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Chaplot V.,Center Ird Dile Of France | Chaplot V.,University of KwaZulu - Natal | Jewitt G.,University of KwaZulu - Natal | Lorentz S.,University of KwaZulu - Natal
Physics and Chemistry of the Earth | Year: 2011

The identification of runoff source areas is essential for Integrated Water and Resources Management (IWRM). Although direct methods for the determination of steady-state water infiltration in soils (Inf) do exist, these are tedious and time-consuming. Geophysical techniques offer an alternative, however, geophysical data are often misinterpreted, especially in terms of the inter-relationships between soil apparent electrical resistivity (Rho) and Inf and several other soil physical or chemical properties. This paper evaluates the magnitude of the extend Rho measurements might allow prediction of Inf. This study was conducted in the Kwazulu-Natal province of South Africa where surface runoff arising from the steep slopes has a large impact in land degradation. Measurements of Rho with an RM-15 resistance meter were taken within a 10×30m plot showing similar sandy-loam Acrisols but different proportions of soil surface coverage by plants (from 0-5% to 75-100%), depth to the clayey Bw horizon (D2B), top-soil (0-0.1m) water content (θ) and bulk density (BD). There was a low correlation between Rho and Inf obtained under controlled conditions of rainfall (30mmh-1during 45min) at fifteen 1m2 micro-plots (r2=0.30). However, the correlation with the normalized Rho (Rhon) as if D2B, θ, and BD were constant over the study plot and equal to their average value, was much higher (r2=0.66), pointing out the need to consider the complex and multiple correlations between soil properties and Rho in an attempt to map the spatial variations of Inf. Finally, the use of Rhon as a co-kriging co-variate appeared to significantly improve the short range spatial prediction of water infiltration in soils and thus IWRM implementation. © 2011 Elsevier Ltd. Source

Rumpel C.,French National Center for Scientific Research | Chaplot V.,University Pierre and Marie Curie | Ciais P.,French Climate and Environment Sciences Laboratory | Chabbi A.,French National Institute for Agricultural Research | And 2 more authors.
Biogeosciences | Year: 2014

In order to assess whether eroded carbon is a net source or sink of atmospheric CO2, characterisation of the chemical composition and residence time of eroded organic matter (EOM) at the landscape level is needed. This information is crucial to evaluate (1) how fast EOM can be decomposed by soil microbes during its lateral transport and (2) its impact at deposition sites. This study considers a continuum of scales to measure the composition of EOM across a steep hillslope landscape of the Mekong basin with intense erosion. We sampled suspended sediments eroded during rainfall events from runoff plots (1 and 2.5 m2) and the outlets of four nested watersheds (0.6 × 104 to 1 × 107 m2). Here we show that changes in the chemical composition of EOM (measured by nuclear magnetic resonance spectroscopy) and in its 13C and 15N isotope composition from plot scale through to landscape scale provide consistent evidence for enrichment of more decomposed EOM across distances of 10 km. Between individual soil units (1 m2) to a small watershed (107 m2), the observed 28% decrease of the C/N ratio, the enrichment of 13C and 15N isotopes as well as O-alkyl C in EOM is of similar magnitude as changes recorded with depth in soil profiles due to soil organic matter "vertical" decomposition. Radiocarbon measurements indicated ageing of EOM from the plot to the watershed scale. Therefore transport of EOM may lead to enrichment of stabilised soil organic matter compounds, eventually being subject to export from the watershed. © Author(s) 2014. Source

Kim J.-H.,Netherlands Institute for Sea Research | Zell C.,Netherlands Institute for Sea Research | Moreira-Turcq P.,Center Ird Dile Of France | Perez M.A.P.,Federal University of Amazonas | And 6 more authors.
Geochimica et Cosmochimica Acta | Year: 2012

In order to trace the transport of soil organic carbon (OC) in the lower Amazon basin, we investigated the distributions of crenarchaeol and branched glycerol dialkyl glycerol tetraethers (GDGTs) by analyzing riverbed sediments and river suspended particulate matter (SPM) collected in the Solimões-Amazon River mainstem and its tributaries. The Branched and Isoprenoid Tetraether (BIT) index, a proxy for river-transported soil OC into the ocean, was determined from the distributions of these GDGTs. The GDGT-derived parameters were compared with other bulk geochemical data (i.e. C:N ratio and stable carbon isotopic composition). The GDGT-derived and bulk geochemical data indicate that riverine SPM and riverbed sediments in the lower Amazon River and its tributaries are a mixture of C 3 plant-derived soil OC and aquatic-derived OC. The branched GDGTs in the SPM and riverbed sediments did not predominantly originate from the high Andes soils (>2500m in altitude) as was suggested previously. However, further constraint on the soil source area of branched GDGTs was hampered due to the deficiency of soil data from the lower montane forest areas in the Andes. Our study also revealed seasonal and interannual variation in GDGT composition as well as soil OC discharge, which was closely related to the hydrological cycle. By way of a simple binary mixing model using the flux-weighted BIT values at óbidos, the last gauging station in the Amazon River, we estimated that 70-80% of the POC pool in the river was derived of soil OC. However, care should be taken to use the BIT index since it showed a non-conservative behaviour along the river continuum due to the aquatic production of crenarchaeol. Further investigation using a continuous sampling strategy following the full hydrological cycle is required to fully understand how soil-derived GDGT signals are transformed in large tropical river systems through their transport pathway to the ocean. © 2012 Elsevier Ltd. Source

Chaplot V.,Center Ird Dile Of France | Chaplot V.,University of KwaZulu - Natal | Bouahom B.,SSLCC NAFRI | Valentin C.,Center Ird Dile Of France
Global Change Biology | Year: 2010

Surface soils, which contain the largest pool of terrestrial organic carbon (C), may be able to sequester atmospheric C and thus mitigate climate change. However, this remains controversial, largely due to insufficient data and knowledge gaps in respect of organic C contents and stocks in soils and the main factors of their control. Up to now and despite numerous evaluations of soil organic carbon (SOC) stocks worldwide, the sloping lands of southeast Asia, one of the most biogeochemically active regions of the world, remain uninvestigated. Our main objective was to quantify SOC stocks and to evaluate the impact of various environmental factors. We, therefore, selected Laos with 230 566 km2 of mostly forested steep slopes, and where cultivation is still mainly traditional, i.e. a system of shifting cultivation without fertilization or mechanical tillage. Analytical data from 3471 soil profiles demonstrated that the top 1 m of soil depth holds an estimated 4.64 billion tons of SOC, 65% of which is in the first 0.3 m. SOC stocks to 0.3 m exhibit a high coefficient of variation (CV=62%) with values from 1.8 to 771 Mg C ha-1 and a mean at 129 Mg C ha-1. Furthermore, these stocks are significantly (at P<0.05 level) affected by land use as shown by principal components analysis and t-tests with the largest amount being found under forest, less under shifting cultivation and the smallest under continuous cultivation. Moreover, SOC stocks correlated regionally to total annual rainfalls and latitude, and locally at the hill-slope level to the distance to the stream network and the slope angle. It is hypothesized that this correlation is through actions on mineral weathering, soil clay content, soil fertility and SOC redistributions in landscapes. These relationships between SOC stocks and environmental factors may be of further use in (1) predicting the impact of global changes on future SOC stocks; and (2) identifying optimal strategies for land use planning so as to minimize soil C emissions to the atmosphere while maximizing carbon sequestration in soils. © 2009 Blackwell Publishing Ltd. Source

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