Search

The spatial distribution of soil constituents at the micrometer scale is of great importance to understand processes controlling the formation of micro-aggregates and the stabilization of organic carbon. Here, the spatial distribution of organic and mineral constituents in Podzol horizons is studied by concerted measurements of (i) the content of various forms of Fe, Al, Si and C determined by selective extraction in the fine earth fraction of soil (f < 2 mm); (ii) the elemental composition of the clay fraction (f < 2 um) with lateral resolution using scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS), and with surface selectivity using X-ray photoelectron spectroscopy (XPS); (iii) the specific surface area (SSA) of fine earth and clay fractions by krypton physisorption. The SSA of the fine earth in illuvial horizons is predominantly due to finely divided Fe oxides, including goethite, characterized by an equivalent particle size of about 10 mu m. Kaolinite platelets of about 2 gm size account for a large volume proportion in the clay fraction but have a minor contribution to SSA. Fe oxides and organic matter (OM) are intimately associated. Heterogeneity at the um scale is created by local variations in the relative amounts of kaolinite and Fe-OM associations. These two kinds of physical entities are in random mixture. Moreover, variation of C/Fe atomic ratios reveals sub-mu m scale heterogeneity. The latter is due to variation in the relative proportion of organic compounds and Fe oxides, indicating that aggregation of nanoparticles, and not only mere adsorption or pore filling, plays a role in these associations. In this regard, our results highlight that OM associated with Fe protects Fe oxides against physical displacement and that part of this associated OM is oxidizable by NaOCl treatment. These findings demonstrate that the concept of OM stabilization through association with Fe must be revisited when considering the sub-mu m scale level because fine Fe oxide particles can be easily dispersed during oxidation of associated carbon. Combination of physical fractionation and microanalysis (e.g. SEM-EDS, vibrational spectroscopy) offer promising perspectives to clarify the relationship between chemical composition and sub-inn scale architecture, and to better understand soil processes.

Soil organic matter (OM) stability plays an important role in the global carbon cycle. The molecular characterisation of soil OM may contribute to an understanding of the feedback mechanisms between soil OM and climate. Umbric Ferralsols with humic properties (humic Ferralsols) are characterised by a thick dark carbon-rich A horizon, and co-occur next to Umbric Ferralsols without humic characteristics. They are concentrated in south-eastern Brazil which suggests an environmental control. In order to understand OM stability in these soils, humic Ferralsols from three Brazilian biomes were studied, including Cerrado (savanna), subtropical forest and tropical forest. The studied soils were developed on several geological substrates, including basalt, gneiss, chamokites, phyllite and tertiary sediments. The molecular composition of the free light fraction (FLF), occluded light fraction (OLF) and 0.1 M NaOH extractable OM of samples from topsoil and subsoil from eight profiles was examined using analytical pyrolysis (pyrolysis-GC/MS). The light fraction reflects litter and charcoal, obtained by density fractionation before (FLF) and after (OLF) ultrasonic disruption; the NaOH extractable fraction (humic acid + fulvic acid) reflects more decomposed material. The chemical differences among soils from different Brazilian biomes were much smaller than the vertical gradient within each profile. The results showed that, within the light fractions, the content of which varied between 2% and 22% of the total carbon content (C t ), black carbon (BC) derived pyrolysis products showed a higher abundance in the topsoil. In the extractable OM, which varied between 46% and 88% of C t , BC derived pyrolysis products showed a higher abundance in the subsoil. This suggests a continuous input of BC and an increase in the degree of decomposition with depth. Pyrolysis products associated with BC contributed up to 10% to the extractable pyrolysable fraction in the subsoil. The results suggest that degradation of charcoal, which is translocated to the subsoil by macroscopic soil organisms, results in a relative high contribution from naphthalene, 2-ethenylnaphthalene, phenanthrene, benzene, dibenzofuran and benzonitrile (subsoil) compared to (other) polyaromatic hydrocarbons (PAHs; C 1 -, C 2 - and C 3 -naphthalenes and fluorene) in relatively fresh charcoal (topsoil). The results highlight the importance of the subsoil in studies of the long term effects of fire on soil OM and suggest that BC is an important factor that underlies carbon stability in humic Ferralsols.

Background : African tropical rainforests are one of the most important hotspots to look for changes in the upcoming decades when it comes to C storage and release. The focus of studying C dynamics in these systems lies traditionally on living aboveground biomass. Belowground soil organic carbon stocks have received little attention and estimates of the size, controls and distribution of soil organic carbon stocks are highly uncertain. In our study on lowland rainforest in the central Congo basin, we combine both an assessment of the aboveground C stock with an assessment of the belowground C stock and analyze the latter in terms of functional pools and controlling factors. Principal Findings : Our study shows that despite similar vegetation, soil and climatic conditions, soil organic carbon stocks in an area with greater tree height (=larger aboveground carbon stock) were only half compared to an area with lower tree height (=smaller aboveground carbon stock). This suggests that substantial variability in the aboveground vs. belowground C allocation strategy and/or C turnover in two similar tropical forest systems can lead to significant differences in total soil organic C content and C fractions with important consequences for the assessment of the total C stock of the system. Conclusions/Significance : We suggest nutrient limitation, especially potassium, as the driver for aboveground versus belowground C allocation. However, other drivers such as C turnover, tree functional traits or demographic considerations cannot be excluded. We argue that large and unaccounted variability in C stocks is to be expected in African tropical rain-forests. Currently, these differences in aboveground and belowground C stocks are not adequately verified and implemented mechanistically into Earth System Models. This will, hence, introduce additional uncertainty to models and predictions of the response of C storage of the Congo basin forest to climate change and its contribution to the terrestrial C budget.