Erosion Crusts Research Articles

Soil crusting impact on soil organic carbon losses by water erosion

Abstract The Sahelian region, characterized by erratic, heavy rainfalls and low soil organic carbon (SOC) stocks, is highly vulnerable to land degradation. While water erosion is recognized as being a main mechanism of SOC losses, little research has yet been done to investigate the role which soil surface crusting might have on SOC losses. The main objective of this study was to evaluate the impact of soil surface crusting on SOC losses. This study was conducted in Tougou Catchment (37 km 2 ), northwest of Burkina Faso, which receives a cumulative mean annual rainfall of 500 mm y − 1 . The area is characterized by sandy soils with varying types of surface crusts. The four different crust types studied were: structural crusts (STRU), which were found under cultivated soils, which were plowed annually; perennial desiccation crusts (DES), gravel (GRAV) and erosion (ERO) crusts, generally found in the degraded semi-arid savannas. Three micro-scale runoff plot (1 × 1 m 2 ) replicates were installed on each of the different types of surface crusts observed in the catchment. Water and sediment samples were collected from the runoff plots after every rainfall event (n = 10) of the 2011 rainy season. The sediment samples were analyzed for organic carbon (OCsed), while the water samples were examined for dissolved organic carbon (DOC). The average of organic carbon losses with sediment (OC L sed), was 0.37 g C m − 2 y − 1 for ERO, 0.36 g C m − 2 y − 1 for DES, 0.24 g C m − 2 y − 1 for STRU and 0.15 g C m − 2 y − 1 for GRAV. DOC accounted for a minute contribution to SOC losses i.e. less than 0.05%. STRU with 10.42 mg C l − 1 showed the highest DOC content, followed by GRAV (6.13 mg C l − 1 ), DES (5.06 mg C l − 1 ) and ERO (4.92 mg C l − 1 ). The OC enrichment ratio (ER) of sediments to that of the 0–0.1 m bulk soil was less than one for DES, GRAV and ERO (0.39, 0.69 and 0.75, respectively) and reached 1.14 for STRU. This pointed to a greater SOC protection from erosion by the perennial crusts of the degraded savannas (DES, GRAV and ERO), as compared to crusts of cultivated fields. Thick, sand-enriched crusts, DES and GRAV, seemed to provide the greatest OC protection. This study pointed out a significant relationship between soils crusting on SOC erosion. It showed that the formation of loose and sandy crusts provides greater SOC protection from water erosion, which in turn may improve SOC stabilization and associated soil functions, such as soil fertility, water-holding capacity and sequestration of atmospheric carbon.

Temporal evolution of the hydrodynamic behaviour of sandy deposits in the Sahelian part of Burkina Faso

Sahelian areas of Burkina Faso are confronted with various natural and anthropogenic processes that can lead to degraded areas where the vegetation is localized mainly in units called deposits”. Those deposits are subject to temporal dynamics that greatly impact their hydrodynamic behaviour; the initial drying crust evolves frequently to a drying crust in transition, and ultimately to an erosion crust (according to the crust classification established by Casenave and Valentin (1992)). To better apprehend the temporal evolution and the hydrodynamic behaviour of the sandy deposits, various field measurements have been performed on seven monitoring sites installed in three contrasting zones corresponding to the three crust types. The sites located on erosion crusts and on drying crusts in transition are characterized by a low infiltration capacity which favours run-off (run-off coefficients between 50 and 80%). The sites situated on drying crusts are characterized by a high hydraulic conductivity and infiltration capacity, as well as, relatively low run-off coefficients (smaller than 40%); the water storage in the root zone is more important and water infiltrates deeper into the soil and, occasionally, drains below the depth of 50 cm. The study confirms that soil properties are subject to important temporal evolution, resulting in a strong degradation. Key words: Aeolian sandy deposits, infiltration, run-off, soil hydrodynamic, soil physical properties, soil surface crusting, Sahel.

Runoff generation fostered by physical and biological crusts in semi-arid sandy soils

Biological soil crusts occur extensively in semi-arid regions. In the western part of Niger, they are associated with various types of physical soil crusts in fallows and in the “tiger bush ecosystem” (a landscape with a typical banded pattern consisting of densely vegetated bands of small trees and shrubs alternating with bare soil bands). Despite their widespread occurrence, not much research has been done regarding their role on runoff generation in the Sahelian environment. The present study aims at: (i) highlighting the relative contribution of biological soil crusts in infiltration and runoff on crusted soil in respect with other soil factors; (ii) assessing the impact of underlying physical crusts associated with a wide range of microbial cover; and (iii) gathering the data required for an evaluation of the potential of microbial cover in water redistribution in an arid environment and for modelling purposes. The study was based on runoff measurements performed in situ at the level of ten plots, 1 m 2 each, under two simulated rainfalls. Those experiments were coupled with the visual characterization and image analysis of soil surface conditions as well as micromorphological examinations, using a light microscope, both on the field and at the laboratory. Visual estimation of crust cover was found consistent with the estimation based on the image analysis. Biological soil crusts capped the summit of pre-existing physical soil crusts, i.e. structural crusts, formed in situ by the impact of water drop and erosion crusts, resulting from water or wind erosion of structural crusts. The percentage of microbial cover in the 1-m 2 plots range from 39 to 80% (55% in average) on structural crusts compared to 4 to 29% (17% in average) on erosion crusts. Runoff coefficient varied with both the level of microbial cover and the type of physical soil crust. Higher runoff coefficients were found where biological soil crusts had developed on erosion crusts as compared to structural crusts. Runoff coefficients recorded from erosion crusts increased with the level of microbial cover while negative correlations were found on structural crusts. They show that the clogging effect of microbial cover for the plasmic layer of erosion crusts is higher than that of the sand layer of structural crusts. Separating biological soil crusts according to the types of the underlying physical crusts explain, therefore, the contradictory findings related to their role on infiltration and runoff generation. Those data will help to evaluate the potential role of biological soil crusts in the modelling of water redistribution in the arid environment of the western part of Niger.

Impact of slope gradient on soil surface features and infiltration on steep slopes in northern Laos

It was recently demonstrated that, infiltration into mountain-tilled soils with highly stable microaggregates, increases with increasing slope gradient. In this work we investigate the processes that underpin this phenomenon by means of field experiments and modelling. The study area is located in northern Laos. Rainfall simulations were conducted in two 1-m 2 plots using a portable field simulator. The drop size distribution and kinetic energy were similar to that occurring on the occasion of tropical downpours. Soils exhibited a clay loam texture and very similar organic matter contents across experimental plots, but differed greatly in slope gradient (30% and 75%). Runoff water samples were collected at intervals ranging from 1 to 3 min, depending on the runoff intensity. Plots microtopography was measured before and after rainfall simulations using an automatic surface roughness meter on a 1-cm grid. High-resolution bulk density images were obtained from soil slices using a standard X-ray generator. Final infiltration rates of 6 and 21 mm h −1; soil detachment of 667 and 310 g m −2; surface lowering due to soil loss of 0.82 and 0.38 mm; surface lowering due to compaction of 1.21 and 0.90 mm; percentage area with sieving crust of 36% and 90%; percentage area with erosion crust of 63% and 0%; were obtained for the 30% and 75% slopes, respectively. Three main conclusions can be drawn from this work: (1) high intensity rainfall can rapidly transform soil surface features of steep bare soil; (2) on steeper slopes, the micro-relief tends to form micro-terraces much more pervious and less erodible than the ripple-like roughness that formed on gentler slopes; and (3) there was a more pronounced lowering of the soil surface due to compaction and denser microlayers on gentler slopes. The latter conclusion confirms the hypothesis that higher effective rainfall intensity is responsible for the formation of less permeable erosion crusts under 30% slope gradients while more permeable structural crusts develop under 75% slope gradients. The runoff results were modelled with the Green and Ampt model which accounts for time evolution of soil hydraulic conductivity. This modelling shows that soil is undoubtedly non homogeneous, evolves with time and that infiltration kinetics is slower and soil permeability greater for the 75% slope.

Soil detachment and transport on field‐ and laboratory‐scale interrill areas: erosion processes and the size‐selectivity of eroded sediment

AbstractField‐ and laboratory‐scale rainfall simulation experiments were carried out in an investigation of the temporal variability of erosion processes on interrill areas, and the effects of such variation upon sediment size characteristics. Poorly aggregated sandy soils from the semi‐arid environment of Senegal, West Africa, were used on both a 40 m2 field plot and a 0·25 m2 laboratory plot; rainfall intensity for all experiments was 70 mm h−1 with a duration of 1 to 2 hours. Time‐series measurements were made of the quantity and the size distribution of eroded material: these permitted an estimate of the changing temporal balance between the main erosion processes (splash and wash). Results from both spatial scales showed a similar temporal pattern of runoff generation and sediment concentration. For both spatial scales, the dominant erosional process was detachment by raindrops; this resulted in a dynamic evolution of the soil surface under raindrop impact, with the rapid formation of a sieving crust followed by an erosion crust. However, a clear difference was observed between the two scales regarding the size of particles detached by both splash and wash. While all measured values were lower than the mean weight diameter (MWD) value of the original soil (mean 0·32 mm), demonstrating the size‐selective nature of wash and splash processes, the MWD values of washed and splashed particles at the field scale ranged from 0·08 to 0·16 mm and from 0·12 to 0·30 mm respectively, whereas the MWD values of washed and splashed particles at the laboratory scale ranged from 0·13 to 0·29 mm and from 0·21 to 0·32 mm respectively. Thus only at the field scale were the soil particles detached by splash notably coarser than those transported by wash. This suggests a transport‐limited erosion process at the field scale. Differences were also observed between the dynamics of the soil loss by wash at the two scales, since results showed wider scatter in the field compared to the laboratory experiments. This scatter is probably related to the change in soil surface characteristics due to the size‐selectivity of the erosion processes at this spatial scale. Copyright © 2006 John Wiley & Sons, Ltd.

Study of sulphation of Candoglia marble by means of micro X-ray diffraction experiments

This research is aimed at assessing the particular kind of damage caused by the interaction between atmospheric pollution and the marble surface of a magnificent late gothic church, Milan Cathedral, restored in the early 1970s. On the basis of visual inspections and diagnostic investigations the facade appears to be seriously damaged by surface erosion and thick black crusts; biological crusts, micro-fractures and detachments are widely present.

Responses of soil crusting, runoff and erosion to fallowing in the sub-humid and semi-arid regions of West Africa

Abstract Systems including a long fallow period have proven to be sustainable in the tropics for they enable the transfer of the rich store of nutrients from the vegetation to the ensuing crops. These systems have been mainly studied in the forest zone with a peculiar focus on the maintenance of organic and chemical fertility. By contrast, the effects of the fallow on soil physical properties have not been widely documented especially in the drier areas. The objectives of this paper are: (i) to summarise the fragmented and not readily accessible information on the responses of soil crusting, runoff, water and wind erosion to fallowing in the sub-humid and semi-arid regions of West Africa; (ii) to infer possible scenarios of land use change from the recent past. Two main examples have been taken in northern Ivory Coast and southern Niger. In the Ivorian example, physical properties are restored after a fallow duration of 10 years on sandy clay loam whilst crusts persist over a longer period on sandy soils, partly due to the foraging activities of termites. In the sandy Sahelian soils, cultivation destroys the erosion crusts that develop as the fallow proceeds as a result of dust deposition and colonisation by blue green algae. This crust development is inherent to the Sahelian ecosystem and favours the natural concentration of water resources. The net balance of dust is negative in millet fields (−0.25 t ha −1 per year) and positive in young fallows (+0.68 t ha −1 per year) so that a slight increase in the field/fallow ratio can transform the region from an accumulation zone to a source zone with consequences on regional fertility transfer. Finally, in both regions the sandy soils have specific dynamics that should be accounted in land management planning. Despite a higher water erosion risk, the sub-humid zone offers a better potential for intensification than the semi-arid zone.

The role of surface water redistribution in an area of patterned vegetation in a semi-arid environment, south-west Niger

The surface hydrology of a semi-arid area of patterned vegetation in south-west Niger is described. In this region alternating bands of vegetation and bare ground aligned along the contours of a gently sloping terrain give rise to a phenomenon known as `brousse tigrée' (tiger bush). At the selected study site the vegetation bands are 10–30 m wide, separated by 50–100-m-wide bands of bare ground. Five species of shrub dominate, Guiera senegalensis, Combretum micranthum, C. nigricans, Acacia ataxacantha and A. macrostachya. Herbaceous vegetation is generally limited to the upslope edges of vegetation bands. A comprehensive field programme was undertaken to investigate the hydrology. Topographic, vegetation and surface feature surveys were carried out in conjunction with the measurement of rainfall, surface and subsurface hydraulic conductivity, particle size and soil moisture content. Four types of vegetation class are recognised, each tending to occupy a constant position relative to the others and to the regional slope. In a downslope direction the classes are: bare ground, grassy open bush, closed bush, bare open bush, bare ground etc. The nature of the ground surface is closely linked to the vegetation class. Over the bare, bare open and grassy open classes various types of surface crust are present with each type of crust tending to occupy a constant position on the regional slope relative to the vegetation class and other crust types. Below closed bush crusts are generally absent. The typical downslope sequence from the downslope boundary of a vegetation band is: structural (sieving) crust→erosion crust→(gravel crust)→sedimentation crust→microphytic sedimentation crust→no crust→sieving crust, etc. It is also shown that these crust types are dynamic and evolve from one to the other as hydrological conditions change. Hydraulic conductivities of surface crusts are low, typically falling within the range 10 −6–10 −7 m s −1. The presence of large expanses of crust over bare regions tends to generate run-off, which moves down the regional slope to be intercepted and pond within and just upslope of vegetated areas. Such run-off concentrates rainfall by a factor of up to 3.7 below vegetated areas. This concentration combined with an absence of crust development in closed bush areas promotes rapid infiltration below and just upslope of vegetation bands. In this way the hydrology of the area operates to ensure that the bulk of the rain which falls is directed as quickly as possible to the areas where it is most needed to support the existing vegetation.

Hillslope dynamics of on-farm generation of surface water flows: The case of rain-fed cultivation of pearl millet on sandy soil in the Sahel

Results from on-farm measurements of surface overland flow of both runon ( R on) and runoff ( R off) are presented for a continuously cultivated pearl millet ( Pennisetum glaucum (L.) R.Br.) field, along a characteristic catena, in the Sahel (Niger). Despite the deep sandy soil along the studied hillslope, classified as a Typic Haplustult, non-negligible depths of R off were measured during two rainy seasons: 12–13% of total rainfall for the upslope position and 6–8% for the downslope position. This was explained by the presence of surface crusts with low infiltrability. Upslope, more than 30% of the soil surface was covered with erosion crusts (runoff coefficient (K r) = 60 – 70% for moist soil conditions) during 40% of the rainy season. The mean coverage of erosion crust for two rainy seasons was 26% upslope, compared with 11% and 15% midslope and downslope, respectively. Weeding, carried out manually with hand hoes, had a very positive effect on soil infiltrability. The destruction of surface crusts resulted in zero Roff (Kr = 0) for rainstorms directly following weeding operations. However, surface crusts were quickly re-established as a result of the erosive effect of the following storms. Large volumes of runon (Ron) water were measured at the upslope limit of the field (14.4 m 3 m −1 in 1995, corresponding to a rainfall depth of 47.5 mm if redistributed over the 8.5 ha millet field). Small volumes were registered downslope (R on = 0.75 m 3 m −1), indicating redistribution of R on along the hillslope. The Ron originated from the upstream degraded fallow zone and the adjacent sparsely vegetated plateau, which in effect functioned as water-harvesting zones. R off was modelled based on field observations of surface coverage of soil crusts, data on hydraulic characteristics for the observed crusts, rainfall data and estimates of antecedent soil moisture. Despite high variance in the observed R off, the model provides a reasonable estimate of R off, indicating the possibility of predicting R off on cultivated fields where crust coverage changes quickly over time. The large volumes of measured overland flow suggest that R on and R off cannot be excluded from on-farm water-balance studies. The findings also indicate the potential of developing water-harvesting systems for protective irrigation during the frequent periods of water stress in the rain-fed Sahelian agriculture.

Soil variability and effectiveness of soil and water conservation in the sahel

Sahelian sylvopastoral lands often degrade into bare and crusted areas where regeneration of soil and vegetation is impossible in the short term unless soil and water conservation measures are implemented. Five combinations of tillage with and without mulch on three crust type/soil type combinations were evaluated. The texture and organic matter content of the crusts are determined more by crust type than by soil type. Differences in crust characteristics are linked with the genesis of the crusts. Germination of Cassia tora L. was lower on runoff crusts than on erosion crusts. Combining soil tillage with mulching resulted in a higher average germination than soil tillage or mulching as single measures. We also monitored the mortality of C. tora seedlings 4 weeks after germination. Seedling survival showed a similar, though less pronounced, trend to germination. Average survival was slightly better on erosion crusts than on runoff crusts. Although tillage is necessary to trigger germination of herbaceous species, it may simultaneously inhibit establishment of these species by bringing about severe soil crusting. Soil tillage combined with mulching provided the best conditions for the ultimate establishment (the product of germination and survival) of C. tora. It appears that soil differences have less influence on germination and survival than do differences in surface conditions.

Tillage-Induced Spatial Distribution of Surface Crusts on a Sandy Paleustult from Togo

The spatial distribution of crusts in coarse-textured soils and the processes affecting it are poorly documented, despite the potential impact of crusts on water infiltration. This study addresses the influence of tillage-induced microrelief on the morphology and spatial distribution of surface crusts in an Oxic Paleustult from southern Togo (West Africa). Replicate 1-m2 plots were exposed to 217 mm of natural rainfall during a 6-wk period, during which the surface topography was measured three times. Subsequently, 24 undisturbed crust samples were used for micromorphological analysis. The crusts exhibited a range of morphologies but were nevertheless adequately mapped and characterized according to two main types. Type 1 crusts (=runoff crusts) showed several superposed clay bands, 100 to 500 mm thick, buried within a micromass-depleted sand layer 5 12 mm thick. Type 2 crusts (=erosion crusts) had an exposed clay band a few tenths of a millimeter thick. The spatial distribution of crusts at the time of sampling appeared better correlated with the initial than with the final microtopography of the plots. These findings suggest that crust distribution should be regarded as history dependent and that erosion and deposition processes largely governed the development of the crusts. This latter aspect is in agreement with the recent crust genesis model of Valentin and Bresson, as is the fact that clay bands in our plots were laterally continuous at all observational scales 50.1 m. Other mechanisms proposed in the literature for the development of clay bands did not seem able to account adequately for the observed pattern.

Kinetics of the development of cracks and the nature of ?Erosion crusts? in gun barrels

Kinetics of the development of cracks and the nature of ?Erosion crusts? in gun barrels