Inferences from the vertical distribution of Fe isotopic compositions on pedogenetic processes in soils

International audienceThe isotopic compositions of major elements in soils can help understand the mechanisms and processes that control the evolution of soils and the nature and dynamics of the soil constituents. In this study, we investigated the variations of the Fe concentrations and isotopic compositions combined with classical soil parameters, such as granulometry, pH, and C and N concentrations. We selected three soils submitted to different hydrodynamic functioning along a toposequence: a well-drained Cambisol and two hydromorphic soils, an Albeluvisol and a Gleysol. In the Cambisol, the isotopic variations were small indicating little redistribution of Fe which we attributed to centimetric-scale exchanges from the Si-bound to the weakly-bound iron pools and insignificant subsurface Fe export. In contrast, the hydromorphic soils showed an overall variation of 0.37‰for δ56Fe and an inverse correlation between the Fe isotopic compositions and the oxide-bound Fe concentrations. We suggest that, in the uppermost horizon, the mobilisation of oxide-bound Fe was due to the reducing conditions and predominantly involved the light Fe isotopes. Similarly, within the Bt horizon of the Albeluvisol, the fluctuations of the water table level induced changes in the redox conditions and thus Fe dissolution and transport of isotopically light Fe. The Fe isotopic composition profile in the B/C horizon of the Gleysol is dominated by the signature of the parental material. Overall, the variations of the underground water table combined with topography-driven water flow were suggested to be the main mechanisms of Fe translocation in these hydromorphic soils. Finally, the comparison between Fe isotope profiles in worldwide soils allows us to show that Fe isotopic variations can help discriminate between various mechanisms and scales of Fe transfer in soils and, accordingly, provide information on the evolution of soils, when used in combination with pedological, geochemical, geographical, and environmental characterisations

Isotope and molecular evidence for direct input of maize leaf wax n-alkanes into crop soils

International audienceThe contribution of plant carbon to crop soils can be followed by isotope labelling at natural abundance, such as growing a C4 plant on a soil which was previously under C3 vegetation. For this purpose, carbon isotope compositions and relative abundances of n alkanes of maize leaf waxes and maize crop soils were compared. Isotope values of soil n alkanes increased with time of maize cultivation as the result of maize carbon integration into soil organic matter. With increasing time of cultivation, the increase in isotopic difference between n-heptacosane (C27) and n nonacosane (C29) is explained, at least partly, by a direct input of maize leaf n-alkanes. The amount of maize-derived carbon within each n-alkane has been calculated by isotopic means

Unexpected 13C-enrichment of organic components from wheat crop soils: evidence for the in situ origin of soil organic matter

International audienceVarious organic constituents extracted from wheat and from soil organic matter have been analyzed for their carbon content, their absolute concentration and their stable carbon isotope ratios. Most organic subfractions from plants, or soil, are 13C-depleted by up to 9.4‰ relative to bulk organic matter, mainly as a result of their higher lipid content. Furthermore, soil organic constituents are unexpectedly 13C-enriched by +1.5‰ to +4.3‰ relative to homologous plant constituents. Indeed, the selective preservation of plant lignin and lipids, following incorporation into the soil biomass, should have led to the accumulation of 13C-depleted compounds. Hence, these results favour the in situ formation of soil organic matter either by recondensation of small molecules or by selective preservation of biopolymers from soil microorganisms

Molecular, 13C, and 14C evidence for the allochthonous and ancient origin of C16-C18 n-alkanes in modern soils

International audienceThe heterogeneous isotopic composition of C3 and C4 plants can be used to to follow the fate of plant carbon into soil organic molecules. Thus, after 23 years of cropping of maize (C4) on a soil which was previously under C3 vegetation, C25 C33 soil n-alkanes are 13C-enriched up to 9‰ relatively to the initial C3 soil, reflecting the input of 13C-enriched n-alkanes from maize waxes. In sharp contrast, C16-C18 soil n alkanes do not show any significant 13C/12C variation over the same time interval. This absence of isotopic variation, along with consideration of their relative concentration, absolute concentration and biodegradability, demonstrate that these substances must represent a regular input from an external source. Evidence of a large contribution of an ancient source, amounting to more than 65% of the alkane fraction, is given by a 14C-age of 8510 yrs BP. Moreover, short-chain n-alkanes from soils, diesel fuel, diesel automobile exhaust and petroleum products exhibit similar distributions and δ13C values. These findings suggests that C16-C18 soil n alkanes represent a non-point source pollution of ancient hydrocarbons either carried by aerosols or entering the soil via continuous hydrocarbon seepage from the deep sedimentary rocks of the Paris basin

A survey of rare event simulation methods for static input–output models

International audienceCrude Monte-Carlo or quasi Monte-Carlo methods are well suited to characterize events of which associated probabilities are not too low with respect to the simulation budget. For very seldom observed events, such as the collision probability between two aircraft in airspace, these approaches do not lead to accurate results. Indeed, the number of available samples is often insufficient to estimate such low probabilities (at least 10^6 samples are needed to estimate a probability of order 10^-4with 10% relative error with Monte-Carlo simulations). In this article, one reviewed different appropriate techniques to estimate rare event probabilities that require a fewer number of samples. These methods can be divided into four main categories: parameterization techniques of probability density function tails, simulation techniques such as importance sampling or importance splitting, geometric methods to approximate input failure space and finally, surrogate modeling. Each technique is detailed, its advantages and drawbacks are described and a synthesis that aims at giving some clues to the following question is given: “which technique to use for which problem?”

Zn speciation in the organic horizon of a contaminated soil by micro X-ray fluorescence, micro and powder EXAFS spectroscopy and isotopic dilution.

Soils which have been acutely contaminated by heavy-metals show distinct characteristics, such as colonization by metal-tolerant plant species and topsoil enrichment in weakly degraded plant debris because biodegradation processes are strongly inhibited by contamination. Such an organic topsoil, located downwind of an active zinc smelter and extremely rich in Zn (~ 2%, dry weight), was investigated by X-ray diffraction (XRD), synchrotron-based X-ray microfluorescence (μSXRF), and powder and micro extended X-ray absorption fine structure (EXAFS) spectroscopy for Zn speciation, and by isotopic dilution for Zn lability. EXAFS spectra recorded on size fractions and on selected spots of thin sections were analyzed by principal component analysis (PCA) and linear combination fits (LCFs). Although Zn primary minerals (franklinite, sphalerite and willemite) are still present (~ 15% of total Zn) in the bulk soil, Zn was found to be predominantly speciated as Zn-organic matter complexes (~ 45%), outer-sphere complexes (~ 20%), Zn-sorbed phosphate (~ 10%) and Znsorbed iron oxyhydroxides (~ 10%). The bioaccumulated Zn fraction is likely complexed to soil organic matter after the plants' death. The proportion of labile Zn ranges from 54 to 92%, depending on the soil fraction, in agreement with the high proportion of organically-bound Zn. Despite its marked lability, Zn seems to be retained in the topsoil thanks to the huge content of organic matter, which confers to this horizon a high sorption capacity. The speciation of Zn in this organic soil horizon is compared with that found in other types of soils

Some Bayesian insights for statistical tolerance analysis

Functionality of assembled products mostly rely on the ability of the manufacturer to produce under some quality requirements. Parts which do not meet these requirements represent a manufacturing waste which can be at the origin of substantial losses in terms of money and credibility. Quality con- trol and defect detection are two keypoints of predictive process management. At the design stage, a statistical tolerance analysis can be performed to predict the process quality. This imply to estimate a so-called defect probability which quantifies the probability that the final assembly does not meet functional requirements. In general, this quantity depends on a number of process specifications (toler- ances, capability levels) set a priori by the manufacturer, but also on the monitoring of the process itself since the process parameters (mean shift value and standard deviation) vary statistically for different batches. In this paper, we give an alternative point of view on an existing method, namely the Advanced Probability-based Tolerance Analysis of products (APTA), proposed in literature to estimate the defect probability. This method, originally relying on a double-loop sampling strategy, is revisited within the Bayesian framework, and an augmented approach is proposed to estimate the defect probability in a more efficient way. The efficiency of the augmented approach for solving tolerancing problems with APTA is illustrated on a linear reference test-case