Carbon isotope data on soil organic matter (SOM) were collected along an ecosystem transect 90 m in length that includes a tropical forest on the plateau, a transitional forest-savanna and savanna in a depression. Total organic carbon data show a significant increase in carbon content from sites representing forest ecosystem to sites representing savanna ecosystem. It was hypothesized that carbon accumulation in the depression is controlled by flooding conditions that slow down carbon decomposition and in part by carbon transport from the upper part of the transect (the savanna and the transition forest-savanna areas) into the depressions by water during the rainy season. The origin of the carbon was confirmed by using soil 13 C analysis. The savanna sites located in the depression showed δ 13C values between –19.5‰ and –22.5‰ indicating a mixture of C3 and C4 plants. The vegetation cover in the depression is predominantly C3 grasses with d 13C values of about –27‰ and –26‰. In the site under savanna located at an elevation slightly higher, the d 13C value was more enriched (–16‰) showing the predominance of C4 plants (d 13C of –13.6‰). At the forest-savanna transition and in the forest ecosystem the d 13C values were characteristic of C3 plants (–25‰ and –28.1‰). 14C and 13C data indicate that the organic matter of mixed origin has been deposited for at least the last 7000 years in the savanna depressions. The 13C pattern observed in the soil organic matter profiles indicate a predominance of C3 plants in the early part of the Holocene. About 7000 to 4000 years ago, the data show the influence of C4 plants, indicating forest regression associated with a drier climate than at present. The more recent 13 C records suggest forest expansion, and the return to a climate similar to the present.

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A mottled horizon in a laterite cover (without any duricrust) was studied by microscopy and quantitative chemical microanalysis. Apart from the voids, light red spots consisting of Fe-rich particles (≈ 2 µm) are set in clayey plasma. Dark red spots consisted of concentrations of Fe-rich particles. These patterns are inherited. On the border of structural or biological voids, where Fe-depletion features are systematic, gray or yellow spots result from dissolution of the Fe-rich particles and impregnation of the plasma by iron, respectively. The present Fe-depletion is the dominant process that explains the mottled differentiation and the absence of lateritic duricrust. To cite this article: V. Rosolen et al., C. R. Geoscience 334 (2002) 187-195. © 2002 Académie des sciences / Éditions scientifiques et médicales Elsevier SAS microscopy / scanning electron microscopy / quantitative chemical microprobe / iron / plinthic Cambisol / Brazil Résumé-Genèse d'un horizon tacheté par déferruginisation dans une couverture à latérite du Bassin amazonien. Un horizon tacheté d'une couverture à latérite (sans cuirasse) d'Amazonie a été étudié par microscopie et microanalyse chimique quantitative. Éloignées des pores, les taches rouges sont constituées de particules ferrugineuses (≈ 2 µm), enchâssées et isolées au sein d'une matrice argilo-quarzeuse ou en amas. Ce faciès est hérité. En bordure des pores, les taches grises ou jaunes témoignent respectivement d'une dissolution des particules ferrugineuses et d'une migration du fer au sein d'une matrice semblable. L'évolution est polyphasée avec, antérieurement, une ségrégation ferrugineuse, puis, actuellement, une déferruginisation, qui provoque une différenciation tachetée. La déferruginisation explique l'absence de cuirassement. Pour citer cet article : V.

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This paper shows the geographic distribution in Germany of iron (Fe), nickel (Ni) and lead (Pb) analyzed in mosses in 1995/96 and compares it with the results of the 1990/91 pilot study within a European moss-monitoring programme. Other elements (As, Cd, Cr, Cu, Ti, V, Zn) are compared on basis of the overall element medians for Germany of the 1990/91 and 1995/96 survey. Samples of Pleurozium schreberi, Scleropodium purum, Hypnum cupressiforme and Hylocomium splendens were taken at a total of 1026 sites. In the 1995/96 monitoring campaign, 95% of the original sites of the 1990/91 study were resampled. The results from 1995/96 display local elevated values and many cases of areas affected by known sources of heavy-metal emissions. The industrialized and urban regions of Germany are shown up clearly by the 1995/96 moss-monitoring results: the Ruhr area, parts of Saarland and Baden-Württemberg, as well as areas in eastern Germany. Relatively low values for many elements were found in large areas of Lower Saxony and Bavaria. A comparison of the results of the 1990/91 and 1995/96 moss-monitoring programmes shows a fall in the concentration of the presented elements (except cadmium, copper and zinc) over the relevant period. Especially in the former GDR, chromium (Cr), iron (Fe), titanium (Ti) and vanadium (V) decreased significantly. This is, firstly, a reflection of the closure of and/or technological improvements to large power plants; secondly it is due to the fact that lignite has given way to other fuels. Vanadium (V) and nickel (Ni), typical constituents of crude oil, also show a decrease in the western part and thus document changes in the type of fuel consumed. The significant fall in lead concentration in 1995/96 as compared to 1990/91 in what used to be East and West Germany probably results from the increasing use of lead-free petrol. A comparison of the median values for 1990/91 and 1995/96 in mosses to the rate of emission of heavy metals in Germany for 1990 and 1995 shows similar trends in the case of elements such as arsenic (As), chromium (Cr), nickel (Ni), and lead (Pb). The comparison of the medians of the elements analyzed for 19 European countries indicates for most of the elements a general tendency to lower values in 1995, except for Lithuania, Netherlands, Portugal, Italy and United Kingdom.

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In seasonal flooding isolated wetlands, the degree of wetness suggests a close synergy between soil processes, landscape evolution and hydrology along space and time. Until now, that subject has received insufficient attention despite natural wetlands supply essential environmental services to society and are surrounded by intensive agriculture that uses agrochemicals and fertilizers in their management. The objectives of this study were to propose an infiltration architecture model based on local surface and subsurface water-fluxes in isolated wetland embedded in lateritic plateau covered by savanna and qualify the environmental sensitivity as an area of aquifer recharge. Grain size, soil bulk density, and hydraulic conductivity were determined in five profiles in a soil catena. Unmanned Aerial Vehicle high-resolution images were obtained to generate a digital elevation model and discriminate areas with different vegetation, water accumulation, and environmental sensitivity. Electrical tomography was performed to unveil the soil architecture and infiltration. The soils (Plinthosols) developed on aquic conditions determine the linkage between the surface–subsurface hydrodynamics with the soil's physical properties. We have identified vertical and lateral water-flows in the soil architecture. Vertical flow occurs exclusively at the center, where the wetland is characterized as a recharge zone. Lateral flow towards the borders characterizes a discharge zone. The recharge zone is a depression surrounded by crops; therefore, it is a point of high environmental sensitivity. This hydrodynamic model is essential to support studies related to the dispersion of contaminants since soybean agriculture dominates the whole area of well-drained soils in the Brazilian Cerrado.

Wetland soils are an important component of the Global Carbon Cycle because they store about 20-25% of the terrestrial soil organic carbon (SOC). Wetlands occupy about 6% of the global land surface and any change in their use or management has potentially dramatic consequences on greenhouse gases emissions. However, the capacity of wetland soils to store carbon (C) differs from place to place due to reasons still not well understood. The objective of this review was to evaluate the global variations in wetlands SOC content (SOC C) and to relate it to key soil and environmental factors such as soil texture, intensity of soil hydromorphy, metallic element content and climate. A comprehensive data analysis was performed using 122 soil profiles from 29 studies performed under temperate, humid, sub-humid, tropical and sub-arctic conditions. The results point to average SOC C of 53.5 ± 15.8 g C kg −1 with a maximum of 540 g C kg −1. SOC C increased with increase in intensity of soil hydromorphy (r = −0.52), Al (r = 0.19) and Fe content (r = 0.21), and decreased with soil pH (r = −0.24). There was also a surprising tendency for intensity of soil hydromorphy, and thus SOC C , to decrease with increasing mean annual precipitation and soil clay content. These results contribute to a better understanding of the impact of soil hydromorphy in wetlands on organic C stabilization in the soils. However, further studies with additional information on soil bulk density to assess carbon C stocks, still need to be performed.