Copyright © 1999 John Wiley & Sons, Ltd.     

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.     

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.     

Tillage translocation and tillage erosivity by planting, hilling and harvesting operations common to potato production in Atlantic Canada

In Canada, the negative impacts of tillage erosion is a growing concern, especially in regions where highly erosive cropping and tillage systems are practiced on highly erodible, topographically complex landscapes. To date, tillage erosion studies have focused primarily on the movement of soil by primary and secondary tillage operations. However, in potato (Solanum tuberosum L.) production there is often considerable soil disturbance that occurs during “tertiary” field operations conducted during the growing season. Therefore, the objective of this project was to generate tillage translocation and erosivity values for implements common to planting, hilling and harvesting operations within intensive potato production systems in Atlantic Canada. Our results show that tertiary tillage operations result in significant soil displacement and can be equally as erosive as primary and secondary tillage operations. Both the planting, cultivating and hilling (PCH) sequence and the harvester moved soil extremely large distances (up to 23.6 and 6.0 m, respectively). In fact, the mean translocated distance of the tilled layer (T_L) and the mass of translocated soil (T_M) of the PCH sequence (0.42 m and 115.9 kg m⁻¹, respectively) and the harvester (0.55 m and 71.7 kg m⁻¹, respectively) are larger than those reported previously for primary and secondary tillage operations in New Brunswick. In addition, the net downslope movement of soil for the PCH sequence and the harvester was approximately 36 and 26 kg m⁻¹, respectively, suggesting that both tertiary tillage operations have the potential to be erosive. A direct relationship was observed between both T_L and T_M and slope gradient for the PCH sequence, but similar relationships were not found for the harvester, even though the harvester moved approximately 30 % more soil downslope than upslope. Linear regression functions were generally improved after including slope curvature in the model, but these results were not always significant. Soil movement by the PCH sequence and harvester were also largely influenced by tillage speed and tillage depth, and future research is needed under controlled conditions to determine whether it is changing topography or the variability in tillage speed and depth across the landscape in response to changing topography that is driving tillage erosion within mechanized agricultural systems. It is clear that tertiary tillage operations must be considered when developing best management practices to improve soil conservation strategies for potato production systems in Canada and worldwide.     

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.     

Assessing soil erosion and control factors by the radiometric technique in the Boussouab catchment,Eastern Rif,Morocco

In the Eastern Rif of N Morocco, soil conservation is seriously threatened by water erosion. Large areas of soil have reached an irreversible state of degradation. In this study, the ¹³⁷Cs technique was used to quantify erosion rates and identify the main factors involved in the erosion process based on a representative catchment of the Eastern Rif. To estimate erosion rates in terms of the main factors affecting soil losses, samples were collected taking into account the lithology, slope and land use along six selected transects within the Boussouab catchment. The transects were representative of the main land uses and physiographic characteristics of that Rif sector. The reference inventory for the area was established at a stable, well preserved, matorral site (value of 4250 Bq m^− 2). All the sampling sites were eroded and ¹³⁷Cs inventories varied widely (between 245 and 3670 Bq m^− 2). The effective soil losses were also highly variable (between 5.1 and 48.8 t ha^− 1 yr^− 1). Soil losses varied with land use. The lowest average values were on matorral and fallow land (10.5 and 15.2 t ha^− 1 yr^− 1, respectively) but much higher with alfa vegetation or cereal crops (31.6 and 27.3, respectively). The highest erosion rate was on a badland transect at the more eroded part of the catchment, with rates exceeding 40 t ha^− 1 yr^− 1 and reaching a maximum of 48.8 t ha^− 1 yr^− 1.The average soil losses increased by more than 100% when the slope increased from 10° (17.7 t ha^− 1 yr^− 1) to 25° (40. 8 t ha^− 1 yr^− 1). Similar results were obtained when comparing erosion rates in soils that were covered by matorral with respect to those under cultivation. Lithology was also a key factor affecting soil loss. Soils on marls were more erodible and the average erosion rates reached 29.36 t ha^− 1 yr^− 1, which was twice as high as soils on the glacis and old fluvial terraces (average rates of 14.98 t ha^− 1 yr^− 1). The radiometric approach was very useful to quantify erosion rates and to examine the pattern of soil movement. The analysis of main erosion factors can help to promote rational soil use and establish conservation strategies in the study area.     

Soil erosion evaluation in a small basin through the use of Cs technique

Soil erosion significantly affects the most productive lands in Argentina, particularly the region called “Pampa Ondulada”. Quantification of the actual rates and patterns of soil loss is necessary for designing efficient degradation control strategies. The aim of this investigation was to gather using the ¹³⁷Cs technique a reliable set of data of erosion and sedimentation rates, in order to describe the long-term erosive landscape dynamic in a 300 ha basin representative for the “Pampa Ondulada” region of Argentina. The general topography of the basin is undulated with slopes gradients between 0 and 2.5% and slope lengths up to 800 m long. The main land use consisted in annual cropping under conventional tillage.For the soil erosion study in the basin the ¹³⁷Cs technique was used, which is based on the comparison between the ¹³⁷Cs inventories surveyed with a local reference ¹³⁷Cs profile. The sampling strategy was based on a multiple transect approach.The estimated mean soil erosion rates obtained applying Mass Balance Model 2 for the studied hillslopes ranged between −11.5 and −36 t ha⁻¹ per year and fitted the low and moderate erosion classes according to FAO. These values ranged beyond the admitted tolerance. Sedimentation was observed at the lower landscape positions probably related to changes from convex to concave slopes. The application of the ¹³⁷Cs technique in the studied basin proved to be a useful and sensible tool for assessing erosion/deposition rates. In areas with low topographic gradients like the Pampa Ondulada region, the slope length appears to be an important property for predicting spatial patterns of erosion rates.     

黔西北麻窝山岩溶盆地沉积的新资料

给出麻窝山岩溶盆地环境近期变化的调查结果和沉积剖面137Cs深度分布的新资料,以回复吴攀先生的《论黔西北麻窝山岩溶盆地土壤堆积记录———兼答张信宝先生》一文(简称"吴文")。新资料表明:1)麻窝山岩溶盆地20世纪70年代前不是长期的湖泊环境。1958年落水洞阻塞,盆地下部低洼地段积水成湖,1973年泄流洞打通后,湖泊消失,复为农田。2)吴文和本文的盆地沉积物137Cs深度分布空间格局基本一致,其差异主要是:吴文的盆地下部3个孔,除剖面中下部部分层位含137Cs,且137Cs含量多高于2 Bq/kg外,其他大部分层位不含137C;本文的盆地下部3个孔,除剖面中下部个别层位不含137Cs外,其他大部分层位含137Cs,且活度多低于2 Bq/kg。因吴文未给出有关137Cs量测精度的探头类型和型号、检测限值、样品测量、量测时间和测试误差等技术参数,本文没有讨论吴文和我们的剖面土层137Cs活度存在差异的原因。     

Outcomes of longterm deciduous forest restoration in southwestern Ontario, Canada

At present, forest cover in southwestern Ontario, Canada, remains at less than 5% due to intensive agricultural and urban land use. Although much of the extant forest is increasingly protected by legislation, remnants continue to be degraded by the spread of non-native plant species, overgrazing, and recreational use. Some parks in the region have adopted management programs aimed at mitigating this degradation. Over the last 35 years, cottages and roads at Point Pelée National Park have been removed and sites either passively restored (i.e. road or cottage eliminated and vegetation allowed to regenerate) or actively restored (i.e. road or cottage eliminated, exotic vegetation removed, and native species planted). In 1994 and 1995, we assessed the effectiveness of restoration by comparing the understorey plant communities in 28 restored sites with those in less disturbed reference sites. There was a significant increase (P<0.0001) in the similarity of understorey plant communities between restored and reference sites as time-since-restoration increased. Soil moisture, canopy cover, distance to continuous forest, and site-shape all significantly affected plant species composition. Former road sites recovered significantly (P<0.05) more rapidly than former cottage sites, and the former lawns of passively restored cottage sites were the slowest to recover. Five years following active restoration, non-native ruderal species continued to dominate restored sites. The observed recovery of understorey plant communities in restored sites is attributed to their proximity to natural vegetation, and its function as a seed source. In some sites, recovery is substantial and, assuming present trajectories of change are maintained, we predict that recovery could occur in many mesic sites within the next 20 years. Restoration activity facilitates forest recovery and would appear to have a valuable function in mitigating ongoing conflicts between conservation and human use in this region.