Revegetation has been proven to significantly affect soil erodibility of gully heads, and climate conditions are expected to affect soil erodibility by determining the vegetation characteristic. However, there are crucial scientific/knowledge gaps regarding the change in the response of soil erodibility of gully heads to revegetation along a vegetation zone gradient. Therefore, we selected the gully heads with different restoration years along a vegetation zone gradient encompassing the steppe zone (SZ), forest-steppe zone (FSZ), and forest zone (FZ) on the Chinese Loess Plateau to clarify the variation in soil erodibility of gully head and its response to soil and vegetation properties from SZ to FZ. Furtherly, we systematically and comprehensively reveal driving factors of changes in soil erodibility in three vegetation zones. Results showed that: (1) Vegetation and soil properties were affected positively by revegetation and differed significantly in three vegetation zones. (2) Soil erodibility of gully heads in SZ was significantly higher than in FSZ and FZ, by 3.3 % and 6.7 % on average, respectively, and it showed a significantly different decrease with restoration years in three vegetation zones. (3) Standardized major axis analysis proved that the sensitivity of response soil erodibility to vegetation characteristics and soil characteristics presented a significant difference as the revegetation proceeded. Vegetation roots were the primary driver in SZ, but soil organic matter content dominated the change in soil erodibility in FSZ and FZ. (4) Structural equation modeling indicated that climate conditions played an indirect role in regulating soil erodibility of gully heads by mediating vegetation characteristics. This study offers essential insights for assessing the ecological functions of revegetation in the gully heads of the Chinese Loess Plateau under different climatic scenarios.
We carried out a time domain reflectometry (TDR) calibration for 25 forest floor samples of five different forest stands. Linear regression was used to estimate the volumetric water content from the refraction index. Due to the presence of bound water, it was impossible to predict the calibration line parameters from theoretical values with a refractive mixing model. However, when the apparent dielectric constant of the water was considered, it was possible to predict the offset parameter because the low forest floor bulk density caused the calibration line slope and offset to be almost reversely proportional. Calibration parameters for different forest floor materials are presented. Allowing a somewhat higher margin of error we could also establish general calibration curves which were in close agreement with a published calibration curve (Malicki et al., 1994). An error analysis showed that decomposition of organic matter, residual water and temperature effects have negligible effects on the calibration parameters. Shrinkage of the organic material significantly influenced both the volumetric water content and the TDR reflection times. If not corrected, both effects yielded systematic errors of approximately 0.02 cm3 cm−3 for a strongly shrinking H horizon.
Dye-tracer studies in the field using Brilliant Blue FCF as tracer were performed to investigate the effect of irrigation intensity and soil heterogeneity on preferential flow. In two fields, both level and newly tilled in terms of seed bed preparation, to plots of 1.6 × 1.6 m were applied 50 mm of dye solution at rates of 10 and 50 mm h−1. In the second year level, plots of grass of similar size were applied with 25 mm dye solution at a rate of 3.1, 6.2, 12.5, and 25 mm h−1. For all plots the stained patterns were examined one or two days after application of dye solution by the excavation of 11 vertical cross sections of 100 × 100 cm and 10 cm apart from each other. Flow patterns were digitized and depth functions for the degree of dye coverage and the number of activated flow channels were calculated. Furthermore, the structural features of each cross section were examined visually. The results show that deep penetration of water into the soil profile took place as preferential flow through macropores, mainly earthworm channels, with much of the water thus bypassing the soil matrix. In the top 0–25 cm layer, the degree of dye coverage tended to be larger for the lower irrigation intensities indicating that water flow in the top soil took place through a relatively great proportion of the pores in the soil matrix. In the 35–100 cm subsoil layer the number of stained macropores tended to be larger for the higher irrigation intensities. Thus, at higher irrigation intensity a positive pressure potential apparently developed more extensively in the topsoil initiating preferential flow through a greater number of macropores in the subsoil. In the newly tilled soil, water flow took place through a relatively great part of the topsoil matrix. Deeply penetrating stained earthworm channels originated, predominantly, in the well defined transition zone between topsoil and subsoil. In the soil left untilled and grass covered for about one year the continuity of macropores was more pronounced, and stained channels could frequently be traced from the subsoil all the way to the soil surface, in particular at low irrigation intensity.
The main conclusions of the research project on soil erosion and sedimentation in Swaziland are as follows. (1) Soil erosion has worsened over the last 20 years. The proportion of an 1800 km 2 study area in the Middleveld classified as ‘high erosion class’ has increased from 6.7 to 13.6% between 1972 and 1990. (2) Gully erosion is the main process and is concentrated within the Manzini, Jabuleni, Lobamba and Chibidze land systems, all characterised by soil-saprolite complexes. (3) Overgrazing and compaction along paths and tracks lower the infiltration rate of the clay-rich ferralitic soils, promoting surface runoff and the formation of rills. (4) Once the rill deepens and cuts through the soil on to the underlying saprolite, gullies develop rapidly because of the low shear strength of the material. (5) Soil erosion problems are compounded by the existing system of land tenure and increasing pressure of livestock and population on the land. (6) In some instances, population pressure can lead to improvements in land management and better soil protection. Recommendations for controlling soil erosion and for further research are presented.
The authors study clay mineral and main nutrient element losses in runoff water generated by rainfall events in 1990–1991 from five field plots on a Petric Calcisol or Xerollic Paleorthid. Illite is the main layer silicate in the clay Ap horizon, with a smaller proportion of kaolinite and a minor amount of chlorite and smectite. Soil samples taken in different seasons show that plots under shrub vegetation are very much alike and, with the exception of available Fe, Cu, Mn and Zn, they are markedly different from arable soil plots, being much richer in organic C, total N and available K, but poorer in available P. Annual and seasonal losses of the main nutrient elements are small, with the highest amounts in the plots cultivated for barley and with cut shrub; losses are extremely low for the plot with shrub cover.
Pulverised fuel ash (PFA) applied at an application rate of 8.3% by weight, incorporated by ploughing to a badly compacted minespoil was shown to reduce bulk density significantly over a one year period. A second ameliorative treatment with 4.8% PFA, and a third treatment with ploughing alone both reduced bulk density after four months, but by twelve months displayed no statistically significant differences in bulk density compared to an untreated control.
The problem of soil erosion in Swaziland is reviewed in the context of land management of communal grazing lands. Aspects of farm operations, cropping patterns and range management are considered in a typical Swazi traditional land management scenario. The location of farm plots, existing land tenure arrangements, farmers' awareness of the erosion problem and available expert advice on land management are analysed in the context of results of a questionnaire survey. It is concluded that land tenure conditions constrain land management, while the social stigma on livestock ownership and limited advice on soil conservation constrain positive measures toward erosion control. Most constraints are beyond the control of individual land users.
The weathering mantle of the Middleveld of Swaziland consists of thick soil-saprolite complexes. The isovolumetric chemical weathering of the saprolites has led to mass losses of more than 50%. Compared with saprolites from quartz-diorite and granodiorite, those from diorite have higher portions of easily weatherable plagioclases and amphiboles and 20–30% greater total pore space. The macro pore space reaches a maximum (4.6–7.0%) in the central saprolite zone, corresponding to saturated hydraulic conductivities of 6.02–11.81 × 10−7 m/s. Similar to the saprolites, the overlying ferrallitic soils show total pore volumes ranging from 39 to 52%. Compared to the soils, the available water capacity of the underlying saprolites is two to four times higher and the saturated hydraulic conductivity is about two times higher due to the high portion of medium pores which amount to 70% of total pore space. In the areas affected by sheet erosion, most of the soil cover is denuded and the underlying saprolites essentially determine the site properties. The hydrological properties of the saprolites are therefore of great importance with respect to erosion during wet periods and plant growth during drought periods. The low structural stability of the saprolites, indicated by shear strength values < 5 kPa, results from a silty texture, absence of organic matter, and low contents of Fe- and Al-oxides. As a result, saprolites are highly susceptible to erosion and represent an essential precondition for the development and rapid expansion of deep incising erosion gullies in areas with magmatic rocks. In contrast, the clay-rich ferrallitic soils developed from saprolite are comparatively stable, indicated by shear strength values ranging from 7 to 12 kPa. The inherent stabilizing properties of the soil are altered by overgrazing and unwise land use leading to infiltration capacities below 65 cm/day and high overland flow potentials at low rainfall intensities.
A knowledge of the physical properties influencing the quasi-steady infiltration rates (Ic) of soils is useful in the hydrological modelling of the infiltration process. In this study, the Ic of 18 highly permeable soils in the derived savannah zone of south-eastern Nigeria were characterized and related to land use, bulk density and pore size distribution. Unmulched soils with or without conventional tillage and soils in continuous pasture/grass cover had relatively slow to moderate Ic, whereas mulched soils, soils amended with plant residues or under secondary forests/legume cover had rapid to very rapid Ic. The saturated porosity and void ratio gave low correlation with Ic with correlation coefficients (r) of 0.388 and 0.217, respectively. Mesoporosity (i.e. pores with equivalent radius of 1.5–15 μm) and microporosity (i.e. pores with equivalent radius of 0.1–1.5 μm) had negative influence on Ic, with respective ’r’ values of −0.566 and −0.404. Preferential porosity (Pe) (i.e. pores with equivalent radius > 15 μm) and dry bulk density (ϱb) were the most important soil physical properties influencing Ic. Their correlation coefficients (r) with Ic were respectively, 0.852 and −0.806 (p = 0.001). This shows that soil management systems which increase the bulk density due to compaction with concomitant reduction in the proportion of the preferential pores will reduce Ic substantially on these soils. The Philip (1957) model, Ic = A + 12St−12 (where A and S are the fitting parameters), could not predict the measured quasi-steady infiltration rates very satisfactorily.
Soil erosion has worsened considerably in the Middleveld of Swaziland over the last 20 years. Most of the erosion has occurred on deep colluvial and saprolitic materials and is associated with convex-slope breaks and rejuvenation shoulders on valley sides. Although the risk of erosion seems to reflect natural instability in the landscape, the way in which the land is used determines the ultimate severity of the problem. Land systems can serve as functional units as far as erosion assessment is concerned. The most severely eroded land occurs within the Manzini, Jabuleni, Lobamba and Chibidze land systems. The universal soil loss equation (USLE) and the soil loss estimator for southern Africa (SLEMSA) give vastly different estimates of the rate of soil loss but no information is available to validate or refute the predictions.
The measurement of fall-cone penetration on air-dried and slowly rewetted subsamples (< 1 mm) at different water contents characterizes hardsetting and non-hardsetting soils by differences in their maximum resistance to penetration (RPmax), the water content (WCmax) at RPmax, and the amount of water adsorbed between the two values of 0.5 × RPmax, called ‘insensitivity’ I. The insensitivity I is strongly correlated with the contents of organic matter and iron oxides which are considered as soil components improving aggregation. The insensitivity may be used to identify hardsetting soils. These have I-values < 7, whereas non-hardsetting soils have I-values > 10.
Rainfall simulation studies on rangeland in the Ntondozi area of Swaziland showed that soil loss decreased exponentially with increasing vegetation cover. Vegetation exerted an important hydrological control by increasing the infiltration capacity of the soil which, in turn, influenced the time to and duration of runoff. The expected effects of vegetation on soil protection and soil strength were not demonstrated. Instead, the amount of soil loss occurring in an individual storm appeared to depend on the supply of loose material on the surface which could be transported by the runoff.
African tropical mountains are often overcrowded because the climate is healthy and favorable to intensive agriculture. Consequently the density of population in the mountains of Rwanda and Burundi has reached an exceptional level (150 to 800 inhabitants/km2) that leads to delicate problems of soil protection against runoff and various types of erosion on steep cultivated hillslopes. Previous measurements on runoff plots have shown that sheet and rill erosion risks have reached 300 to 700 t/ha/year on 20 to 60% slopes with regional rainfall erosivity (Rusa = 250 to 700), very resistant ferrallitic soils (K = 0.01 to 0.20) and traditional farming systems (C = 0.8 to 0.3). Curiously, the runoff rate (10 to 30%) is relatively moderate so that it is possible to restrict erosion with a natural or leguminous fallow, a pine plantation (litter effect) or by mulching coffee, banana or cassava plantations. The problem is now to produce enough biomass to mulch the whole surface with the help of agroforestry. A new strategy (GCES = land husbandry) was suggested to meet the major farmer problems: what should be done to increase the soil productivity rapidly and protect the rural environment? A part of the answer is to be found in the efficient management of water, organic matter and soil fertility restoration (Roose et al., 1988). This strategy was first tested in 9 runoff plots (5 × 20 m) on a 23% slope of a very acid ferrallitic soil (pH = 4). Three types of living hedges (leucaena, calliandra, calliandra + setaria) twice replicated, were compared with the international bare standard plot and with the regional farming system (maize + beans during the first season, and sorghum during the second season). After 2 years, living hedges reduced runoff to less than 2% and erosion to 2 t/ha/year: they produced fire wood and high quality leguminous forage (3 to 8 kg/m) and return to the soil as much as 80 to 120 kg/ha/year of nitrogen, 3 kg/ha/year of phosphorus, 30 to 60 kg/ha/year of calcium and potassium, 10 to 20 kg/ha/year of magnesium. Thanks to agroforestry it was possible to reduce erosion hazard but not to restore the soil productivity. Without 2.5 t/ha/3 years of lime to increase the pH up to 5 and reduce the aluminium toxicity, without 10 t/ha/2 years of farm manure and mineral fertilizers to nourish the crops, the yield remains very low (800 kg/ha/season of cereals). Thanks to agroforestry and a mineral fertilizer complementation, erosion hazard was controlled and the productivity of soil and labour intensified more than 3 times.
The processes of the action of wind and/or water on the soil surface as well as the processes of soil movement caused by wind and/or water are complex. In view of the great complexity of the phenomena involved in aerodynamics and hydraulics, they are not always described theoretically with sufficient accuracy, and the experimental approach is often more reliable. Processes of water erosion are often studied under rainfall simulation whereas wind erosion processes are investigated by means of wind tunnels. At the International Centre for Eremology (I.C.E.), Ghent University, Belgium, a wind tunnel is installed which not only enables to study wind and water erosion processes separately, but also to investigate the interaction and the combined effect of wind and water on soil movement.
Accurate measurements of soil heat flux are important for energy balance studies on bare soils. Measurements are usually made with passive transducers that transform the vertical soil heat flux into an e.m.f.. Measurement errors or bias result from differences between the calibration coefficients provided by the manufacturer and those determined in the field. These differences result from bad thermal contact between the soil and the fluxmeter and/or changes in soil thermal conductivity. New printed circuit heat fluxmeters are very thin (0.2 mm) and they provide a better thermal contact with the soil because they have an external copper layer instead of an insulating resin. We carried out a theoretical analysis to identify properties of the transducers (geometrical, thermal or electrical) most important for reducing the calibration variability. The transducer thickness was found to reduce the calibration variability due to large soil thermal conductivity variations. Transducer thermal conductivity is also important when the soil thermal conductivity is accurately known. The printed circuit transducers and classical soil heat flux transducers (thermopiles) where then compared in three different soils, a sandy loam, a loamy and a chalky soil under changing climatic conditions in spring. The outputs of both transducers were compared to reference soil heat flux measurements obtained by the heat storage method. The thermopile transducers were more sensitive (4.1 μV W−1 m2) than the printed circuit transducers (1.6 μV W−1 m2). Both transducers gave similar responses when the soil thermal conductivity varied over a narrow range. The total variation of the calibration coefficients of the printed circuit transducer was smaller for all three soils and for days where the soil thermal conductivity varied widely. We conclude that the printed circuit transducers should be used when field calibration is not possible, or when the calibration is not stable following large soil thermal conductivity variations. The experiment also showed that the theory does not completely describe the interaction between calibration coefficients and soil properties. We have therefore developed a new interpretation of the experimental data that takes into account the thermal contact between the soil and the transducer.
Cation exchange capacity (CEC) recoveries from a leaching procedure utilizing intact soil columns were compared to extracts from disturbed soil samples to determine the influence of macropores and preferential flow on ion exchange. Eleven soils representing eight soil series with a variety of morphological and physicochemical characteristics were used in the study. Leachate was introduced into duplicate undisturbed soil columns following the 1 M NH4OAc, pH 7.0 procedure at a weight to volume ratio equivalent to that used for the disturbed soil samples. Effluents from disturbed and undisturbed samples were collected and analyzed for CEC and total extractable bases. Average CEC values for the intact columns were 49.1% lower than those measured by routine analysis. Regressional analysis indicated a significant difference between the two methods (p < 0.1). Particle size distribution was identified through multiple linear regression analysis as the most influential physicochemical property contributing to the difference between methods. Soil columns with sandier textures displayed high recovery rates attributable to uniform porosity and low CEC. As the silt fraction increased, the difference between methods increased due to formation of macropores and associated preferential flow through the soil matrix. However, increased clay levels allowed more thorough hydration of the matrix which apparently restricted flow, increased residence time and promoted exposure of more exchange sites, thus resulting in intermediate recovery rates. These results suggest that routine CEC measurements based on batch extractions of disturbed soil samples may overestimate ion exchange interactions, and therefore, overestimate true contaminant sorption capacities of soils.
Non-irrigated and irrigated Ordinary Chernozems of Stavropol Upland, Central Precaucasus (Russia), were examined. A number of morphological and instrumental methods was used for detailed investigation of the carbonates in the soils. This approach allowed us to reveal the essential change of the carbonate status of the irrigated Chernozems as a result of changing the soil moisture regime and the predominance of leaching processes under 30 years irrigation by fresh water. The changes included distinct enhancement of spatial variability and instability of carbonate-illuvial horizon deposition, the highest data scatter on carbonate content within this horizon, the signs of increased mobility of carbonate material and the change of the mechanism of the carbonate accumulations. Carbonates moving with soil solutions and mobile migrational forms of carbonate accumulations as well as segregating forms and lithogenic concretions were transformed under the irrigation. Irrigation for 30 years resulted in unfavorable reclamative changes of the Ordinary Chernozems carbonate status: decalcification of the upper part of the solum, disappearance of the mobile carbonate accumulations from the upper horizons, the initial point of the compact carbonate-hardened horizon in the low part of profile and the increase of the calcium and magnesium content in the soil solution. The problem studied is complicated because of the inherent variability of carbonate parameters and the multitude of factors having an influence on carbonate status. Much more work is needed because of the importance of the topic for sustainability of agricultural production systems, for prediction and overcoming of unfavorable consequences of irrigation, for the up-to-date problem of the greenhouse effect and the response of carbonate reserves to climate changes.
In August of 1992 a wildfire affected 9498 ha of pine forest and shrub in Sierra Calderona (Valencia, Spain). The results obtained in six stations of erosion control located in the burned are since the end of the fire are reported. Topographical, edaphological and vegetation characteristics of each station are described. Seven episodes of erosive rain with production of runoff and sediments, between August 1992 and April 1993 were studied. The data suggest that the highest soil losses were produced in the period immediately after the fire. Runoff and sediments produced in each station have been quantified. It has been observed that under similar conditions, the taxonomical and textural characteristics are determinant factors in the production of runoff and so, of soil loss.
