Françoise Watteau

Characterization of organic matter microstructure dynamics during co-composting of sewage sludge, barks and green waste

A microstructure characterization study using transmission electron microscopy (TEM) was conducted to specify organic matter dynamics during the co-composting process of sewage sludge, green waste and barks. TEM results showed that ligneous and polyphenolic compounds brought by barks were not biodegraded during composting. Green waste brought more or less biodegraded ligneous constituents and also an active microbial potential. Chloroplasts and sludge flocs appeared to be relevant indicators of green waste and sewage sludge in composted products, as they were still viewable at the end of the process. TEM characterization of the final product highlighted two main fractions of organic matter, one easily available and a more recalcitrant one, and also a remaining microbial activity. Thus microstructure characterization appeared to be an appropriate way of taking the heterogeneity of the organic constituents size and composition into account when attempting to specify such compost quality parameters as maturity and stability.

In situ ageing of fine beech roots (Fagus sylvatica) assessed by transmission electron microscopy and electron energy loss spectroscopy: description of microsites and evolution of polyphenolic substances.

Root biomass is quantitatively and qualitatively important in most ecosystems, but its contribution to the pool of organic matter in the soil is not clear. This work was designed to specify root ageing on an ultrastructural scale by transmission electron microscopy combined with microanalysis by electron energy loss spectroscopy. This approach is very suitable for studying the soil/plant interface, and for semi‐quantitative analysis of the evolution of polyphenolic substances during root evolution. Three root segments were studied according to a gradient of root senescence: the apical and basal segments of the mycorrhiza and the mycorrhiza‐carrier root. Each segment contained a certain proportion of senescent cells, some of which were of fungal origin, and this proportion increased as the root aged. In the three segments, the soil/plant interfaces were differentiated, and the micro‐organisms observed in situ were described. Senescent root cells contained many polyphenolic substances and our results showed that these substances were, according to the root segment, differently associated with Ca, N and Si. When all these ultrastructural data are correlated with more global data, they can be usefully applied to root cell physiology, microbiology and pedology. This approach makes it possible to specify the evolution of organic matter in situ in soils whatever its origin.

Dynamic observation of the biodegradation of lignocellulosic tissue under solid-state anaerobic conditions

The solid-state anaerobic digestion (SS-AD) of wheat straw was characterized under low inoculated batch tests during 244 days. High levels of degradation of the cellulose (52%±1) and hemicelluloses (55%±2) were observed at the final stages and associated to a methane yield of 204±16 NmL gTS(-1). Ultrastructural observations, using transmission electronic microscopy, indicated that microorganisms degraded wheat straw from the central to the outer tissue (i.e. parenchyma to epidermis), depending on cell chemical, physical accessibility and the degree of lignification. Furthermore, major degradation of sclerenchyma secondary walls was observed. The bioaccessibility of lignocellulosic structures of wheat straw is mainly limited by the external waxy layer (cuticle), tertiary cell walls, high silica content and access to the cell lumen.

Localization and characterization by electron energy loss spectroscopy (EELS) of the brown cellular substances of beech roots

Abstract Degradation of roots from beech trees was investigated by a novel approach: localization and elemental characterization of predominant brown substances were performed at ultrastructural scale using electron energy loss spectroscopy (EELS). This technique allowed us to detect light elements such as C and N, main constituents concerned in the degradation of plant tissues, within cellular constituents observed with a transmission electronic microscope, after having localized them by light microscopy. The results showed that there was no relationship between the brown pigmentation of the cellular substances and the presence of N, a relationship previously found in studies of degrading leaves. Other elements such as Si and Ca were also found to be involved in these root degradation processes. This approach seem powerful for detailed descriptions of the process of root degradation in soil.

Impact of fresh organic matter incorporation on PAH fate in a contaminated industrial soil.

The impacts of fresh organic matter (OM) incorporation in an industrial PAH-contaminated soil on its structure and contaminant concentrations (available and total) were monitored. A control soil and a soil amended with the equivalent of 10 years maize residue input were incubated in laboratory-controlled conditions over 15 months. The structure of the amended soil showed an aggregation process trend which is attributable to (i) the enhanced microbial activity resulting from fresh OM input itself and (ii) the fresh OM and its degradation products. Initially the added organic matter was evenly distributed among all granulodensimetric fractions, and then rapidly degraded in the sand fraction, while stabilizing and accumulating in the silts. PAH degradation remained slight, despite the enhanced microbial biomass activity, which was similar to kinetics of the turnover rate of OM in an uncontaminated soil. The silts stabilized the anthropogenic OM and associated PAH. The addition of fresh OM tended to contribute to this stabilization process. Thus, in a context of plant growth on this soil two opposing processes might occur: rhizodegradation of the available contaminant and enhanced stabilization of the less available fraction due to carbon input.

Nondestructive monitoring of the effect of biological activity on the pedogenesis of a Technosol

PurposePedogenesis is a set of steps which leads to the formation and evolution of soils under pedogenetic factors and processes (e.g., aggregation, weathering, transfer). To describe them quantitatively for a modeling end, constructed Technosols are suitable candidates to be studied, because their initial composition can be controlled. The challenging objective of our work was then to monitor and study nondestructively, visually, and quantitatively the effect of biological agents on the evolution of a constructed Technosol.Materials and methodsThe Technosol is constructed in three horizons. From bottom to top of the mesocosms, horizons are: (1) gravels, (2) treated industrial soil mixed with paper mill sludge (2/3, 1/3 mass ratio), and (3) green waste compost. Pedogenetic factors are organized according to two modalities each repeated three times: “Plant and Fauna,” where six adult earthworms, Lumbricus castaneus, and five seeds of white lupin, Lupinus albus, are inoculated, and a “control” without any plant and macrofauna. Moisture of 60/80xa0% field capacity is maintained in all treatments throughout a 14-month experiment. Soil evolution is studied by recurrent image acquisition of the soil profile.Results and discussionAt the beginning, roots grew preferentially through fissures and cracks at 10xa0mm·day-1 speed during the first 3xa0weeks. Then they grew exponentially until reaching a plateau and decreased when plants were at the end of their life cycle. Earthworms prospected the top of the soil first before exploring the deeper horizons preferentially along roots. During their round-trip between the two horizons, earthworms translocated compost. The porosity increased in the first hours of experience and decreased when the system was irrigated. In the Control, porosity continuously decreased while it increased in Fauna–Plant treatment. The evolution of aggregation is root system-dependent. Aggregation was constant in control but significantly increased in Fauna–Plant treatment (about 10 times at 268 days compared with the control).ConclusionsThe use of nondestructive observation of soil profiles is therefore an innovative way of monitoring and quantifying the impact of pedogenetic factors on the functioning and evolution of Technosols. Porosity and aggregation increased with time under the influence of biological factors. Constructed Technosols could be used as model soils for studying the dynamics of soil structure. Although their composition is likely to be different from natural soils, the pedogenetic evolution of Technosols is similar to that of natural soils when facing the impact of biological factors.

Metal Immobilization on Wood-Derived Biochars: Distribution and Reactivity of Carbonate Phases

Metals can be immobilized on biochars by precipitation with carbonate. The distribution of metal-carbonate phases at the surface of biochars and the conditions of their formation, however, are unknown. Electron microscopy and X-photon spectroscopy were used to characterize carbonate phases in various morphological groups of particles of a wood-derived biochar, both before and after a metal-sorption experiment. Our results showed that the distribution of metals at the surface of biochar particles depended on the corresponding wood tissues and the presence of carbonate phases. Metals were particularly concentrated (i) within calcium carbonate crystals in bark-derived particles, which originated from calcium oxalate crystals formed prior to pyrolysis, and (ii) as new phases formed by the reprecipitation of carbonate on specific tissues of biochar. The formation of biochar carbonate phases and their redistribution by dissolution-precipitation mechanisms may primarily control the localization of metals on biochar particles and the durability of metals immobilization.

Soil Microstructures Examined Through Transmission Electron Microscopy Reveal Soil-Microorganisms Interactions

Research over the last few decades has shown that the characterization of microaggregates at the micrometer scale using Transmission Electron Microscopy (TEM) provides useful information on the influence of microorganisms on soil functioning. By taking soil heterogeneity into account, TEM provides qualitative information about the state of bacteria and fungi (e.g., intact state of living organisms, spores, residues) at the sampling date within organo-mineral associations, from the soil-root interface to the bulk soil, and in biogenic structures such as casts. The degree of degradation of organic matter can be related to the visualized enzymatic potential of microorganisms that degrade them, thus indicating organic matter dynamics within soil aggregates. In addition, analytical TEM characterization of microaggregates by EELS (Electron Energy Loss Spectroscopy) or EDX (Energy Dispersive X-rays spectroscopy) provides in situ identification of microbial involvement in the biogeochemical cycles of elements. Furthermore, micrometer characterization associated with other methodologies such as Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) or soil fractionation, enables monitoring both incorporation of biodegraded litter within soil aggregates and impacts of microbial dynamics on soil aggregation, particularly due to production of extracellular polymeric substances. The present focused review suggests that such an approach using micrometer characterization of soil microhabitats provides relevant qualitative and quantitative information when monitoring and modelling microbial processes in dynamics of organo-mineral associations.