Earthworm Cast Formation and Development: A Shift From Plant Litter to Mineral Associated Organic Matter

Earthworms play a major role in litter decomposition, in processing soil organic matter and driving soil structure formation. Earthworm casts represent hot spots for carbon turnover and formation of biogeochemical interfaces in soils. Due to the complex microscale architecture of casts, understanding the mechanisms of cast formation and development at a process relevant scale, i.e., within microaggregates and at the interface between plant residues, microorganisms and mineral particles, remains challenging. We used stable isotope enrichment to trace the fate of shoot and root litter in intact earthworm cast samples. Surface casts produced by epi-anecic earthworms (Lumbricus terrestris) were collected after 8 and 54 weeks of soil incubation in mesocosms, in the presence of 13C-labeled Ryegrass shoot or root litter deposited onto the soil surface. To study the alteration in the chemical composition from initial litter to particulate organic matter (POM) and mineral-associated organic matter (MOM) in cast samples, we used solid-state 13C Nuclear Magnetic Resonance spectroscopy (13C-CPMAS-NMR) and isotopic ratio mass spectrometry (EA-IRMS). We used spectromicroscopic approach to identify plant tissues and microorganisms involved in plant decomposition within casts. A combination of transmission electron microscopy (TEM) and nano-scale secondary ion mass spectrometry (NanoSIMS) was used to obtain the distribution of organic carbon and δ13C within intact cast sample structures. We clearly demonstrate a different fate of shoot- and root-derived organic carbon in earthworm casts, with a higher abundance of less degraded root residues recovered as particulate organic matter on the short-term (8 weeks) (73 mg·g−1 in Cast-Root vs. 44 mg·g−1 in Cast-Shoot). At the early stages of litter decomposition, the chemical composition of the initial litter was the main factor controlling the composition and distribution of soil organic matter within casts. At later stages, we can demonstrate a clear reduction of structural and chemical differences in root and shoot-derived organic products. After 1 year, MOM clearly dominated the casts (more than 85% of the total OC in the MOM fraction). We were able to highlight the shift from a system dominated by free plant residues to a system dominated by MOM during cast formation and development

Analytical pyrolysis as a tool to probe soil organic matter

International audienceThe environmental importance of soil organic matter (SOM) in the ecosystems and in the C biogeochemical cycle is well established. Indeed, it represents the main terrestrial carbon pool and due to its vulnerability, it plays a key role in the global carbon cycle. However, as SOM is mainly composed of products resulting from microbial and physicochemical transformations of vegetal, microbial and animal biomass, it results in a heterogeneous mixture. This complexity, along with organo-mineral interactions, makes challenging the characterization of SOM composition at the molecular scale. Nevertheless, its precise characterization is essential to determine its fate in the environment and eventually to provide recommendations on sustainable practices. Among the available techniques to analyse SOM, thermal degradations appear as especially efficient as they are less selective than some chemical ones, leading to a larger view of the SOM chemical structure. Analytical pyrolysis was thus used in a wide range of soil science fields including studies on pedogenesis and anthropic effects. It allows to characterize SOM at the molecular level, including identification of biomarkers, and to compare different soils and/or different horizons in a given soil profile under various impacts (land use, evolution, …). The review of recent developments in data acquisition and/or processing leads us to provide guidelines to select the most appropriate method and to avoid possible pitfalls. Examples will illustrate the wide range of soil science applications and show the potential and limitations of this approach

Etude structurale et dynamique des fractions lipidiques et organiques réfractaires de sols d'une chronoséquence forêt/maïs (Cestas, sud ouest de la France)

PARIS-BIUSJ-Thèses (751052125) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Are chemical oxidation methods relevant to isolate a stable pool of centenial carbon ? Using soils from a long term bare fallow to answer this question

Several mechanisms are developed to explain carbon stabilization in soils: (i) physical protection, (ii) protection through interaction with the soil matrix and (iii) chemical recalcitrance. To date there is still no successful experimental way to isolate the stable pool of soil organic matter (SOM), i.e., the SOM that has a residence time of centuries to millennia. Moreover, the relevance of the latter mechanism is currently being discussed at this time scale (Schmidt et al., 2011).A recent study showed that long term bare fallows (LTBF) offer a unique opportunity to study the stable compartment of SOM (Barré et al., 2011), as without carbon inputs and with continuing biodegradation and mineralization, SOM becomes progressively enriched in its most stable components. At the INRA 42-plot experiment, initiated in 1928 in Versailles, over 80% of total SOM is stable.Here, we take advantage of the 42-plot LTBF to test the relevance of chemical oxidation methods to isolate a stable pool of carbon (Mikkuta et al., 2005). We selected two reagents -hydrogen peroxide, H2O2, and sodium hypochlorite, NaOCl- and studied their effect on soil carbon content and SOM chemical nature after 0 and 80 years of LTBF. If these methods can isolate the stable C, then chemical oxidation resistant-C from 0 and 80 years of bare fallow should equate, both in amount and chemical nature, TOC after 80 years of bare fallow.Materials and methods. We used 0-20 cm depth soil samples from the 42 plots archive collection. In order to prevent the influence of decomposing plant debris, we fractionated the soil samples, and separated the fraction of particles <50μm (i.e. silt+clay). These were subjected to H2O2, and NaOCl oxidation according to (Jagadamma et al., 2010; Bruun et al., 2008). We used Curie point pyrolysis coupled with gas chromatography and mass spectroscopy (Py-GC-MS) to analyse SOM chemical nature.Results showed that, after chemical oxidation, both soils contained approximately the same amount of chemical oxidation resistant OC (≈ 1 mgC g-1 soil), with slightly more for 1929 soils than for 2008 soils. This amount was five times smaller than TOC in the untreated 2008 soils, after 80 years of biological oxidation. Moreover the chemical signatures of the soils, as seen with Py-GC-MS, showed that, after chemical oxidation, very fewidentifiable pyrolysis products remained. The characteristic alkanes and alkenes peaks of 1929 samples and the alkylbenzenes of 2008 samples had disappeared.In conclusion, the two reagents isolated a subset of total soil organic carbon, but removed too much SOM. The resistant C fraction was smaller than century stable C in this soil and its chemical nature was different. We conclude that chemical oxidation methods cannot efficiently isolate a stable pool of carbon.Références bibliographiques :Barré et al., 2010, Biogeosciences, Vol 7: 3839-3850, Quantifying and isolating stable soil organic carbon using long-term bare fallow experimentsBruun et al., 2008, European Journal of Soil Science, Vol 59, Issue 2: 247–256, In search of stable soil organic carbon fractions: a comparison of methods applied to soils labelled with 14C for 40 days or 40 yearsJagadamma et al., 2010, Biogeochemistry, Vol 98: 29–44, Evaluation of structural chemistry and isotopic signatures of refractory soil organic carbon fraction isolated by wet oxidation methods 22Mikutta et al., 2005, Soil Science Society of America Journal, Vol 69: 120-135, Organic matter removal from soil using hydrogen peroxide, sodium hypochlorite, and disodium peroxodisulfateSchmidt et al., 2011, Nature, Vol 478: 49-56, Persistence of soil organic matter as an ecosystem propert

Incorporation of 13C labelled shoot residues in Lumbricus terrestris casts: A combination of transmission electron microscopy and nanoscale secondary ion mass spectrometry

Earthworms transform organo-mineral associations in soil, especially by incorporating fresh residuesinside casts where the microbial abundance and activity are enhanced. The heterogeneous distribution oforganic carbon in these structures influences decomposition levels at the microscale. The incorporationof 13C labelled plant residues by Lumbricus terrestris inside cast was investigated, through the innovativecombination of two fine scale imaging techniques: transmission electron microscopy and nanoscalesecondary ion mass spectrometry (NanoSIMS). The association of these methods sheds new lights onorgano-mineral structures. Different types of organic matter (plant residues, microbial remains) wereidentified in the casts and the freshly incorporated residues could be differentiated from the indigenousorganic matter thanks to d13C NanoSIMS mapping. 13C labelled bacteria and fungi abundance and diversityhighlight their preeminent role in litter decomposition within casts. Labelled plant residuesobserved at various stages of decomposition and microorganisms presented highly variable d13C values,emphasizing the complexity of organic matter dynamics and the importance of microscale analyses todescribe this variability. Thus, the combination of NanoSIMS and TEM shows great potential to relateorganic matter stages of decomposition with their 13C signature

The Organic Signature of an Experimental Meat-cooking Fireplace: the Identification of Nitrogen Compounds and their Archaeological Potential

International audienceA better understanding of the operation and use of prehistoric fires is fundamental to interpreting the organization of living spaces. Following a previous study that focused on the organic signatures of fireplaces fueled with wood and/or bones, this study targeted the completion of an experimental reference database through the addition of a wood-fueled fireplace dedicated to the cooking of meat. Different sedimentary features of this experimental fireplace were visually identified (e.g. darkening, reddening), sampled, and subjected to geochemical analysis. Corg and N contents were quantified, samples were extracted with organic solvents and analyzed through GC-MS and bulk organic matter was characterized through py(TMAH)-GC-MS. Five different samples were studied and compared with a control sample, representative of the local background. A significant increase of Corg and N contents was measured for the three samples presenting darkened or charred caracteristics. The meat-cooking fireplace seems to be characterized by the strong contribution of nitrogen, which was visible in elementary analyses as well as in the molecular composition of solvent extracts, and bulk organic matter. More specifically, compounds containing nitrile functions, amides, N-heterocyclic and N-aromatic compounds could be detected in solvent extracts. Amines, amides, N-heterocyclic and N-aromatic compounds could be identified in py(TMAH)-GC-MS. Some of these compounds present a relative stability in soils and could therefore aid in our comprehension and functional interpretations of archaeological fireplaces, and may, more particularly, make it possible to highlight the use of fireplaces for the cooking of meat

Influence of earthworms on apolar lipidfeatures in soils after 1 year of incubation

International audienceMolecular and compound specific isotope compositions of apolar lipids were characterized in soil mesocosms incubated for 1 year with or without 13C-labelled plant residues and earthworms, in order to investigate, at the molecular scale, the effect of earthworms on the fate of organic matter (OM) in soils. Molecular and isotope composition of long chain alkanes in casts confirmed that earthworms preferentially ingest soil fractions rich in plant debris. Apolar lipid specific isotope composition allowed calculation of the proportion of carbon derived from the labelled residues (Clab). Casts displayed higher Clab values than surrounding soil while soil without earthworm exhibited intermediate Clab. The odd-over-even predominance (OEP) of alkanes suggested they are probably less degraded in casts than in the surrounding soil. Taken together, OEP and Clab values suggested that besides high incorporation of plant residues, earthworms may also favor the preservation of plant apolar lipids in their casts. Additionally, chain length and isotope pattern of alkanes further suggested root lipids were probably less degraded than shoot lipids. High 13C-incorporation level for the bacterial biomarker hopene provided evidence for intense recycling of plant OM and suggested further contribution of bacterial necromass to soil OM

Compréhension et amélioration des mécanismes intervenant dans la bioremédiation du benzo[a]pyrène dans les sols par le champignon Talaromyces helicus

International audienceEn France, 6800 sites pollués ou potentiellement pollués ont été recensé en 2018 par la base de données en ligne BASOL du BRGM (Bureau de recherches géologiques et minières). 31% de ces sites présentent une pollution aux hydrocarbures, et 12 % une contamination par des hydrocarbures aromatiques polycycliques (HAP). Ces polluants organiques représentent une problématique récurrente dans les écosystèmes du fait de leur prévalence, de leur faible biodégradabilité, de leur faible biodisponibilité, de leurs propriétés toxiques et de leur accumulation dans les chaînes alimentaires. En effet, les HAP s'adsorbent sur la matière organique du sol et sont peu biodisponibles, ce qui signifie une moindre exposition aux microorganismes susceptibles de les dégrader

Soil tillage impact on the relative contribution of dissolved, particulate and gaseous (CO₂) carbon losses during rainstorms

Although the impact of water erosion on soil carbon losses has been widely investigated, little is known about the relative contributions of dissolved, particulate and gaseous losses, a prerequisite for understanding the mechanisms of carbon (C) export from soils and designing mitigation procedures. The main objective of this study was to quantify the losses of dissolved organic and inorganic C (DOC, DIC), particulate organic C (POC) and soil CO2 from runoff microplots on tilled (T) and no-tilled (NT) soils. The study was performed in the Beauce region in central France under Luvisols using 45 and 80 mmh−1 artificial rains. At 45 mm h-1, T plots produced C erosion at an average of 1189.7 +- 114.8 mg C m-2 h-1 with 76.9% of it being POC (915.0 +- 100.0 mg C m-2 h-1), 21.7% DOC (258.3.0 +- 7.6 mg C m-2 h-1), 1.4% (16.3 +- 7.2 mg C m-2 h-1), DIC and 0.01% CO2. NT decreased total soil C losses by 95% (from 0.8 to 0.038 g C m-2 h-1) and soil C losses were as CO2 only. At 80 mm h-1, NT surprisingly increased C erosion by 40% compared to T (from 39.4 to 55.3 g C m2 h-1), with 95.5% of the C losses being POC vs 88.7% for T. These results on rainstorm-induced C fluxes from soils controlled by tillage are expected to be of future value: (1) for selecting appropriate land management that will mitigate against C losses from soils and improve soil carbon sequestration and; (2) to better understand the Global Carbon Cycle and further develop the existing models