Carbon isotope evidence for sedimentary miliacin as a tracer of Panicum miliaceum (broomcorn millet) in the sediments of Lake le Bourget (French Alps).

International audienceWe here report on the determination of the carbon isotopic composition of miliacin (olean-18-en-3βol methyl ether), extracted from the sediments of Lake le Bourget (French Alps). It is compared to the δ13C of miliacin extracted from Panicum miliaceum (broomcorn millet, a C4 plant) and Chionochloa sp. (a C3 plant). The δ13C of sedimentary miliacin (-21.5 ‰) is very close to that of miliacin extracted from bran (-23 ‰) and seeds (-23.5 ‰) of P. miliaceum and significantly different from that of Chionochloa sp. (-33 ‰). These results provide additional support for the use of sedimentary miliacin as a tracer of broomcorn millet, a C4 cereal cultivated since the Bronze Age around Lake le Bourget. These findings illustrate the potential of this compound to reconstruct past agriculture from lake sediment archives. Finally, considering the high abundances of miliacin in the bran of P. miliaceum this compound could have been wind-transported to the sediment during threshing and winnowing on the lake shore

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

Impacts of new agricultural practices on soil erosion during the Bronze Age in the French Prealps

International audienceIn order to better understand the evolution of past climate-human-environment interactions in the North-western Alps during the Holocene, we have analysed the lipid content of two cores taken from the sediments of Lake le Bourget (French Alps). By using a specific molecular biomarker of Panicum miliaceum (broomcorn millet) previously defined and a new molecular marker of soil erosion, we demonstrate that the onset of millet cultivation coincides with the onset of major soils erosion in the catchment during the Middle Bronze Age. Although archaeological and archaeobotanical investigations indicate a discrete human occupation of the lakeshores at this period, they also point to a regional change in agricultural practices that deeply affected soils. The evolution of millet cultivation appears in strong connection with climatic variations, estimated in the same cores from the variations in titanium, a proxy of hydrological changes in the region. Social and cultural triggers cannot be discarded at this stage. Such an approach applied to more sedimentary archives shows high potential to unravel the temporal and spatial dynamics of human land-use

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

Effects of soil mineral matrix on the analysis of plant-and soil-derived polysaccharides after acid hydrolysis

International audienceRATIONALE: The efficiency of extraction procedures for the determination of organic compounds in soil may be affected by the presence of the mineral phase. Our aim was to analyse the magnitude of such an effect on both total polysaccharide content and C-13-isotopic signature of the polysaccharides. METHODSAfter acid hydrolysis of C-13-labelled wheat, soil and a mixture of these, sugars were quantified and analysed isotopically. Measured values were compared with theoretical contents. RESULTSNo matrix effect was apparent for total sugar-C content of the mixture. However, a matrix effect was observed for the contribution of C-13-labelled wheat sugars. For the soil+plant mixture C-13-labelled wheat sugar contribution was overestimated. Soil-derived sugar-C contribution to the mixture was underestimated. CONCLUSIONSStudies using stable isotopes to follow the fate of added plant-derived compounds in soil need to take into account matrix effects. Further studies have to elaborate on correction procedures and/or the development of extraction procedures to overcome the influence of matrix effects and/or acid hydrolysis extraction on sugar-C contents

A miniaturised method to quantify microbial mineralisation of C-13-labelled organic compounds in small soil samples

International audienceA miniaturised method developed to measure the mineralisation of C-13-labelled organic compounds in small soil samples is presented. Soil samples (<0.5 g) were placed in wells of microtiter plates with CO2 traps (NaOH-soaked glass microfiber filters) and amended with C-13-labelled-labelled substrate. The microtiter plate was covered with a seal and placed in a microplate clamp system to ensure that each well was airtight. After incubation, the CO2 traps were transferred to tightly sealed glass phials under CO2-free atmosphere and the C-13-labelled-labelled CO2 was released by addition of H3PO4. The CO2 was measured by micro-GC and its isotopic signature was determined using a GC-IRMS. The qualitative and quantitative efficiency of the microplate system was demonstrated by comparison with direct measurement of CO2 in the headspace of phials in which similarly treated soil samples had been incubated. The two methods showed similar mineralisation rates for added C-13-labelled-substrates but the apparent mineralisation of soil organic matter was greater with the microtiter plate method. The microplate system presented here is suitable for studying the mineralisation of different kinds of C-13-labelled-labelled substrates in small soil samples and allows analysis of functional and molecular characteristics on the same micro-samples

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of the Abiotic Sinks of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies.

In this study we investigated the isotope fractionation of the abiotic sink (hydrolysis, halide exchange) of methyl halides in water.<br /

Evidence that stable C is as vulnerable to priming effect as is more labile C in soil

WOS:000305660900006International audienceA significant fraction of soil organic carbon, named stable organic carbon (C) pool, has residence times longer than centuries and its vulnerability to land use or climatic changes is virtually unknown. Long-term bare fallows offer a unique opportunity to isolate the stable organic pool of soils and study its properties. We investigated the vulnerability of the stable organic C pool to fresh organic matter inputs by comparing the mineralization in a long-term bare fallow soil with that of an adjacent arable soil, containing stable C as well as more labile C. For this, we amended or not the soil samples with two different C-13-labelled fresh organic matter (straw or cellulose). In all cases we found a positive priming effect (i.e. an increased mineralization of soil organic carbon) when fresh organic matter was added. By comparing the results obtained on both soils, we estimated that half of the "primed" C in the arable soil due to straw addition as fresh organic matter, originated from the stable C pool. Our results suggest that under such conditions, which frequently occur, the stable pool of soil organic matter may largely contribute to soil extra-CO2 emissions clue to priming effect. Consequently, the C storage potential of this pool may be modified by changes in land use and/or biomass production that might change the priming of the mineralization of the stable pool of soil organic carbon. (C) 2012 Elsevier Ltd. All rights reserved

Negative priming effect on mineralization in a soil free of vegetation for 80 years

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Effet de la structure du sol sur la biodégradation de la matière organique native du sol et sur la matière organique apportée

Effet de la structure du sol sur la biodégradation de la matière organique native du sol et sur la matière organique apportée. 11. Journées d'Etude des Sols (JES