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Sugarcane (Saccharum officinarum L.), an intensive, long-term, monoculture, economical crop in South Africa, is known to degrade soil characteristics. Soil structure, a key indicator of soil health and biomass production potential, is manageable by agricultural practices. This study aims to evaluate the effect of crop residue management practices (mulching, burning with residues scattered or removed) and mineral fertilization (with or without) on soil structure by analyzing soil shrinkage curves (SSCs) and other soil physical and chemical properties in a long-term sugarcane trial established in 1939. The SSC provides descriptive structural soil data by differentiating and characterizing two pore systems (plasma and structural pores). By analyzing the SSC of 24 plots (four replicates each treatment), residue management and fertilization practices were found to have statistically and physically significant effects on hydrostructural variables. Partial redundancy analysis showed that residue management practices had a slightly higher effect (19/100 of total variance) on hydrostructural variables compared with fertilization (12/100). The main hydrostructural variables representing the management effect were total soil shrinkage, specific volume, and swelling capacity of the plasma, which were higher in mulched and/or unfertilized plots, indicating that soil was less compact, and shrinkage was more intense, including at the plasma level. The stronger structural dynamics and aggregate stability of the soil were explained by the behavior of the primary aggregates (peds), which were more porous and reactive during the drying process. This study highlights the importance of mulching and limited fertilization to maintain soil structure in the long term while still ensuring yield production.

The oases in the arid the Nefzaoua region in central-western Tunisia are associated with spring mounds. Spring mounds result from an exceptional succession of geomorphological, hydrogeological and climatic conditions and processes that follow each other in a specific order. They look like small volcanoes that range from 200 m to more than 2000 m in diameter and 3-30 m in height, which centres were once occupied by a pond fed by an artesian spring. They are at the origin of each oasis's irrigation system, which radiates from the mound's foot. Due to intense irrigation of intensive date palm plantations (Phoenix dactylifera) in recent decades, all artesian springs dried out. We established a complete inventory and typology (i.e. location, dimension, and types of degradation) of all spring mounds in the Nefzaoua region. Overall, in an area of 3000 km2, 126 spring mounds of 7 types were identified, of which 112 (89%) were degraded. Due to the low salt content in spring mound sediments, they are excavated and used as a soil amendment to expand new palm plantations. Only 14 mounds (11%) are presently preserved. Their qualification as a geomorphosite could preserve them from complete destruction.

International audience; Termites are key soil bioturbators in tropical ecosystems. Apart from mound nests constructed by some advanced lineages, most of the species use their faeces, oral secretions, debris, or soil aggregates to protect themselves from predators and desiccation when they go out to forage. Although this soil ‘sheeting’ is considered to play a key role in soil functioning, the properties of this termite-made material has been poorly studied. The few available data showed that sheeting properties are highly variable with positive, neutral or negative impacts on soil C and clay content, and consequently on soil aggregate stability. Therefore, the objective of this study was to determine the factors controlling the physical (particle size fractions and structural stability) and chemical (pH, electrical conductivity and carbon content) properties of soil sheeting produced by termite species encompassing all feeding and building categories using a dataset representative of an important diversity of biotopes coming from 21 countries from all continents colonized by termites. We showed that sheeting properties were explained by the properties of their environment, and especially by those of the bulk soil (linear relationships), followed in a lesser extent by the mean annual precipitation and biotope. Classic hypotheses related to termite feeding and building strategies were not hold by our analysis. However, the distinction of termites into fungus-growing and non-fungus growing species was useful when differentiating the impact of termites on soil electrical conductivity, C content, and structural stability. The large variability observed suggests the need to redefine termite functional groups based on their impacts on soil properties using a trait-based approach from morphological, anatomical and/or physiological traits.

The utilization of termite mounds for the improvement of soil fertility is a worldwide practice and usually explained by the specific properties of this biogenic material. In Cambodia, farmers also use termite mound soils as amendments with the aim to improve the fertility of paddy fields. The first objective of this study was, therefore, to describe the physical and chemical properties of this material and, consequently, to determine its potential for improving soil fertility. A second objective was to consider farmer’s perception and to quantify the diversity of services provided by termite mounds. We confirmed the specific soil properties of termite mounds but showed that their positive influence on soil chemical fertility and water retention are only significant in very sandy soil (>80% sand) while they remain limited in less sandy soil (~40 and 60% of sand). However, termite mounds are considered useful by farmers independently of the soil condition, mostly because this soil material is considered to positively increase soil fertility but also because they host a specific biodiversity which can be used for medicinal purposes or because consumed, then increasing population livelihood. Our work shows the discrepancy between the perception of farmers and the real impact of termite mounds on soil fertility as well as the diversity of services delivered by biodiversity in paddy fields.

International audience

Les paysages des tertres oasiens du Nefzaoua sont disposés à l’est du Chott el Jerid en Tunisie aride. Les palmeraies s’étalent au pied des tertres, monticules de forme conique creusés au centre par une source artésienne d’eau douce. Ils ont été aménagés et ont été à la base du développement des oasis. Aujourd’hui, les sources sont taries par la prolifération des forages et les tertres, fruits d’une conjonction de conditions géomorphologiques, hydrogéologiques et climatiques sont à l’abandon, voire détruits pour étendre les plantations de palmiers dattiers. Ces édifices peuvent être considérés à la fois comme un géopatrimoine et un patrimoine culturel en péril et certains mériteraient d’être préservés.

International audience; The formation of deep gullies (called ‘dongas’ locally) in rangeland in KwaZulu‐Natal Province in South Africa is a natural phenomenon. These U‐shaped, very wide gullies have considerable lateral expansion due to the episodic collapse of sidewalls.The dongas have developed in duplex soils such as Luvisols and Lixisols formed on Permian sedimentary rocks or unconsolidated Quaternary colluvium. This study combined morphological, mineralogical and chemical characterization with measurements of grain‐size content, structural stability and the complete shrinkage curve to detect changes in soil properties of the different horizons located in the gully banks.The different soil horizons present clear and sharp differences in physical and mineralogical properties. The topsoil with complete grass cover is very resistant to soil detachment. However, the leached E horizon and the BC horizon have low structural stability. The soil profile down to and including the Bt horizon contains exclusively illite in the clay fraction, while the BC colluvial layer and the C horizon (mudstone) contain expandable interstratified illite–smectite. The Bt horizon has a high water content at saturation and high shrinkage, while the BC and C horizons have a high residual shrinkage and a very low water content at saturation.Because this type of gully expansion is not significantly linked to slope value or the stream power index (SPI) at the gully head, to land‐use change, high rainfall intensities or the threshold of concentrated runoff being exceeded at the gully head, other causes were investigated. It was concluded that the heterogeneity between horizons with different mineralogical properties and structural stabilities, soil types and parent material, anisotropic water‐saturation and shrink‐swell properties are of major importance. This heterogeneity between different soil horizon morphologies and their physical properties can explain why the relationship between the critical slope and the drainage area for gully initiation showed a threshold for gullying much lower than that found elsewhere.

Si is one of the most abundant element on earth and an abundant literature shows its beneficial effects on plant growth and resistance. We here question the influence of termites, as key soil bioturbators, on the distribution of Si in a tropical soil. The abundance and forms of Si in termite mounds build by Odontotermes obelus (TM) or in the soil eroded from TM but redistributed on the ground surface (EROD) were compared to those measured in the 0-5 (Ctrl(0-5)) and 70-120 cm soil layers (Ctrl(70-120)). Although termites use the soil from Ctrl(70)-(1)(20) for building their mounds, we found that TM and EROD had intermediate soil physical, chemical and mineralogical properties between Ctrl(0-5) and Ctrl(70)(-1)(20). Clay content was not significantly different between soil materials. However, the lower variability measured in TM than in the soil suggested that termites used soil layers with higher amounts of clay fraction and with a preference especially for layers enriched in 2:1 clay minerals (smectite) most likely because they provide better physical properties in terms of plasticity and water retention than kaolinite. Finally, phytoliths and bioavailable Si (Si-CC) contents were increased in TM in comparison with Ctrl(70)(-1)(20), suggesting an incorporation of phytoliths in termite construction through their saliva and/or an increasing availability of Si-CC from the minerals. In conclusion, this study highlights how termites, through their feeding and building activities, impact Si distribution in tropical soils.

International audience; Among policy institutions, researchers in other fields, and the public, there is an enduring misunderstanding of the nature of desertification. To a considerable extent, its meaning has been reduced to just two eye-catching images: sand dunes encroaching on productive land and habitations and bare, cracked soil surfaces. Yet neither of these is an indicator of desertification-sometimes called dryland degradation. Rather desertification results in a wide range of changes, including erosion , loss of biodiversity, decline in soil fertility, and reduced carbon storage. Surprisingly, it is the pictures themselves, not the scientific literature, that have fixed this erroneous concept in the minds of even some who work in the field of land degradation. The results of this confusion include inappropriate management efforts, mistaken premises in research, and ill-informed policies at local to global scales including misleading prognostications of institutions-even at the level of United Nations agencies.

International audience; In experimental trials the spatial distribution of soil properties is a key factor to evaluate and determine the credibility of the research outcomes. To evaluate the spatial distribution of soil properties at different depths in the present study, the electrical resistivity method was selected because of its non-destructive nature and high spatial resolution. The investigation was conducted on BT1 (Burning versus mulching) the world's oldest sugarcane trial (established 25 October 1939) located on the premises of the South African Sugarcane Research Institute (SASRI). The experimental area, covering 7200 m(2), is divided in 32 plots with three crop residue retention treatments. These treatments are either fertilized and not fertilized. The RM15 resistance-meter equipped with a multiplexer was used to collect geophysical data. The latter was used to generate apparent electrical resistivity maps corresponding to three approximate depth ranges: (0-0.5 m, 0-1.0 m and 0-2.0 m). The spatial resolution of the shallowest map (0-0.5 m), was 0.5 m x 1.0 m. Bulk density and soil water content, for the 0-0.1 m depths, were also determined from samples collected on a grid of 4 m x 5 m. All data collected were georeferenced using a spatial resolution of 4 m x 1 m with the Digital Elevation Model (DEM). The results of this survey showed the effectiveness of geophysical mapping to reveal detailed information regarding soil spatial variability and to understand and establish the relationships between soil fertility and soil physical properties. Detailed analysis of these results led to the identification of two different soil types on the trial site: a Mollic Cambisol in the upper slope of the trial with a depth of < 1.0 m and a deeper Mollic Nitisol in the lower slope. Additionally, close to the southern edge of the site, higher values of electrical resistivity revealed a layer of colluvial deposits, that has not been considered in the past. The relationships between electrical resistivity and investigated soil properties (soil water content, bulk density, aggregate stability and fertilizer) initially seemed paradoxical. Although there is still a need for further investigations, the current study decently establishes the rather unexpected relationships. The most striking finding was the high resistivity in fertilized plots associated with the application of potassium chloride fertilizer which led to a decreased soil aggregate stability.

L'unité BIOEMCO est devenue IEES en janvier 2014.; International audience; These last twenty years have seen the development of an abundant literature on the influence of soil macrofauna on soil structure. Amongst these organisms, earthworms, termites and ants are considered to play a key role in regulating the physical, chemical and microbiological properties of soils. Due to these influential impacts, soil ecologists consider these soil macro-invertebrates as 'soil engineers' and their diversity and abundance are nowadays considered as relevant bioindicators of soil quality by many scientists and policy makers. Despite this abundant literature, the soil engineering concept remains a 'preach to the choir' and bioturbation only perceived as important for soil ecologists. We discussed in this article the main mechanisms by which soil engineers impact soil structure and proposed to classify soil engineers with respect to their capacity to produce biostructures and modify them. We underlined the lack of studies considering biostructure dynamics and presented recent techniques in this purpose. We discussed why soil engineering concept is mainly considered by soil ecologists and call for a better collaboration between soil ecologists and soil physicists. Finally, we summarized main challenges and questions that need to be answered to integrate soil engineers activities in soil structure studies.

EA SPE BIOmE AGROSUP SUPDAT; International audience; ggregation is an important physical process to study during the early formation of Technosols. It is known to be influenced both by the organic matter content and soil biota. Constructed Technosols represent good models to test the importance of these factors since their composition can be easily manipulated by mixing different proportions of parent materials and introducing soil organisms. In this study, we performed a 5 month mesocosm experiment, using excavated deep horizons of soils (EDH) as mineral material mixed with green waste compost (GWC) at six different proportions (from 0 to 50%) in the presence or absence of plants and/or earthworms. After 21 weeks of incubation, aggregation was characterized by: 1) determining the size fraction and morphology, 2) measuring the distribution of organic carbon (OC) in each fraction and 3) testing the aggregate stability. Results showed that organisms accounted for 50% of soil aggregation variance while GWC was responsible for only 5% of the variance. The percentage of total variance of OC distribution in aggregates explained by organisms, GWC, and the interaction of the two was similar (28%, 22% and 26%, respectively). The effect of GWC on structural stability was negligible (2%) compared to that of organisms (70%). The effect of earthworms combination with plants was complex: plants had a dominant effect on the distribution of the size of aggregates by disrupting earthworm casts, but earthworms had a dominant effect over plants for aggregate stability under fast wetting only when the percentage of compost was low. This study underlines the importance of considering the interaction of the organic matter and soil biota: in this case, increasing compost proportion in a Technosol has significant effects on aggregation only in the presence of plants or earthworms.

International audience; Gully erosion and woody plant encroachment are frequently observed in grasslands worldwide. Soil aggregate stability is one of the drivers of gully erosion and needs to be studied, especially in the context of both tree encroachment and gully erosion. This study compared the effects of tree encroachment [paperbark thorn; Vachellia sieberiana (DC.) Kyal. & Boatwr. previously called Acacia sieberiana DC.] and soil surface horizon properties on the aggregate stability (as measured by mean weight diameter, MWD) in a degraded semihumid grassland in the KwaZulu-Natal province of South Africa. Soil properties from under trees (UT) and open areas (OA) of both an Entisol within deep gullies (locally called dongas) and Alfisol from grasslands outside the gullies (grassland). The MWD values were higher in the grassland (3.06 +/- 0.21 mm) than in the dongas (1.84 +/- 1.03 mm). In the grassland, the MWD was significantly higher in the OA than UT, associated with more fine roots in the OA. In the donga, soil properties UT were not significantly different than those in OA. The soil aggregate stability showed strong local variability that depends on the age of the donga and the nature of the material that composes the pediments. Illite was associated with higher MWD values. The presence of interstratified illite-vermiculite located in active donga increases the shrink-swell potential of the aggregates, exacerbating their dispersion reflected in the low MWD values measured, especially when C and root contents were also low. These results suggest that the local soil clay mineralogy controls the orientation and extension of gullies rather than soil chemistry, topography, or the size of the drainage area.

Because of technical difficulties in measuring soil properties at a large scale, little is known about the effect of soil properties on the spatial distribution of trees in grasslands. We were interested in the associations of soil properties with the phenomenon of tree encroachment, where trees increase in density at the expense of grasses. The spatial variation of soil properties and especially soil texture may modify the properties of hydraulic conductivity, and the availability of soil water and mineral nutrients, which in turn may affect the spatial distribution of encroaching trees. Through the development of a geophysical method (Slingram) using an electromagnetic device EM38 and Bayesian inversion, we were able to accurately map soil electrical conductivity (EC) of a Luvisol in a grassland of South Africa. EC measured at the 08 to 2m depth on a 15ha area is a proxy for clay content and was correlated with the spatial distribution of four size classes of the encroaching Acacia sieberiana. Tree location (all sizes considered) was significantly correlated with EC. Tall acacias (>3m height) were totally absent from patches with EC >24mSm(-1). For all other size classes from medium trees to seedlings, tree density decreased with increasing EC. This suggests that high clay contents at depth associated with high EC values may prevent the establishment and/or survival of trees and influence the spatial distribution of A.sieberiana. This result also shows that geophysical tools may be useful for demonstrating important ecological processes. Copyright (c) 2014 John Wiley & Sons, Ltd.

Distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs) in soils depends on environmental parameters such as mean annual air temperature (MAAT) and soil pH. MBT′/MBT′5ME (methylation index of branched tetraethers) and CBT/CBT′ (cyclization ratio of branched tetraethers) are ratios based on the relative abundances of brGDGTs. Using these ratios, global and regional/local calibrations have been established using surface soils, but generally without any preliminary study of soil types. In this study, we reconsider global MAAT–pH/brGDGT calibrations by assigning soil types to the 358 components of a composite soil dataset. Additionally, we investigate brGDGT-derived proxies in five well-described soil profiles along an altitudinal transect in the Mount Fan Si Pan National Park, northwestern Vietnam. Our results show that at the global scale, traditional MAAT–pH/brGDGT relationships per soil type are significantly different and that MAAT (and pH) residuals per soil type may differ as well, for example between Alisols and Podzols. This effect persists when 5- and 6-methyl brGDGTs are quantified separately. Moreover, MAAT (and pH) residuals per soil type are generally not clearly reduced, including in Leptosols, in which 6-methyl brGDGTs are present. In the Fan Si Pan transect, MBT′/CBT–MAAT estimates showed no significant deviation from expected MAATs. We find, however, that soil type effect – related to vegetation changes and contrasting soil organic carbon properties – may bias MAAT–pH estimates in the Vietnamese soil transect. Furthermore, soil depth plays a role which differs between the different Fan Si Pan soil profiles, likely determined by soil type and history.

EABIOmEAGROSUP; Constructed Technosols may be an alternative for creating urban green spaces. However, the hydro-structural properties emerging from the assembly of artefacts have never been documented. The soil shrinkage curve (SSC) could provide relevant structural information about constructed Technosols, such as the water holding capacity of each pore system (macropores and micropores). The objectives of this study were (i) to evaluate the SSC and water retention curve (WRC) to describe the structure of constructed Technosols and (ii) to understand the influence of organic matter content on soil hydro-structural properties. In this study, Technosols were obtained by mixing green waste compost (GWC) with the material excavated from deep horizons of soil (EDH). The GWC was mixed with EDH in six different volumetric percentages from 0% to 50% (GWC/total). The GWC and EDH exhibited highly divergent hydro-structural properties: the SSC was hyperbolic for GWC and sigmoid for EDH. All six mixture treatments (0%, 10%, 20%, 30%, 40% and 50% GWC) exhibited the classical sigmoid shape, revealing two embedded levels of pore systems. The 20% GWC treatment was hydro-structurally similar to the 30% and 40% GWC treatments; so, a large quantity of expansive GWC is unnecessary. The relation with the GWC percentage was a second-degree equation for volumetric available water in micropores, but was linear for volumetric available water in macropores and total volumetric available water. Total volumetric available water in the 50% GWC treatment was twice as high as that in the 0% GWC treatment. By combining SSCs and WRCs, increasing the GWC percentage increased water holding capacity by decreasing the maximum equivalent size of water-saturated micropores at the shrinkage limit and increasing the maximum equivalent size of water-saturated macropores, resulting in an increased range of pore diameter able to retain available water.

International audience; This manuscript focuses on the effects of above-ground earthworm casts on water runoff and soil erosion in steep-slope ecosystems in Northern Vietnam. We investigated the effects of Amynthas khami, an anecic species producing above-ground casts of prominent size, on water infiltration and soil detachment along a land-use intensification gradient: a cultivation of cassava (Mahinot esculenta; CAS), a plantation of Bracharia (Bracharia ruzziziensis; BRA), a fallow (FAL), a fallow after a forest of Eucalyptus sp. (EUC) and a plantation of trees (Acacia mangium and Venicia Montana; FOR). Two scales of studies were considered: (i) at the structure scale (cm2), a water runoff simulation was used to differentiate the effects of casts, free biogenic aggregates that previously belong to casts, and free physicogenic aggregates; (ii) at the station levels, 1-m2 plots were used to determine runoff and soil detachment rates during the rainy season in 2005. A. khami was sensitive to land-use management. Earthworm density was low in all the fields (0–1 ind m−2). The highest densities were found in EUC and FOR and no individual was found in CAS. As a consequence, soil surface in EUC and FOR was covered with casts and free biogenic aggregates (approximately 22 and 8 kgm−2, respectively). In FAL and BRA, casts covered the soil only sparsely with b3 kgm−2. In CAS, soil surface was characterized by free physicogenic aggregates that might be produced by human activity or endogeic earthworms through tillage (approximately 1 kg m−2). Water runoff simulation clearly showed an enhancement of water infiltration with earthworm casting activity. Water runoff was more decreased with casts (R2=0.26) than free biogenic aggregates (R2=0.49). Conversely, physicogenic aggregates were not associated with higher water infiltration. Analyses of runoff and soil detachment rates during the rainy season underlined that the more land-use type have aggregates on soil surface and the less important is surface runoff (R2=0.922). Conversely, no relation occurred between aggregates and soil detachment rate. While above-ground casting activity decreased surface runoff, they were not involved in soil detachment, and therefore soil erosion.

Summary In the Ecuadorian Cordillera, the hardened volcanic ashes (cangahuas) account for 15% of the cultivated area. The soil resulting from the fragmentation of these materials, generally by heavy machinery, shows an apparent stable millimetric structure. However, this new structure is highly susceptible to disintegration under rain, because it contains no organic matter and little clay, and the material itself is readily eroded in consequence. We studied the evolution of soil aggregate stability in two factorial experiments during five cultivation cycles with two kinds of soil preparation and five fertilization treatments. The aggregate stability was not influenced by either kind of soil preparation, nor by large additions of cattle manure (80 t ha−1) or green manure (10 t ha−1), nor by growing a perennial grass. The variation in the aggregate stability seemed to depend on the components inherited from the original volcanic material: in the plots with larger clay content, and with swelling clay minerals, the aggregates were less stable than those composed of isometric fine silt particles.

Summary Water repellency of volcanic ash soils from the Ecuadorian paramo was studied by a combination of extraction and analysis of water-repellent products, Molarity Ethanol Droplet values, water contact-angle measurements by capillary rise, and N2 adsorption isotherms. The undried samples studied are hydrophilic, but exhibit water repellency after moderate drying (48 hours at 30°C). The advancing water contact-angle measured by capillary rise varies from 78° to 89°. These water contact-angles decrease strongly after extraction of organic materials by an isopropanol–water mixture. Elemental analysis, infrared spectra and gas chromatography-mass spectrometry analyses were used to characterize the extracts. The results show that long-chain fatty acids and more complex non-polar alkyl components (waxes) are the main water-repellent materials. The deposition of such extracted materials onto hydrophilic sand leads to the rapid increase of water contact-angle until values close to those measured on the soil samples are achieved. Assuming a coating of the mineral surface by organic hydrophobic products and using Cassie's law, the water contact-angle of extracted materials was computed. The values ranged from 100° to 157°. Nitrogen specific surface areas of the soils studied were very small, indicating a low adsorbent–adsorbate interaction on hydrophobic surfaces. These results partially validate the hypothesis of water-repellent materials that occur as coatings at least after a drying process.

So far, the occurrence of hydric properties (i.e. water retention values at 1500 kPa matrix potential larger than 1000 g kg(-1)) has generally been reported in Andisols characterised by advanced stages of weathering. Moreover, as these properties are due to the presence of short-range-ordered (SRO) minerals, they are frequently better expressed in the subsurface than in the surface horizons of these Andisols. The present paper describes the characteristics of three Hydric Melanudands whose properties distinctly depart from those briefly summarised above. These soils have been found in some of high-altitude (3200-4000 m a.s.l.) paramos of Ecuador where they are likely to cover several thousands of km(2). They either derived from or have been rejuvenated by volcanic ash deposited more recently than 3000 years BP and they develop under climatic and vegetation conditions favouring exceptionally large accumulations of organic matter. As a consequence, the deep (50-80 cm) epipedons of these Melanudands exhibit an unusual combination of properties. Though rich in primary weatherable minerals and virtually devoid of allophane, these epipedons have nevertheless 1500 kPa water contents either close to or even much larger than 1000 g kg(-1). In these Hydric Melanudands, organic colloids are the constituents responsible for their large porosity. Hydric Andisols are known to experience important irreversible physical changes on drying and this behaviour, in turn, makes them very fragile. As the hydric properties occur here in surface horizons, these paramos soils are thus expected to be especially susceptible to any land use change. (C) 2003 Elsevier Science B.V. All rights reserved.

Gypsum minerals forming in pedogenic environments in New Caledonia were examined for their S and Sr isotope compositions and REE distributions. Three possible sources were identified: volcanic, soil-derived and marine. However, differentiation based only on the REE contents and normalized patterns was not possible, but, the best preserved volcanic sample yields a negative EuEu∗ anomaly and the lowest GdNYbN ratio due to the high content of organic matter. Mixing calculations of the δ34S values and 87Sr86Sr ratios confirmed that the marine gypsum has a δ34S value close to + 21‰ and an 87Sr86Sr ratio close to 0.70912, and that the volcanic gypsum has a δ34S value of 0‰ and an 87Sr86Sr ratio of about 0.70331. The soil-derived gypsum samples yield intermediate values, as a result of a mixing between the marine and volcanic end members. Negative δ34S values for the volcanic gypsum are explained by low fO2 of the source rocks, while high 87Sr86Sr ratios for the same gypsum are explained by probable atmospheric input. The data confirm also that biological fractionation of δ34S, if any, is limited.

Quantifying the magnitude of soil organic carbon loss linked to the conversion of forest to arable land in tropical environments requires long-term estimates of soil redistribution trends within the catchment. In order to assess the cumulative soil carbon loss that occurred since deforestation and cropping started in 1967, a multidisciplinary study was carried out in a small catchment (67 ha) with a swamp area in its middle part, characterized by steep slopes under shifting cultivation of upland rice, typical for northern Laos's landscapes. Using 137Cs and total organic carbon (TOC) inventories, it was possible to estimate a soil TOC depletion rate of 0.039 kgC m−2 yr−1 in 42 years (1963–2005), corresponding to ca. 21% of its initial content in the top 10 cm. Because farmers preferred clearing more fertile soils located downhill and leave forest on poorer soils on the crests, the redistribution of 137Cs and TOC along slopes of the catchment was mainly controlled by the topographic position of soils and the cultivation frequency. Estimates of soil TOC accumulation in wetlands with δ13C measurements and sediment volumes by electrical resistivity tomography showed that TOC accumulation rates were linked to land use change since the 1960s with three successive periods corresponding to undisturbed forest, deforestation-first rice crop and rice-fallow rotations. Forest clearing triggered a higher soil TOC delivery to the swamp (19.6 ± 5.5 kgC ha−1 yr−1) over 8 years (1967–1975) than the former undisturbed forest (8.5 ± 1.8 kgC ha−1 yr−1) and crop-fallow rotations over 30 years (1975–2005, 6.4 ± 4.8 kgC ha−1 yr−1). Due to the small size of the area covered by the swamp, only a limited fraction of eroded soil organic carbon (ca. 7 wt.%) was stored in wetlands of the catchment. Mineralization apparently played a key role for soil TOC depletion since first forest clearing.

International audience; Woody plant encroachment in grasslands is a worldwide phenomenon. Despite many studies, the consequences of woody plant encroachment on sub-canopy vegetation and soil properties are still unclear. To better understand the impacts of trees on grassland properties we examined the following questions using a mountainous sub-tropical grassland of South Africa encroached by an indigenous tree, Acacia sieberiana as a case study: (1) Do trees increase sub-canopy herbaceous diversity, quality and biomass and soil nitrogen content? (2) Do large trees have a stronger effect than medium-sized trees on grass and soil properties? (3) Does the impact of trees change with the presence of livestock and position of trees in a catena? We studied grass and non-graminoid species diversity and biomass, grass quality and soil properties during the wet season of 2009. Nitrogen in grass leaves, soil cation exchange capacity and calcium and magnesium ion concentrations in the soil increased under tall Acacia versus open areas. Medium-sized Acacia decreased the gross energy content, digestibility and neutral detergent fibre of grasses but increased the species richness of non-graminoids. Tall and medium Acacia trees were associated with the presence of Senecio inaequidens, an indigenous species that is toxic to horses and cattle. The presence of livestock resulted in a decrease in herbaceous root biomass and an increase in soil carbon and leaf biomass of grass under Acacia. Tree position in the catena did not modify the impact of trees on the herbaceous layer and soil properties. For management of livestock we recommend retaining tall Acacia trees and partially removing medium-sized Acacia trees because the latter had negative effects on grass quality.

International audience; We use the Bayesian method to invert a simple two-layer pedological horizon (1-D with a topsoil and a subsoil) of a surveyed site to be assessed. We show how the Bayesian method is well suited to the determination of topsoil/subsoil features, and can be used in particular as a tool for the analysis of parameters to be retrieved in terms of information content. Our approach is devoted mainly to the assessment of topsoil thickness, and of topsoil and subsoil conductivities, which are provided in terms of probability density functions. We first summarize the methodology implemented with the Geonics EM38-MK2 conductivity meter, and discuss the adaptation of field procedures and post-processing methods to mitigate the effects of drift and bias. We briefly review some non-Bayesian approaches, and then develop the Bayesian approach for the context of our geophysical survey, highlighting its merits. Positivity constraints (on thickness and conductivity) are included in the form of log parameters. A priori knowledge, based on an objective choice made by the geophysicist, is naturally included in the Bayesian scheme. We discuss the equivalence problem associated with the application of the Slingram method to soil structure analysis. The survey of a luvisol at the Kwazulu-Natal (South Africa) site of Potshini is used to illustrate an ecological application of the Slingram and Bayesian methods, used to define the geo-electrical structure of the near-surface soil. These algorithms have demonstrated their usefulness in mapping the clay content of the Bt horizon associated with the control of encroaching trees. (C) 2013 Elsevier B.V. All rights reserved.

Gully erosion and woody plant encroachment are frequently observed in grasslands worldwide. Gully erosion driven by water processes is usually affected by topography, land-use change and vegetation cover. We hypothesised that trees, through their potential link with overland and subsurface flow, may have an impact on gully extension. However, very few studies have simultaneously considered tree encroachment and gullies. We used aerial photographs to study Acacia sieberiana encroachment and gully erosion in a South African grassland (KwaZulu-Natal Province) for a period lasting 64 years. At the catchment scale, results showed that acacias started invading after 1976 and transformed the grassland into a savanna with 9.45% of tree cover in 2009. Gully area increased by 3.9% in the last 64 years and represented 12.76% of catchment area in 2009. Mean estimated sediment loss was 200 Mg ha− 1 of gully y− 1, indicating a high erosion rate mainly due to the collapse of gully banks after swelling and shrinking. Volumetric retreat rate (V) of 15 gully heads was correlated with drainage area (Drain.A) by a power function explaining 64% of the variance: V = 0.02*Drain.A0.83. A positive correlation between gully retreat rate and Acacia canopy area was measured between 2001 and 2009 when established tree encroachment was observed. These results, associated with the susceptibility of this soil to subsurface flow and the observation of pipe erosion systems in the field, showed that both surface and subsurface processes occur in this sub-humid grassland and that trees can be indirectly associated with increased gully erosion.

Gypsum crystals occur in Vertisols on the west coast of New Caledonia. These soils are derived from the weathering of basic rocks that contain no gypsum. The sulphur isotopic ratio 34 S/ 32 S was used as tracer to identify the origin of the gypsum. A total of 26 measurements of δ 34 S were made on gypsum crystals from the Vertisols and compared to those of continental sulphates and to marine gypsum in tidal flats. The δ 34 S value of the crystals located in New Caledonian Vertisols decreases with increasing distance from the coast. This suggests that the gypsum in the Vertisols comes from two sources: an oceanic source where the major part of the sulphur is introduced by atmospheric precipitation, and a continental source where sulphides disseminated in various subsurface deposits are oxidized. Gypsum from atmospheric precipitation penetrates the Vertisols through vertical cracks that probably formed in dry seasons during the last glacial period.

International audience; Gully erosion is one of the main causes of soil loss in drylands. Understanding the dominant mechanisms of erosion is important to achieve effective erosion control, thus in this study our main objective was to quantify the mechanisms involved in gully bank retreat as a result of three processes, falling of entire soil aggregates, transport of soil material by splash and by water running along gully banks (runoff). during rainfall events. The study was conducted in the sloping lands of the KwaZulu-Natal province, a region that is highly affected by gully erosion. Artificial rain was applied at 60 mm h(-1) for 45 min at the vertical wall of a gully bank typical to the area. The splash material was collected by using a network of 0.045 m(2) buckets. The sediments in the running water were assessed by sampling the runoff collected from a microplot inserted within the base of the bank, and collecting the fallen aggregates after the rainfall simulation was complete. Results indicated that the overall erosion for the simulation was 721 g m(-2) h(-1). Runoff erosion proved to be the dominant mechanism and amounted to 450g m(-2) h(-1), followed by splash and fall down of aggregates (about 170 g m(-2) h(-1)). Gully bank retreat occurred at a rate of 0.55 mm h(-1) and assuming that the soil bulk density is 1.3 g cm(-3), this corresponds to a retreat of 8.8 mm y(-1). Extrapolations to the watershed level, where about 500 m(2) of gully bank are observed per hectare, would lead to an erosion rate of 4.8 t ha(-1) y(-1). These limited results based on a simulated storm show that the three main mechanisms (runoff, splash and fall down of aggregates) are responsible for the retreat of gully banks and that to mitigate gully erosion, appropriate measures are required to control all three mechanisms. Further research studies are needed to confirm and to scale up, both in time and space, as these data are obtained at one location and from a single artificial storm. (C) 2010 Elsevier B.V. All rights reserved.

High mountain ecosystems are generally considered to be particularly sensitive to global climate change. Studies of pedogenesis associated with altitudinal variation, vegetation type and soil carbon content on the same type of parent rock are very limited in inter-tropical mountain areas. Therefore the altitudinal variation of soil pedogenesis through 9 selected profiles from the altitude of 1340 m to 3143 m asl, the summit of the Fan Si Pan Mountain, in the north of Vietnam was examined. Fan Si Pan Mountain is composed of a homogenous alkaline granite rock and is the highest point of the Inter-tropical Continental Asia. The Soil Organic Matter properties (C, N, δ 13 C and δ 15 N contents) of the different grain-size fractions of the topsoil of 4 selected profiles corresponding to different ecosystems were also examined. Three zones of different soil forming processes were present: Acrisols and Alisols at lower altitudes in sub montane forest, Podzols formed in montane and upper montane forest while Umbrisols formed at high altitude where the forest vegetation had given way to a shrubby vegetation or a steppe composed of dwarfed bamboo. With altitude, soils become sandier, have higher concentration of SiO 2 and are lower in Al 2 O 3 . The selective Fe and Al oxalate (Fe o and Al o ) and pyrophosphate (Fe p and Al p ) extracts show a clear discrepancy between Acrisols or Umbrisols with no clear variation with soil depth and Podzols with high enrichment in their Bs horizon. The SOM status is highly dependent of the organic matter input by the vegetation. In Acrisols, the SOC is linked to the fine fraction within the soil profile with rapid turnover and low C/N values. Podzols are formed by the accumulation on soil surface of OM enriched in the coarse fraction with depleted δ 15 N and high C/N values. The organic matter input is exogenic and probably seasonal from leaves forming the surface litter while in Acrisols or Umbrisols the SOC is mainly linked to the fine fraction, and with constant δ 15 N values at depth. In Umbrisols, the SOC origin seems to be linked with endogenic inputs deep in the soil profile mostly produced by the decay of bamboo roots. In this tropical mountainous soil catena, the soil carbon mineralization depends not only on temperature and organo-metallic complexes that stabilize the non labile carbon pool but also is controlled by the pedogenetic process, which is linked with the vegetal ecosystem change with altitude.

In most regions of the world overgrazing plays a major role in land degradation and thus creates a major threat to natural ecosystems. Several feedbacks exist between overgrazing, vegetation, soil infiltration by water and soil erosion that need to be better understood. In this study of a sub-humid overgrazed rangeland in South Africa, the main objective was to evaluate the impact of grass cover on soil infiltration by water and soil detachment. Artificial rains of 30 and 60 mm h−1 were applied for 30 min on 1 m2 micro-plots showing similar sandy-loam Acrisols with different proportions of soil surface coverage by grass (Class A: 75–100%; B: 75–50%; C: 50–25%; D: 25–5%; E: 5–0% with an outcropping A horizon; F: 0% with an outcropping B horizon) to evaluate pre-runoff rainfall (Pr), steady state water infiltration (I), sediment concentration (SC) and soil losses (SL). Whatever the class of vegetal cover and the rainfall intensity, with the exception of two plots probably affected by biological activity, I decreased regularly to a steady rate

Although dung beetles are known to perform a multitude of ecosystem services, their effects on water infiltration, runoff, porosity, moisture and erosion of soil have never been thoroughly researched. Maintenance of these hydrological properties is important in agro-ecosystem functioning where overgrazing results in negative impacts on the soil. The study site was located in the Potshini catchment in Kwazulu-Natal (South Africa), an area heavily grazed by livestock. We conducted two rainfall simulations on three 1 m(-2) control (no dung) and six dung-treated plots in December 2008, and repeated the study in June 2009 on the same plots. Natural populations of dung beetles were allowed to colonise the dung. Simulations were conducted for 30 min at an intensity of 30 mm h(-1). Key variables calculated were pre-runoff amounts (Pi), infiltration ratios (Ki), and soil losses. Samples were collected for bulk density determination during the same time periods in order to measure differences in porosity and moisture in control and dung-treated plots at different depths. Using multivariate statistics we found significant differences between dung-treated and control plots in three of four simulations. After 48 h of beetle activity, Pi and Ki values were significantly increased and remained at elevated levels six months later. Soil losses were initially higher in dung-treated plots than controls, but had declined to less than control values after six months. Bulk density in the A-horizon (0-10 cm) was significantly reduced after 48 h of beetle activity and remained so for six months. No difference in bulk density was observed at greater depths. Soil moisture initially increased significantly in the A-horizon, as well as at 20 and 30 cm depths after six months of activity. We conclude that dung beetles positively influence hydrological properties of the soil by increasing water infiltration and soil porosity, and reducing surface water runoff. Contrasting effects on soil losses are problematic to reconcile from this study. High losses initially observed may be offset in the long-term by reductions associated with the increased infiltration ratios, though this remains to be confirmed.

Conventional soil science methods for the estimation of the spatial variations of soil properties within landscapes are destructive, time consuming, and do not allow the estimation of the short range variability. Recent advances in geomatic global positioning systems and sensors offer new possibilities for the mapping of spatially varying soil patterns. Although geophysical techniques offer an alternative to traditional soil sampling methods, the resulting data are still often misinterpreted, especially in terms of the inter-relationships between, for example, soil apparent electrical resistivity ( ρ ) and several soil physical or chemical properties. Our main objective was to test the suitability of using ρ measurements for mapping the thickness of the organo-mineral A-horizon ( ThickA ). ρ was mapped (0–0.5 m), using an RM15, in a highly water eroded field plot of 300 m² showing large variations in ThickA , but no significant variations in soil clay and nutrient content. The correlations between ρ and ThickA , the top-soil water content ( θ 0.1 ), and the bulk density ( BD 0.1 ) were determined at the nodes of a regular grid of 2 m ( n = 96). Principal Component Analysis (PCA) was used to identify the impact of the different soil properties on the ρ data. ThickA varied between 0.17 and 0.3 m, and ρ between 1046 and 3864 Ω m . There was a significant correlation between ρ and ThickA ( r = 0.56) and also between θ 0.1 ( r = 0.23) and BD 0.1 ( r = 0.22). If it was assumed that θ 0.1 and BD 0.1 were constant over the study plot then there was a correlation between ρ and ThickA with an r of 0.88, thus making possible the accurate mapping of ThickA (MAE = 0.017 m; i.e., 7% of the average value of ThickA ). Prediction (MAE = 0.015 m) was only slightly improved through the use of regression kriging of ThickA with ρ recalculated as the covariate. This result showed that apparent soil electrical resistivity measurements could be very successful for estimating A-horizon thickness provided that limited information on top-soil characteristics is included in the mapping process.

The main objectives of this study were to evaluate soil organic C (SOC) variability in a representative hillslope of Laos and to quantify the impact of some environmental factors. We collected 2348 soil samples from 581 georeferenced soil pits within a hillslope of northern Laos under traditional shifting cultivation at 0- to 0.05-m depth and then every 0.1 m to 0.35-m depth. The SOC stocks at 0- to 0.05-m depth varied between 0.4 kg C m(-2) (standard error of 0.046 kg C m(-2)) and 1.9 (+/- 0.22) kg C m(-2) and stocks in the 0- to 0.35-m depth were between 2.6 (+/- 0.29) and 11.4 (+/- 1.31) kg C m(-2). About 85% of SOC spatial variability occurred at a distance less than 20 m. As expected, SOC content and stocks at 0- to 0.05-m depth were significantly greater with higher soil clay content and shorter durations Of cultivation (P < 0.001). Bur at 0- to 0.35-m depth, the significance of clay content was only P = 0.04 and stocks surprisingly increased with increasing slope gradient (P < 0.001). Thus, it seems that sloping lands under shifting cultivation act as a conveyor that stores atmospheric inorganic C in soils during the regeneration of natural fallows and ultimately transfers it by water erosion to the steepest areas of hillslopes, where it accumulates, probably due to greater infiltration by water. These results on SOC spatial variations under steep slope conditions of the tropics give a better picture of SOC dynamics that may allow development of optimal strategies of land management to foster main soil functions and offset the current rise in atmospheric CO2.

Rapid changes in upland farming systems in Southeast Asia generated predominantly by increased population pressure and 'market forces' have resulted in widespread land degradation that has been well documented at the plot scale. Yet, the links between agricultural activities in the uplands and downstream off-site effects remain largely unknown because of the difficulties in transferring results from plots to a larger scale. Many authors have thus pointed out the need for long-term catchment studies. The objective of this paper is to summarize the results obtained by the Management of Soil Erosion Consortium (MSEC) over the last 5 years from 27 catchments in five countries (Indonesia, Laos, Philippines, Thailand, and Vietnam). The purpose of the study was to assess the impacts of cultivation practices on annual runoff and erosion rates. Initial surveys in each catchment included topography, soils and land use. Monitoring included climatic, hydrologic and erosion (total sediment yield including bed load and suspended sediment load) data, land use and crop yields, and farmers' income. In addition, new land management options were introduced through consultations with farmers and evaluated in terms of runoff and erosion. These included tree plantations, fruit trees, improved fallow with legumes, maize intercropped with legumes, planted fodder, native grass strips and agro-ecological practices (direct sowing and mulch-based conservation agriculture). Regressions analyses showed that runoff during the rainy season, and normalized runoff flow coefficient based on erosive rainfall during the rainy season (rainfall with intensity exceeding 25 mm h(-1)) increase with the percentage of the catchment covered by maize. Both variables decrease with increasing soil depth, standard deviation of catchment slope (that reflects terrain roughness), and the percentages of the catchment covered by fallow (regular and improved), tree plantations and planted fodder. The best predictors of sediment yield were the surface percentages of maize, Job's tears, cassava and footpaths. The main conclusions generated from this study were: (i) soil erosion is predominantly influenced by land use rather than environmental characteristics not only at the plot scale but also at the catchment scale; (ii) slash-and-burn shifting cultivation with sufficiently long rotations (I year of cultivation, 8 years of fallow) is too often unjustly blamed for degradation; (iii) in its place, continuous cropping of maize and cassava promotes high rates of soil erosion at the catchment scale; (iv) conservation technologies are efficient in reducing runoff and total sediment yield at the catchment scale; (v) the adoption of improved soil management technologies by upland farmers is not a function of the degree of intensification of their farming system and/or of their incomes. The results suggest that if expansion of maize and cassava into already degraded upland systems were to occur due to increased demand for biofuels, there is a risk of higher runoff and sediment generation. A failure to adopt appropriate land use management strategies will result in further rapid resource degradation with negative impacts to downstream communities.

The rehabilitation of indurate pyroclastic formations of the Ecuadorian Sierra (cangahua) paves the way for the development of new agricultural areas. The material derived from the fragmentation of the hardened volcanic ashes is strongly prone to pluvial erosion, essentially because it has a fine silty-sandy texture, and because contains no organic matter and no clay minerals. Rainfall simulation was implemented before and after three cycles of cultivation to asses the evolution of soil structure and its susceptibility to erosion. The cultivated plots were < 1% slope and the rainfall simulation tests were conducted after the harvest on bare surfaces. Two soil preparations, (coarse and fine) and four different agricultural practices, (organic matter, green manure, mineral fertilization, and zero fertilization) were evaluated; as well reference/control plots (uncultivated bare plots). Surface soil crusting occurred rapidly within the cultivated plots when compared to the recent tilled cangahua. Runoff and soil loss were generally higher on plots with lower structural stability and generally exhibited higher clay content. In agreement with the structural stability measured in a laboratory, organic matter inputs increase the soil porosity. This condition had no effect on the structural stability of the soil, resistance to crusting, and therefore little impact on surface runoff and erosion. The preferred option for land with these soils would be no tillage and a permanent soil cover (pasture) if they were to be utilised for agriculture.

Earthworms are known to significantly influence soil aggregate stability and hence to regulate underlying ecosystem functions such as soil organic matter (SOM) decomposition and soil erosion. However the, available research information on this aspect in the tropics, and especially in Asia, is rather scanty. The aims of this paper are therefore to study the influence of Amynthas khami, an anecic earthworm species, on soil structural properties on one hand, and to determine the factors involved and the consequences for the soil matrix properties on the other hands. Investigations were carried out in different management systems ranging from low to high above-ground casts density. We measured the physical and chemical properties and water behavior (mean weight diameter: MWD, and water drop penetration time: WDPT) of casts and those of the surrounding soil aggregates (control). Pore space was quantified using a morphological approach and 2D image analysis. Soil aggregate stability was primarily specific for each-land use type and afterwards affected by earthworm activity within each ecosystem. Earthworm casts were always more stable than control surrounding soil aggregates. Casts were characterized by higher pH. C, polysaccharides, Ca2+, Mg2+ and K+ contents. Conversely, the particle size fraction and the content of oxides (Al-cbd and Fe-cbd) were not significantly affected by earthworm activity. Soil porosity was sharply decreased in casts as compared to the surrounding soil aggregates and casts were characterized by the absence of packing void poroids and large pores > 50,000 mu m(2). Regressions between soil physico-chemical properties and MWD underlined the relationships between SOM (C and polysaccharide content), pH, Ca2+ content, soil porosity, and the structural stability of cast aggregates. Surprisingly, the abundance of casts on the soil surface was not related to the bulk density of the soil matrix, suggesting that cast aggregates are not integrated into it or that they are integrated with different soil structural characteristics.

Two consecutive years of investigation on soil surface features, surface runoff and soil detachment within 1-m(2) microplots on 40% slope highlighted the effects of land-use change, vegetation cover and biological activity on the water pathways in Northern Vietnam. Three replicate plots were setup on each of five land-uses: cassava (CAS), grass fodder of Bracharia ruziziensis (BRA), a 3-year old fallow (FAL), tree stands of Acacia mangium and Venicia montana (FOR), and a fallow with regrowth of Eucalyptus regularly cut (EUC). The second year, two of the microplots under FAL and EUC were treated with herbicide (FALh, EUCh), one of them was burnt (FALh+b, EUCh+b). The highest yearly surface runoff coefficient of 16%, and soil detachment rate of 700 g m(-2) yr(-1) in average with a maximum of 1305 g m(-2) yr(-1) have been recorded under CAS. On FALh and FALh+b, runoff ratios were 8.7 and 13.5%, respectively and detachment rates were 86 and 389 g m(-2). On FAL and BRA the yearly runoff ratio varied from 5.9 to 9.8% but the detachment rate was limited at 24 to 35 g m(-2). FOR and EUC annual runoff was

This manuscript focuses on the effects of land-use change and earthworm activities on the dynamics of dissolved nutrients (carbon and mineral nitrogen) in a steep slope ecosystem in Northern Vietnam. We investigated the properties of soil aggregates (casts and surrounding soils) sampled in different agrosystems: a plantation of Bracharia ruzziziensis (BRA), a fallow (FAL) and a plantation of Acacia mangium and Venicia montana (FOR), following a cultivation of cassava (CAS), and a fallow after a forest of Eucalyptus (EUC). Soil physical, chemical and biological properties were determined on the sampling date and dissolved organic C (DOC), and mineral N (N min) contents were followed during a 21-day incubation period. CAS, BRA and FOR were characterised by a high rate of N mineralisation, followed by a rapid loss of N min. Conversely, FAL and EUC were more interesting in terms of soil fertility because these systems had higher soil N min content that could become available to plants. Lost of C through DOC leaching was very low with values ever less than 0.5% of the total soil C content. The greatest lost of C through leaching was in FAL, EUC and FOR. The impacts of earthworms on the soil seemed site-specific. The protection of organic matter in earthworm casts varied with the initial substrate soil and agroecosystem management. Casts were characterised by greater enzymatic activities, except for alkaline phosphatase, than the surrounding soil. While SOM mineralisation was not affected in casts collected in FAL, EUC and FOR, the leaching of N min was increased over the surrounding soil. Conversely, mineralisation of SOM and nitrification activity were less in BRA-casts than in the surrounding soil, causing a greater retention of N min in soil. While the DOC leached from casts increased in BRA, it was reduced in FAL and was similar in the other sites. Our study indicates that while land-use change occurred only 3 years ago, the extent of leaching was mostly regulated by the type of agroecosystem and by earthworm activities.

Soils of tropical regions account for ca. 30 % of the terrestrial organic carbon pool. Present-day fires, deforestation and land use changes enhance soil erosion and soil organic matter mineralization, increasing as much the CO2 flux to the atmosphere and reducing the efficiency of organic carbon storage in soils. This chapter presents the results of several isotopic studies carried out on two cultivated watersheds with steep slopes located in tropical regions (Venezuela, Laos) in order to better: 1) link the erosion status and the organic carbon content of cultivated soils by coupling 137Cs and total organic carbon measurements; 2) constraint the source of suspended organic matter during flood events by monitoring the composition of suspended loads in runoff and stream flows with d15N and d13C measurements and; 3) evaluate the impact of a water reservoir set up at the outlet of a watershed on suspended organic carbon fluxes generated by erosion. The redistribution pattern of fallout 137Cs and the soil organic carbon content in the first 30 - cm layer of cultivated soils of the Houay Pano catchment (Laos) are significantly linked. A common process apparently relates the soil erosion status with the amount of organic matter contained in the topsoil horizons. However, the budgets based on soil organic carbon depletion trends are higher than those derived from 137Cs soil erosion rate models. This difference provides evidence for additional organic carbon losses, most likely controlled by mineralization of soil organic matter and dissolved organic carbon release by runoff. The d15N composition of fine-sized suspended organic matter transported by runoff and stream flows during flood events on the Rio Boconó watershed (Venezuela) highlights the contribution of particulate organic carbon derived from incisive erosion of organic matter bearing rocks (“fossil” organic carbon) of the geological basement (i.e.: shales and argillaceous rocks of the river banks). The extent of “fossil” organic carbon release depends on the intensity of the flood event. During an intense event with a return period of 10 years, the contribution of “fossil” organic carbon to suspended sediments was approximately equivalent to that of soils sensu stricto Monitoring the composition of fine (

The paramo is a high-altitude ecosystem of the northern Andes. The vegetation is continuous, with grasses as the dominant groundcover. Because of their high water retention, paramos play a fundamental role in water availability for all the population of the inter-Andean valleys. There are many studies of this specific ecosystem, but very few are focussed on overgrazing and its effect on vegetation and soil properties. Intensive grazing started less than 20 years ago and was studied in a representative area in the western Cordillera of central Ecuador covered by recent volcanic ash deposits. Intensive sheep grazing has led to a strong decline in the number of plant species, the replacement of the tussock grass vegetation by a short carpet grass vegetation, and an increase of bare land. In that area, the upper 50 cm of Andisols are deeply affected by a convergent decrease of Al and Fe oxalate and pyrophosphate in soil extracts, carbon contents decrease from 100 g kg -1 to less than 50 g kg -1 in the humid zone, from 70 to 40 g kg -1 in the dry zone and a reduction of water-retention capacity at -33 kPa matrix potential from 800 g kg -1 to 200 g kg -1 in humid zones, from 350 to less than 100 g kg -1 in drier areas. They showed also a decrease in the macrostructure and the development of a highly water repellent microstructure. All these important transformations favour the development of aeolian erosion in dry areas, runoff on bare surfaces, and gully erosion on slopes. The role of the paramo in water-regulation of this ecosystem seems to have been adversely affected for the future.

In northern Ecuador, soils of high altitude grasslands (paramos) are mainly non-allophanic Andisols developed on Holocenic volcanic ash. These soils have a high water retention capacity and are the “water tank” of central Ecuador. To assess the effect of land use (burning and tillage) on soil hydrodynamic properties, rainfall simulation was conducted at two different sites. At Pichincha near Quito, the simulation was conducted on a recent volcanic ash soil comparing natural, tilled and burned plots. At El Angel, the simulation was conducted on a mature non-allophanic Andisol comparing natural, recently tilled and formerly cultivated plots. On natural plots, the infiltration rate was very high and sediment loss very low. Results for infiltration rate and runoff indicated that land use change on paramos increased runoff flow and reduced saturated hydraulic conductivity. Superficial reorganisation of the soil surfaces occurred on tilled plots at both sites. This crusting process was fast and resulted in surfaces with very low conductivity at Pichincha. The same processes seemed to be slower at El Angel. The soil surface of recent Andisols at Pichincha was prone to crusting whereas the mature Andisol, at El Angel, with a lower bulk density, was compacted when the kinetic energy of raindrops was high. Water repellency occurred after burning at Pichincha and following long natural air drying after tillage in the non-allophanic A horizon at El Angel. Water repellency combined with the low bulk density of soil aggregates explain the intensity of sediment losses in the abandoned soils after cultivation (Bare fallow plots). Erosion occurred in these areas through floating hydrophobic and stable aggregates.

In the Tamoa Valley, on the western coast of New Caledonia, a Vertisol sequence shows a downslope transition from calcimagnesic Vertisols containing gypsum, aragonite, and magnesium-calcite, derived from flysch colluvium, to hypermagnesic Vertisols, rich in dolomite and magnesite, and derived from peridotite and serpentinite alluvium deposited by the Tamoa River. Gypsum, magnesium-calcite and magnesite were probably formed during the dry period of the last glaciation, between 30,000 and 17,000 yr B.P. Magnesite nodules seem to be allochtonous, and were formed upstream by the weathering of serpentinite. During the more humid period of last Flandrian transgression (5000 yr B.P.), floods of the magnesium-rich waters of the Tamoa River over the alluvial plain brought magnesium to the soils and lead to fluctuations of the water table. These conditions favoured the formation of manganese deposits as mangans, calcite pseudomorphs after lenticular gypsum, crystallization of authigenic barite in calcimagnesic Vertisols, and dolomite formation in magnesic and hypermagnesic Vertisols. During the recent period, apparently drier, aragonite is forming in the lower part of calcimagnesic Vertisols.