Bioenergetic control of soil carbon dynamics across depth.

Soil carbon dynamics is strongly controlled by depth globally, with increasingly slow dynamics found at depth. The mechanistic basis remains however controversial, limiting our ability to predict carbon cycle-climate feedbacks. Here we combine radiocarbon and thermal analyses with long-term incubations in absence/presence of continuously C/C-labelled plants to show that bioenergetic constraints of decomposers consistently drive the depth-dependency of soil carbon dynamics over a range of mineral reactivity contexts.

Can stable isotopes quantify soil carbon build-up from organic fertilizers?

The application of organic fertilizers (OF) can supply carbon (C) to the soil in crop fields. OF-derived C (OF-C) is often estimated using the differential method that can be biased due to indirect effects of OF on soil C. This study tested three methods to quantify OF-C: (i) the widespread differential method, (ii) the synchronic isotope method comparing plots with and without OF and (iii) the asynchronic isotope method mimicking a trial without a control plot.

Can N O emissions offset the benefits from soil organic carbon storage?

To respect the Paris agreement targeting a limitation of global warming below 2°C by 2100, and possibly below 1.5°C, drastic reductions of greenhouse gas emissions are mandatory but not sufficient. Large-scale deployment of other climate mitigation strategies is also necessary.

Including Stable Carbon Isotopes to Evaluate the Dynamics of Soil Carbon in the Land-Surface Model ORCHIDEE.

Soil organic carbon (SOC) is a crucial component of the terrestrial carbon cycle and its turnover time in models is a key source of uncertainty. Studies have highlighted the utility of δC measurements for benchmarking SOC turnover in global models. We used C as a tracer within a vertically discretized soil module of a land-surface model, Organising Carbon and Hydrology In Dynamic Ecosystems- Soil Organic Matter (ORCHIDEE-SOM).

Liquid chromatographic isolation of individual carbohydrates from environmental matrices for stable carbon analysis and radiocarbon dating.

Carbohydrates are among the most abundant organic molecules in both aquatic and terrestrial ecosystems; however, very few studies have addressed their isotopic signature using compound-specific isotope analysis, which provides additional information on their origin (δC) and fate (ΔC). In this study, semi-preparative liquid chromatography with refractive index detection (HPLC-RI) was employed to produce pure carbohydrate targets for subsequent offline δC and ΔC isotopic analysis. δC analysis was performed by elemental analyzer-isotope ratio mass spectrometer (EA-IRMS) whereas ΔC analysis was performed by an innovative measurement procedure based on the direct combustion of the isolated fractions using an elemental analyzer coupled to the gas source of a mini carbon dating system (AixMICADAS).

Effects of soil process formalisms and forcing factors on simulated organic carbon depth-distributions in soils.

Soil organic carbon (OC) sequestration (i.e. the capture and long-term storage of atmospheric CO) is being considered as a possible solution to mitigate climate change, notably through land use change (conversion of cropped land into pasture) and conservation agricultural practices (reduced tillage).

Atmosphere-soil carbon transfer as a function of soil depth.

The exchange of carbon between soil organic carbon (SOC) and the atmosphere affects the climate and-because of the importance of organic matter to soil fertility-agricultural productivity. The dynamics of topsoil carbon has been relatively well quantified, but half of the soil carbon is located in deeper soil layers (below 30 centimetres), and many questions remain regarding the exchange of this deep carbon with the atmosphere. This knowledge gap restricts soil carbon management policies and limits global carbon models.

Labelled microbial culture as a calibration medium for (13) C-isotope measurement of derivatized compounds: application to tert-butyldimethylsilyl amino acids.

Rationale: Compound-specific stable carbon isotope analysis by gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) is widely used in studies of environmental or biological functioning. In the case of derivatized molecules, a calibration might be required due to added non-analyte carbon and in some cases non-stoichiometric recovery by the mass spectrometer.

Methods: Two biological materials of known isotopic composition were produced by microbial cell cultures on either (13) C-labelled glucose or non-labelled glucose as sole source of carbon.

Deep soil carbon dynamics are driven more by soil type than by climate: a worldwide meta-analysis of radiocarbon profiles.

The response of soil carbon dynamics to climate and land-use change will affect both the future climate and the quality of ecosystems. Deep soil carbon (>20 cm) is the primary component of the soil carbon pool, but the dynamics of deep soil carbon remain poorly understood. Therefore, radiocarbon activity (Δ14C), which is a function of the age of carbon, may help to understand the rates of soil carbon biodegradation and stabilization.

Measurement of δ13C values of soil amino acids by GC-C-IRMS using trimethylsilylation: a critical assessment.

In this study, we evaluated trimethylsilyl (TMS) derivatives as derivatization reagents for the compound-specific stable carbon isotope analysis of soil amino acids by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS). We used non-proteinogenic amino acids to show that the extraction-derivatization-analysis procedure provides a reliable method to measure δ(13)C values of amino acids extracted from soil. However, we found a number of drawbacks that significantly increase the final total uncertainty.

Contamination of soil by copper affects the dynamics, diversity, and activity of soil bacterial communities involved in wheat decomposition and carbon storage.

A soil microcosm experiment was conducted to evaluate the influence of copper contamination on the dynamics and diversity of bacterial communities actively involved in wheat residue decomposition. In the presence of copper, a higher level of CO(2) release was observed, which did not arise from greater wheat decomposition but from a higher level of stimulation of soil organic matter mineralization (known as the priming effect). Such functional modifications may be related to significant modifications in the diversity of active bacterial populations characterized using the DNA stable-isotope probing approach.

Molecular turnover time of soil organic matter in particle-size fractions of an arable soil.

The composition and molecular residence time of soil organic matter (SOM) in four particle-size fractions (POM >200 microm, POM 63-200 microm, silt and clay) were determined using Curie-point pyrolysis/gas chromatography coupled on-line to mass spectrometry. The fractions were isolated from soils, either continuously with a C(3) wheat (soil (13)C value = -26.4 per thousand), or transferred to a C(4) maize (soil (13)C value = -20.

Plant host habitat and root exudates shape soil bacterial community structure.

The rhizosphere is active and dynamic in which newly generated carbon, derived from root exudates, and ancient carbon, in soil organic matter (SOM), are available for microbial growth. Stable isotope probing (SIP) was used to determine bacterial communities assimilating each carbon source in the rhizosphere of four plant species. Wheat, maize, rape and barrel clover (Medicago truncatula) were grown separately in the same soil under (13)CO(2) (99% of atom (13)C) and DNA extracted from rhizosphere soil was fractionated by isopycnic centrifugation.

Identification of cellulolytic bacteria in soil by stable isotope probing.

Plant residues, mainly made up of cellulose, are the largest fraction of organic carbon material in terrestrial ecosystems. Soil microorganisms are mainly responsible for the transfer of this carbon to the atmosphere, but their contribution is not accurately known. The aim of the present study was to identify bacterial populations that are actively involved in cellulose degradation, using the DNA-stable isotope probing (DNA-SIP) technique.

Long term changes in the distribution and delta(15)N values of individual soil amino acids in the absence of plant and fertiliser inputs.

The long-term 'biodegradation' on soil amino acids was examined in the control plots of '42 parcelles' experiment, established in 1928 at INRA, Versailles (France). None of the plots is cultivated, but is kept free of weeds, and mixed to a depth of 25 cm twice yearly. Topsoil (0-10 cm depth) samples collected in 1929, 1963 and 1997 were subjected to acid hydrolysis (6 N HCl) for comparison.

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

Soils that 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 (approximately 2%, dry weight), was investigated by X-ray diffraction, synchrotron-based X-ray microfluorescence, 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 and linear combination fits.

Measurement of the 13C/12C ratio of soil-plant individual sugars by gas chromatography/combustion/isotope-ratio mass spectrometry of silylated derivatives.

Carbohydrate is an important pool in the terrestrial carbon cycle. The potential offered by natural and artificial 13C-labelling techniques should therefore be applied to the investigation of the dynamics of individual sugars in soils. For this reason, we evaluated the method of 13C sugar analysis by gas chromatography/combustion/isotope-ratio mass spectrometry (GC/C/IRMS) after hydrolysis and direct trimethylsilylation.

Molecular insight into soil carbon turnover.

Curie-point pyrolysis-gas chromatography coupled on-line to mass spectrometry (Py-GC/MS) and isotope ratio mass spectrometry (Py-GC/IRMS) were used to determine the individual turnover rate of specific carbohydrates, lignin, lipids and N-containing compounds from French arable soils. The analysed soils were cultivated, either continuously with a C3 plant (wheat delta(13)C-value = -25.2 per thousand), or transferred to a C4 plant (maize delta(13)C-value = -11.

Present dynamics of the savanna-forest boundary in the Congolese Mayombe: a pedological, botanical and isotopic (C and C) study.

Isolated savannas enclosed by forest are especially abundant in the eastern part of the Congolese Mayombe. They are about 3000 years old, and were more extensive some centuries ago. The boundary between forest and savanna is very abrupt, as a consequence of the numerous savanna fires lit by hunters.

Earthworm diet related to soil organic matter dynamics through C measurements.

The ability of earthworms to assimilate soil organic pools of different ages was investigated in field conditions through natural C labelling. Provided that C natural abundance of earthworm tissues is determined by their diet, the assimilation of soil organic matter by earthworms was estimated by measuring C/C ratio of tissues of earthworms sampled in soils containing organic pools with differing C/C ratios. Earthworms were collected in two locations where the vegetation shifted from a C3 to a C4 type (France), or from a C4 to a C3 type (Ivory Coast) 3 to 20 years ago.