No abstract

We measured the organic carbon (OC) content, the isotopic composition, the C/N ratios and the photosynthetic pigment composition of suspended matter, sediments, sediments traps and epiphytic and epilithic biofilms at the tropical Sinnamary River system (French Guiana). Our sampling included the mid-stream reservoir lake (Petit Saut) and the estuary on the Atlantic coast. These tracers were complementary and allowed identifying different sources of particulate organic matter (POM) in the system. We found a d 13 C-C/N signature of POM close to that of soils and litters collected in the surrounding forest, both for water column and sediment at the upstream station as well as for the sediment in a littoral zone of the reservoir, which thus indicated a terrestrial origin. Plankton communities at the centre of the reservoir were dominated by Chlorophyceae (chlorophyll a (Chl a), chlorophyll b (Chl b) and lutein) in the oxic epilimnion and by anoxygenic phototrophic bacteria, namely Chlorobiaceae (bacteriochlorophyll d (BChl d) and bacteriochlorophyll c (BChl c)) at and below the oxycline (6 m depth). In addition, this planktonic material was slightly 13 C-depleted due to a contribution of methanotrophic bacteria. Phytoplankton and bacterioplankton were the major source of settling material collected in the traps at all depths in the centre of the reservoir. In the traps, POM was subject to intense degradation, as revealed by C/N and isotopic data and by the presence of pheopigments. In the river downstream of the dam, Chl b, lutein, BChl c and d originating from the reservoir progressively decreased downstream as the result of mineralization. At the estuarine mouth, fucoxanthin showed the presence of diatoms and the d 13 C-C/N signature matched that of POM carried by the Amazonian coastal mobile mud belt. By analysing sedimentation rates in the reservoir and its outflow into the river, we were able to provide a first estimates of POM transfers in this system during the sampling period. POM IN A TROPICAL RESERVOIR 255 Figure 2. Typical chromatograms at 440 and 664 nm showing most of the pigments quantified in the Petit Saut samples. (A) Surface sediment at Station 1; (B) suspended matter at À6 m depth at Station 4 A. DE JUNET ET AL. Ã Indicates that lutein and BChl d coeluted in the samples and lutein concentrations were corrected from the BChl c and d concentration as detailed in the text.

No abstract

Polycyclic Aromatic Hydrocarbons (PAHs) are ubiquitous and persistent soil pollutants. Their fate and the influence of the plant rhizosphere on their dynamics has been extensively studied, but studies mainly focused on their dissipation rate. We conducted a plant-soil mesocosm experiment to study the fate and distribution of PAHs or derived compounds in the extractable fraction, the residual soil, the shoot biomass and the root biomass. The experiment was conducted for 21 days using ryegrass and a forest soil spiked with C-labeled phenanthrene (PHE), using combined IRMS and NanoSIMS for analyses. Almost 90% of the initial extractable PHE content was dissipated within 3 weeks, but no rhizospheric effect was highlighted on PHE dissipation. More than 40% ofC-PHE was still in the soil at the end of the experiment, but not as PHE or PAH-derived compounds. Therefore it was under the form of new compounds (metabolites) and/or had been incorporated into the microbial biomass. About 0.36% of the initial C-PHE was recovered in the root and shoot tissues, representing similarC enrichment (EC) as in the soil (EC ≈ 0.04 at.%). Using NanoSIMS, C was also localized at the microscale in the roots and their close environment. GlobalC enrichment confirmed the results obtained by IRMS. Some hotspots of C enrichment were found, with a highS/CN ratio. Comparing the ratios, sizes and shapes of these hotspots suggested that they could be bacteria.

Abstract. Carbon cycling and organic matter transfers in the tropical Sinnamary river system (French Guiana), including a mid-stream reservoir (Petit Saut) and its estuary on the Atlantic coast, were studied during the dry season by analyzing the organic carbon isotopic ratios (δ13C-OC), C/N ratios and pigments contents of suspended matter, sediments, sediments traps and epiphytic and epilithic biofilms. In the River upstream as well as in surface sediments at the entrance of the reservoir and at the littoral zone of the reservoir, particulate organic matter (POM) was in majority of terrestrial origin, with a δ13C-C/N signature close to the one of soil and litter collected in the surrounding forest and with high OC/total pigments ratios. High concentrations of Pheopigments a and b in these surface sediments showed that this terrestrial POM, either carried by the river and eolian transport or present in the soil before flooding, undergoes intense degradation. Deeper in the sediment, the δ13C profile showed a decreasing trend with depth typical of what is found in soils, showing that the flooded soil still remains present at the reservoir bottom 10 years after flooding. At the center of the reservoir, POM in the water column, in sediment traps and in surface sediments was in majority of aquatic origin with low C/N and OC/total pigments ratios. In the oxic epilimnion at 3 m depth, Chl a, Chl b and Lutein showed the predominance of Chlorophyceae to the phytoplankton community. At this depth, a C/N ratio of 21 suggests a large contribution of transparent exopolymeric particles to the bulk POM, which, in addition, was 13C-depleted due to a significant contribution of methanotrophic bacteria. At 7 m depth, below the oxicline, high concentrations of BChl d and occasionally BChl c revealed the presence of anoxygenic phototrophic bacteria, namely Chlorobiaceae. In the sediment traps, Chl a, Chl b, Lutein and BChl c and BChl d confirmed the contribution of plankton to the sedimentary POM. This material was undergoing intense degradation as revealed by high concentration of pheopigments and by an increase in C/N ratio and an increase in δ13C-OC with trap depth. Scytonemin was found in a biofilm developed on tree trunks at the reservoir surface and in all sediment traps. Other tracers showed however that the contribution of the biofilm to the sedimentary POM was minor compared to the planktonic source. In the Sinnamary downstream of the dam, POM became more 13C-depleted showing a larger contribution of methanotrophic bacteria. Chl b, Lutein and BChl c + BChl d originating from the reservoir progressively decreased downstream as the result of mineralization. At the estuarine mouth, fucoxanthin showed the presence of diatoms and the δ13C-C/N signature matched the one of POM carried by the Amazonian coastal mobile mud belt.

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Interactions between organic matter (OM) and mineral phases represent a fundamental mechanism of carbon (C) sequestration in soils. They result in the formation of organo–mineral associations, which prevent rapid OM biodegradation. The density‐separation technique enables effective isolation of different types of organo–mineral associations in soil. However, analyses of OM in these organo–mineral associations are limited because the C contents are very small (<10 mg OM g−1 of soil) and mineral phases complicate direct studies of OM. Previous studies have shown that hydrofluoric acid (HF) dissolution is an effective treatment for OM analysis since the mineral phases are dissolved and C is concentrated in the extract. Nonetheless, few studies on the effects of HF dissolution have been directly carried out on organo–mineral associations. The main objectives of this study were to determine the effects of HF dissolution on organo–mineral associations and to establish the best HF treatment conditions for OM analysis. We chose two specific tropical soils, an hydric Andosol and an haplic Cambisol (Réunion), that varied in their mineralogy. After densimetric separation, we tested three HF concentrations and four contact times. Mineral removal was 94% for both soils at the most concentrated HF treatments. Aluminium and silicon compounds were largely dissolved after the first treatment, while iron‐rich residue accumulated after repeated HF treatments. XRD analyses confirmed the rapid dissolution of halloysite, gibbsite and quartz and the persistence of iron oxides. For the organo–mineral associations of the Cambisol, HF treatments did not enable us to obtain a usable signal with 13C NMR. For the organo–mineral associations of the Andosol, a four‐times extraction over 2 hours with 5% HF allowed us to acquire an effective signal with 13C NMR for OM qualitative analyses. However, a new compound made of aluminium and fluorine appeared with the 10% HF treatments for both soils. In order to compare and analyse the OM present in organo–mineral associations of these two soil types, we thus recommend using four 2‐hour treatments with 5% HF.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.