La durée d'engorgement peut-elle être déduite de la description morphologique et de la position topographique dans des sols sur schistes en Guyane ?

International audienceDans le contexte de la Guyane française, la formation et le fonctionnement des sols qui constituent la couverture ferrallitique (ferralsols) sont fortement influencés par le substrat géologique, la topographie et les régimes hydriques. Il en résulte une diversité importante des sols qui influencent la répartition de la végétation et la sensibilité aux effets du changement climatique.La genèse des différents types d'horizons est dépendante des régimes hydriques qui déterminent les processus de ferruginisation et déferruginisation (Ferry et al. 2003). Ces régimes hydriques (drainage vertical, drainage latéral, nappe permanente) ont été utilisés pour définir une typologie, dite "classification IRD" (Boulet et al. 1982).Nos objectifs sont de définir le régime d'ennoyage de sols répartis le long d'une toposéquence, pour comparer avec le rattachement typologique et la pédogénèse et établir des liens avec les traces d'hydromorphie.Le site d'étude est situé dans le nord de la Guyane, il fait partie du dispositif de recherche sur les écosystèmes forestiers tropicaux de Paracou (CIRAD). Le substrat géologique est constitué de schistes Bonidoro et le relief est formé de collines à pentes fortes recouvertes de forêt primaire.L'étude a été menée sur 4 sols répartis entre le mi-versant et le bas-fond où coule un ruisseau. D'après la classification IRD, ces sols se rattachent respectivement aux types drainage latéral superficiel et aux systèmes hydromorphes aval, de bas de versant et de bas-fond. La profondeur d'apparition de la nappe a été enregistrée toutes les 12 h par des sondes installées à 0,5 et à 1 ou 2 m de profondeur. Sur 2 années, nos données piézométriques montrent que dans le versant, les nappes sont présentes le plus souvent durant quelques jours, contrairement aux données bibliographiques où les nappes sont données comme durables sur plusieurs mois. Dans les sols de bas-fond, la nappe est présente 10 à 11 mois par an. La comparaison des battements de nappe entre 0 et 50 cm et les battements en profondeur montre que dans certaines conditions de précipitations, une nappe superficielle perchée s'installe à moins de 50 cm de profondeur. Plus rarement, seule la nappe profonde est présente. Dans les périodes les plus pluvieuses, la totalité du profil est engorgé.Nos données tendent à confirmer la présence d'un drainage latéral superficiel dans le versant et d'une nappe quasi permanente dans le bas-fond. En revanche, dans le cas du système hydromorphe aval de bas de versant, la piézométrie ne confirme pas la présence d'un engorgement long. Pour ces sols, les traces d'hydromorphie pourraient ne plus être fonctionnelles. Leur régime d'engorgement serait plus dépendant des précipitations que supposé, donc plus dépendant des effets du changement climatique

Pyrogenic carbon content and dynamics in top and subsoil of French forests

Pyrogenic carbon (PyC) may leave the soil surface where it is deposited, either through degradation, lateral transport or transfer within the profile. However, PyC has been seldom measured in the subsoil. We estimated the quantity and molecular composition of PyC in the topsoil and subsoil of 22 French forests with diverse soil types and vegetation cover. While the absolute PyC content decreased with depth, its proportion to the soil organic carbon remained constant or tended to increase. The benzene polycarboxylic acids pattern indicated that more condensed structures were found in the subsoil. Our results show that PyC transfers through the soil profile, probably as soluble fraction, and tends to accumulate in the subsoil, specifically in podzolic soils

The contribution of deep soil horizons to persistent organic carbon sequestration in French forest soils

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Effects of depth, soil and vegetation types on indicators of soil organic carbon lability in forest soils

Soil organic matter is a key property as it influences soil ecosystem services like productivity, water storage, etc. In particular, the labile soil organic carbon (SOC) fraction plays a central role in short- to medium-term nutrient availability and soil structural stability. There is little evidence to differentiate the relative importance of factors influencing the labile SOC fraction in contrasted pedological and vegetation conditions. Soil respiration tests and particulate organic matter (POM) obtained by different fractionation schemes are considered as classical indicators of the labile soil organic carbon (SOC) pool. Thermal analyses, in particular Rock-Eval 6 (RE6) analysis, have also shown promising results in the determination of SOC biogeochemical stability.Using a large set of samples of French forest soils representing contrasted pedoclimatic conditions we assessed the effects of depth (n = 5; up to 1 m), soil class (entic Podzol; dystric Cambisol; Calcisol) and vegetation types (deciduous; coniferous) on SOC biogeochemical and thermal stability. We explored how respired-C isolated by a 10-week laboratory soil respiration test, POM-C isolated by a physical SOC fractionation scheme (particle-size > 50 μm and d < 1.6 g·cm-3) and four RE6 parameters, correlated to short- or long-term SOC persistence, evolved in a set of 233 soils samples from 53 forest sites. Results showed that depth was the dominant discriminating factor, affecting significantly all parameters. With depth, we observed a decrease of both classical labile SOC indicators and the thermally labile SOC pool and an increase of the thermally stable SOC pool, along with an oxidation and a depletion of hydrogen-rich moieties of the SOC. Soil class and vegetation type had contrasted effects. For instance, entic Podzols and dystric Cambisols had relatively more thermally stable SOC in the deepest layer than Calcisols but more labile SOC in the surface layer than Calcisols. Soils in deciduous stands tend to contain a higher proportion of thermally stable SOC than soils in coniferous stands. This study shows that both vegetation and soil types influenced SOC stability at various depths and thus should be considered when mapping soil climate regulation ecosystem service

Assessing SOC labile fractions through respiration test, density-size fractionation and thermal analysis – A comparison of methods

Assessing SOC labile fractions through respiration test, density-size fractionation and thermal analysis – A comparison of methods. EGU 2017, European Geophysical Union General Assembly 201

Can we calculate the value of Rock-Eval parameters for the 0-50 layer from the measured values on the layers 0-30 and 30-50 cm?

International audienceCurrent studies investigating soils use different sampling methods. Generally, soils are sampled in different soil horizons and the sampling depths may vary across studies or according to the soil profile composition. For some soil properties such as soil organic carbon stock, it is possible to calculate the organic carbon content of a soil profile by adding the values measured in each horizon. Soil organic carbon stock is therefore independent from the sampling strategy. In the recent years, Rock-Eval has been proposed as a reliable method to investigate soil organic carbon stock and its stability. The objective of this study is to determine, whether Rock-Eval parameters of soil organic matter in a given soil horizon, can also be calculated from Rock-Eval parameters measured in subhorizons; an idea which would greatly facilitate the comparison of results of studies using different sampling methods. In this study, samples from 10 French forest sites encompassing a variety of pedoclimates were used. At each site, samples were collected from two depth ranges, 0-30 and 30-50 cm. To test the linearity of the mixing of RE indicators, binary mixtures of surface and deep soil were composed for each site using five different mixing ratios (10:90, 25:75, 50:50, 75:25, 90:10). All 70 samples were then analysed using Rock-Eval, resulting in five classical RE parameters for each sample. The values of the RE parameters measured on composite samples were generally in good agreement with theoretical values, which were calculated using values measured on 0-30 cm and 30-50 cm according to the mixing equation. This is particularly the case for the following parameters: TOCRE6, PC,RC and OI. However, for HI the relationship between measured and calculated values is unsatisfactory. For sites with a clay-rich deep soil horizon layer and a surface layer with a coarser texture the variation was the highest. Retention of hydrocarbons by clay minerals is a common mineral matrix effect in pyrolysis methods and could explain this observation. Future research should include quantification of the mineral matrix effect for different soil types and calculation of a correction factor for the addition of parameters in a soil profile. Therefore, we conclude that in most temperate soils, most classical RE parameters of a soil profile can be indeed calculated as a sum of the different horizons

Is Rock-Eval 6 thermal analysis a good indicator of soil organic carbon lability? – A method-comparison study in forest soils

Soil respiration tests and abundance of particulate organic matter (POM) are considered as classical indicators of the labile soil organic carbon (SOC) pool. However, there is still no widely accepted standard method to assess SOC lability and the pertinence of these two time-consuming methods to characterize SOC turnover can be questioned. Alternate ways of determining the labile SOC fraction are thus much needed. Thermal analyses, in particular Rock-Eval 6 (RE6) analysis has shown promising results in the determination of SOC biogeochemical stability.Using a large set of samples (n = 99) of French forest soils representing contrasted pedoclimatic conditions, including deep samples (up to 0.8 m depth), we compared three different methods used for SOC lability assessment. We explored whether respired-C isolated by a 10-week laboratory soil respiration test, POM-C isolated by a physical SOC fractionation scheme (particle-size > 50 μm and d < 1.6 g cm−3) and several RE6 parameters were comparable and how they correlated.As expected, respired-C (mg CO2-C·g−1 SOC) and POM-C (% of total SOC) fractions strongly decreased with depth. RE6 parameters showed that SOC from deeper soil layers was also thermally less labile, more oxidized and H-depleted. Indeed, SOC from deeper soil layers had lower proportion of thermally labile SOC, higher T50_HC_PYR (temperature at which 50% of the pyrolysable hydrocarbons were effectively pyrolyzed) and T50_CO2_OX (temperature at which 50% of the CO2 gas had evolved during the oxidation phase), larger oxygen index, and smaller hydrogen index. Surprisingly, the two classical indicators of the labile SOC pool (respired-C and POM-C) were only marginally correlated (p = 0.051) and showed layer-specific correlations. Similarly, respired-C was poorly correlated to RE6 parameters. Conversely, the POM-C fraction showed a strong negative correlation with T50_HC_PYR (ρ = −0.73) and good correlations with other RE6 parameters.Our study showed that RE6 parameters were good estimates of the POM-C fraction, which represents a labile SOC pool with a residence time of ca. a couple decades that is meaningful regarding SOC stock changes upon modifications in land management. RE6 thermal analysis could therefore be a fast and cost-effective alternative to more time-consuming methods used in SOC pool determination, and may be integrated into soil monitoring networks to provide high-throughput information on SOC dynamics

Heterogeneity of the chemical composition and thermal stability of particulate organic matter in French forest soils

International audienceIn temperate forests, soils contain a significant part of the ecosystem carbon (C) stock that can be subjected to C losses upon global changes. In forest soils, particulate organic matter (POM) is a major contributor to the labile C pool and its dynamics can significantly influence the overall total soil organic carbon stock. However, POM has been overlooked in forest soils, specifically in deep horizons.We isolated the POM fraction of mineral soil samples collected in 52 French forest sites, using a size- (> 50 μm) and density- (< 1.6 g·cm−3) fractionation scheme. These soil samples presented variability in terms of depth (0–10 cm; 40–80 cm), soil class (dystric Cambisol, eutric Cambisol, entic Podzol) and vegetation type (deciduous, coniferous). First, we determined the POM chemical composition and thermal stability using elemental analysis, mid infrared-attenuated total reflectance spectroscopy and Rock-Eval thermal analysis. Then, we assessed how depth, soil class and vegetation type influenced POM chemistry and thermal stability in these temperate forest soils.Depth, soil class and vegetation type were all important factors influencing POM chemistry and thermal stability. Variations in POM chemistry (higher C/N ratio, lower ether + alcohol and carbonyl + carboxyl ratios and decrease in hydrogen-rich compounds) and increase in thermal stability with depth suggested different POM input sources for the surface and deep soil layers and an increased biogeochemical stability of POM in deep soil layers. Whatever the vegetation, POM in eutric Cambisols had lower aliphatic and higher aromatic ratios than POM in dystric Cambisols. POM in soils under deciduous trees had higher aliphatic and carbonyl + carboxyl ratios and lower aromatic ratio, more hydrogen-rich and less oxygen-rich compounds than POM in soils under coniferous trees, reflecting the difference in litter chemistry between the two vegetation types. POM from deciduous plots was also significantly more thermally stable than from coniferous plots, suggesting a higher biogeochemical stability for POM in deciduous forest soils.This study highlights the variations in POM chemistry and thermal stability existing within and among soil profiles and the role of depth, soil class and vegetation type in these variations. It appears that if POM can be regarded as a labile carbon fraction in soils, its lability varies depending on the ecosystem (soil, vegetation) and depth considered

Environmental factors controlling soil organic carbon stability in French forest soils

International audienceAimsIn temperate forests, soils contain a large part of the ecosystem carbon that can be partially lost or gained upon global change. Our aim was to identify the factors controlling soil organic carbon (SOC) stability in a wide part of French forests.MethodsUsing a set of soils from 53 French forest sites, we assessed the effects of depth (up to 1 m), soil class (dystric Cambisol; eutric Cambisol; entic Podzol), vegetation types (deciduous; coniferous) and climate (continental influence; oceanic influence; mountainous influence) on SOC stability using indicators derived from laboratory incubation, physical fractionation and thermal analysis.ResultsLabile SOC pools decreased while stable SOC pool increased with depth. Soil class also significantly influenced SOC stability. Eutric Cambisols had less labile SOC in surface layers but had more labile SOC at depth (> 40 cm) than the other soil classes. Vegetation influenced thermal indicators of SOC pools mainly in topsoils (0–10 cm). Mountainous climate forest soils had a low thermal SOC stability.ConclusionsOn top of the expected effect of depth, this study also illustrates the noticeable effect of soil class on SOC stability. It suggests that environmental variables should be included when mapping climate regulation soil service