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.     

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.     

利用137Cs示踪技术评价东北黑土侵蚀和沉积过程

Soil and water losses through erosion have been serious in the black soil region of Northeast China. Therefore, a sloping cultivated land in Songnen Plain was selected as a case study to： 1） determine the ^137Cs reference inventory in the study area; 2） calculate erosion and deposition rates of black soil on different slope locations; 3） conduct a sensitivity analysis of some model parameters; and 4） compare overall outputs using four different models. Three transects were set in the field with five slope locations for each transect, including summit, shoulder-slope, back-slope, foot-slope, and toe-slope. Field measurements and model simulation were used to estimate a bomb-derived ^137Cs reference inventory in the study area. Soil erosion and deposition rates were estimated using four ^137Cs models and percentage of ^137Cs loss/gain. The ^137Cs reference value in the study area was 2 232.8 Bq m^-2 with ^137Cs showing a clear topographic pattern, decreasing from the summit to shoulder-slope, then increasing again at the foot-slope and reaching a maximum at the toe-slope, Predicted soil redistribution rates for different slope locations varied. Among models, the Yang Model （YANG-M） overestimated erosion loss but underestimated deposition. However, the standard mass balance model （MBM1） gave predictions similar to a mass balance model incorporating soil movement by tillage （MBM2）. Sensitivity analysis of the proportion factor and distribution pattern of ^137Cs in the surface layer demonstrated the impact of ^137Cs enrichment on calculation of the soil erosion rate. Factors influencing the redistribution of fallout ^137Cs in landscape should be fully considered as calculating soil redistribution rate using ^137Cs technique.     

Modelling tillage erosion in the topographically complex landscapes of southwestern Ontario, Canada

A tillage erosion model was developed for southwestern Ontario based on the relationship between tillage translocation and slope gradient and slope curvature. Two studies of tillage translocation and tillage erosion were used to calibrate this model, one a comparison of upslope and downslope tillage translocation on shoulder slopes, the other an examination of tillage translocation throughout topographically complex landscapes. Two field sites were used for validation of the model. For both sites, past tillage practices were known and past soil erosion was determined using ¹³⁷Cs as an indicator of soil redistribution. The model accurately predicted the pattern of soil redistribution that had occurred within the two field sites. Severe soil loss was observed and predicted on convex landscape positions and soil accumulation was observed and predicted on concave landscape positions. The model accounted for almost all of the soil lost from the convex upper slope positions where tillage erosion was expected to be the dominant erosion process. There was considerable soil loss and accumulation elsewhere in the landscapes which could not be accounted for by the model and was presumed to be primarily the result of water erosion. It was concluded that tillage erosion must be incorporated into soil erosion modelling for the purposes of soil conservation.     

Using Cs measurements to validate the application of the AGNPS and ANSWERS erosion and sediment yield models in two small Devon catchments

Distributed erosion and sediment yield models are being increasingly used for predicting soil erosion and sediment yields in agricultural catchments. In most applications, validation of such models has commonly been restricted to comparison of the predicted and measured sediment output from a catchment, because spatially distributed information on rates and patterns of soil redistribution within the catchment has been lacking. However, such spatially distributed data are needed for rigorous model testing, in order to validate the internal functioning of a model and its applicability at different spatial scales. The study reported in this paper uses two approaches to test the performance of the agricultural non-point source pollution (AGNPS) and areal non-point source watershed environmental response simulation (ANSWERS) erosion and sediment yield models in two small catchments in Devon, UK. These involve, firstly, comparison of observed and predicted runoff and sediment output data for individual storm events monitored at the basin outlets and, secondly, information on the spatial pattern of soil redistribution within the catchments derived from ¹³⁷Cs measurements. The results obtained indicate that catchment outputs simulated by both models are reasonably consistent with the recorded values, although the AGNPS model appears to provide closer agreement between observed and predicted values. However, the spatial patterns of soil redistribution and the sediment delivery ratios predicted for the two catchments by the AGNPS and ANSWERS models differ significantly. Comparison of the catchment sediment delivery ratios and the pattern of soil redistribution in individual fields predicted by the models with equivalent information derived from ¹³⁷Cs measurements indicates that the AGNPS model provides more meaningful predictions of erosion and sediment yield under UK conditions than the ANSWERS model and emphasises the importance of using information on both catchment output and sediment redistribution within the catchment for model validation.     

Soil erosion and deposition rates in a cultivated catchment area in central Greece, estimated using the Cs technique

The spatial variation of soil erosion and deposition rates was studied in a small catchment cultivated by rainfed agriculture, in the Mouriki area, Viotia Greece, using the ¹³⁷Cs technique. A 25 m grid was established parallel to the slope and the ¹³⁷Cs inventories were defined for the grid points. After establishing the local reference inventory, the soil erosion and deposition rates were estimated using the ¹³⁷Cs residuals for individual points on the grid in conjunction with the four conversion (calibration) models described by Walling and He (2001) [Models for converting ¹³⁷Cs measurements to estimates of soils redistribution rates on cultivated and uncultivated soils]. The conversion models were validated by means of sensitivity analysis and using local experimental data. The resulting estimates of soil redistribution rates were interpolated by means of kriging, using Surfer Golden software. The magnitude of the soil erosion rates depend on many factors, including the location of the sampling point, the local slope, and the soil properties. The mass balance model 2 (MBM2) and mass balance model incorporating soil movement by tillage (MBM3) conversion models predict soil redistribution rates of the same order of magnitude as the experimental data and are able to take account of Chernobyl fallout. Predicted soil erosion rates for catchment grid varied from 6.71 to 85.55 t ha⁻¹ per year using MBM2 and from 3.54 to 95.78 t ha⁻¹ per year using MBM3. Deposition rates varied from 1.23 to 168.19 t ha⁻¹ per year using MBM2 and from 3.24 to 189.18 t ha⁻¹ per year using MBM3. High correlation was apparent between erosion/deposition rates (MBM2) and soil P (P<0.001), soil K (P<0.001), soil organic matter % (P<0.05), point slope (P<0.05), clay % (P=0.053) and altitude (P=0.057). The total soil losses from the catchment have been estimated at 18.34 t ha⁻¹ per year using MBM2 and 22.12 t ha⁻¹ per year using MBM3.     

Dual roles of tillage erosion in lateral SOC movement in the landscape

The facts that the global carbon budget cannot be currently balanced and current estimates of agricultural sources and sinks may be inaccurate, may be linked to unaccounted‐for erosion‐induced changes in soil organic carbon (SOC). A closed landscape with field banks and an open landscape without field banks were selected from two sites located in Jianyang County, Sichuan Province, and Zhongxian County, Chongqing Municipality, respectively. In these landscapes, the role of tillage and water erosion was examined using measurements of soil redistribution in relation to ¹³⁷Cs radionuclide depth‐stratigraphy, to elucidate the mechanism of SOC depth distribution in the soil profile and resultant stocks in agricultural landscapes of terraced field systems. Changes in the ¹³⁷Cs inventory at different landscape positions depend on both ¹³⁷Cs concentrations of individual subsample layers (5‐cm depth) and the vertical extent of ¹³⁷Cs depth distribution in the terrace system with field banks, while the changes are only associated with the vertical extent of ¹³⁷Cs depth distribution in the terrace system without field banks because of similar ¹³⁷Cs concentrations of individual subsample layers. The profile shape of SOC depth distribution exhibits notable differences between the upper and lower parts of the terrace in systems with field banks, but no apparent differences were found in the systems without field banks and the SOC profile shape is similar to that of the upper part of the terrace in systems with field banks. It is suggested that SOC depth distribution in these two types of terraced field systems is controlled by different soil erosion patterns. Tillage erosion playing a dominant role in the process of soil erosion within a landscape can increase SOC stocks. However, SOC depletion takes place in situations where the two processes of tillage and water erosion are both important and tillage erosion acts as a delivery mechanism for water erosion. We conclude that tillage erosion plays a dual role: enhancing carbon storage at depositional positions, and accelerating carbon depletion when combined with water erosion within the same landscape.     

137Cs tracing of the spatial patterns in soil redistribution,organic carbon and total nitrogen in the southeastern Tibetan Plateau

The southeastern Tibetan Plateau, which profoundly affects East Asia by helping to maintain the stability of climate systems, biological diversity and clean water, is one of the regions most vulnerable to water erosion, wind erosion, tillage erosion, freeze–thaw erosion and overgrazing under global climate changes and intensive human activities. Spatial variations in soil erosion in terraced farmland (TL), sloping farmland (SL) and grassland (GL) were determined by the ¹³⁷Cs tracing method and compared with spatial variations in soil organic carbon (SOC) and total nitrogen (total N). The ¹³⁷Cs concentration in the GL was higher in the 0–0.03 m soil layer than in the other soil layers due to weak migration and diffusion under low precipitation and temperature conditions, while the ¹³⁷Cs concentration in the soil layer of the SL was generally uniform in the 0–0.18 m soil layer due to tillage-induced mixing. Low ¹³⁷Cs inventories appeared at the summit and toe slope positions in the SL due to soil loss by tillage erosion and water erosion, respectively, while the highest ¹³⁷Cs inventories appeared at the middle slope positions due to soil accumulation under relatively flat landform conditions. In the GL, the ¹³⁷Cs data showed that higher soil erosion rates appeared at the summit due to freeze–thaw erosion and steep slope gradients and at the toe slope position due to wind erosion, gully erosion, freeze–thaw erosion and overgrazing. The ¹³⁷Cs inventory generally increased from upper to lower slope positions within each terrace (except the lowest terrace). The ¹³⁷Cs data along the terrace toposequence showed abrupt changes in soil erosion rates between the lower part of the upper terrace and the upper part of the immediate terrace over a short distance and net deposition on the lower and toe terraces. Hence, tillage erosion played an important role in the soil loss at the summit slope positions of each terrace, while water erosion dominantly transported soil from the upper terrace to the lower terrace and resulted in net soil deposition on the flat lower terrace. The SOC inventories showed similar spatial patterns to the ¹³⁷Cs inventories in the SL, TL and GL, and significant correlations were found between the SOC and ¹³⁷Cs inventories in these slope landscapes. The total N inventories showed similar spatial patterns to the inventories of ¹³⁷Cs and SOC, and significant correlations were also found between the total N and ¹³⁷Cs inventories in the SL, TL and GL. Therefore, ¹³⁷Cs can successfully be used for tracing soil, SOC and total N dynamics within slope landscapes in the southeastern Tibetan Plateau.     

川中丘陵区人工刨地耕作方式下紫色土坡耕地侵蚀速率的137Cs和210Pbex双核素示踪法研究

Purple soils are widely distributed in the Sichuan Hilly Basin and are highly susceptible to erosion, especially on the cultivated slopes. Quantitative assessment of the erosion rates is, however, difficult due to small size of the plots of the manually-tilled land, the complex land use, and steep hillslopes. 137Cs and 210Pbex (excess 210Pb) tracing techniques were used to investigate the spatial pattern of soil erosion rates associated with slope-land under hoe tillage in Neijiang of the Sichuan Hilly Basin. The 137Cs and 210Pbex inventories at the top of the cultivated slope were extremely low, and the highest inventories were found at the bottom of the cultivated slope. By combining the erosion rates estimates provided by both 137Cs and 210Pbex measurements, the weighted mean net soil loss from the study slope was estimated to be 3100 t km-2 year-1, which was significantly less than 6930 t km-2 year-1 reported for runoff plots on a 10°cultivated slope at the Suining Station of soil Erosion. The spatial pattern of soil erosion rates on the steep agricultural land showed that hoe tillage played an important role in soil redistribution along the slope. Also, traditional farming practices had a significant role in reducing soil loss, leading to a lower net erosion rate for the field.     

Simulation and 137Cs tracer show tillage erosion translocating soil organic carbon,phosphorus, and potassium

Research on how tillage‐induced soil redistribution affects soil properties is limited for complex slopes in nonmechanized agricultural areas. The objectives of this study are (1) to examine the vertical redistribution of soil organic C (SOC), extractable P and K induced by tillage on a complex slope, (2) to assess the effects of tillage erosion on soil profile properties, and (3) to elucidate the variations in soil properties induced by both vertical mixture and downslope transport of soil within the landscape. Simulated tillage was conducted in the Yangtze Three Gorges Reservoir Area, China. The ¹³⁷Cs data showed that intense tillage caused the soil vertical mixture and downslope transport. The redistribution of ¹³⁷Cs and soil constituents varied with the number of tillage passes and location in relation to curvature. SOC was completely depleted with the disappearance of soil profiles at the summit position, while SOC concentrations decreased by 26% for the till layer and increased by 29% for the sublayer at the toeslope position for the 15‐tillage operation, as compared with those of pretillage. The vertical redistribution of extractable P and K followed a similar pattern to that of SOC. The gap and variation in soil constituents between the till layer and sublayer declined after tillage, suggesting that the mixing effect of tillage attenuates the variability of soil properties in the vertical direction. Net loss and gain of soil constituents occurred at the summit and toeslope positions, respectively, suggesting that the downslope transport of soil induced by tillage accentuates the variability of soil properties in the lateral direction.