Delphine Derrien

Pulse-labelling trees to study carbon allocation dynamics: a review of methods, current knowledge and future prospects

Pulse-labelling of trees with stable or radioactive carbon (C) isotopes offers the unique opportunity to trace the fate of labelled CO(2) into the tree and its release to the soil and the atmosphere. Thus, pulse-labelling enables the quantification of C partitioning in forests and the assessment of the role of partitioning in tree growth, resource acquisition and C sequestration. However, this is associated with challenges as regards the choice of a tracer, the methods of tracing labelled C in tree and soil compartments and the quantitative analysis of C dynamics. Based on data from 47 studies, the rate of transfer differs between broadleaved and coniferous species and decreases as temperature and soil water content decrease. Labelled C is rapidly transferred belowground-within a few days or less-and this transfer is slowed down by drought. Half-lives of labelled C in phloem sap (transfer pool) and in mature leaves (source organs) are short, while those of sink organs (growing tissues, seasonal storage) are longer. (13)C measurements in respiratory efflux at high temporal resolution provide the best estimate of the mean residence times of C in respiratory substrate pools, and the best basis for compartmental modelling. Seasonal C dynamics and allocation patterns indicate that sink strength variations are important drivers for C fluxes. We propose a conceptual model for temperate and boreal trees, which considers the use of recently assimilated C versus stored C. We recommend best practices for designing and analysing pulse-labelling experiments, and identify several topics which we consider of prime importance for future research on C allocation in trees: (i) whole-tree C source-sink relations, (ii) C allocation to secondary metabolism, (iii) responses to environmental change, (iv) effects of seasonality versus phenology in and across biomes, and (v) carbon-nitrogen interactions. Substantial progress is expected from emerging technologies, but the largest challenge remains to carry out in situ whole-tree labelling experiments on mature trees to improve our understanding of the environmental and physiological controls on C allocation.

NanoSIMS Study of Organic Matter Associated with Soil Aggregates: Advantages, Limitations, and Combination with STXM

Direct observations of processes occurring at the mineral-organic interface are increasingly seen as relevant for the cycling of both natural soil organic matter and organic contaminants in soils and sediments. Advanced analytical tools with the capability to visualize and characterize organic matter at the submicrometer scale, such as Nano Secondary Ion Mass Spectrometry (NanoSIMS) and Scanning Transmission X-ray Microscopy (STXM) coupled to Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), may be combined to locate and characterize mineral-associated organic matter. Taking advantage of samples collected from a decadal (15)N litter labeling experiment in a temperate forest, we demonstrate the potential of NanoSIMS to image intact soil particles and to detect spots of isotopic enrichment even at low levels of (15)N application. We show how microsites of isotopic enrichment detected by NanoSIMS can be speciated by STXM-NEXAFS performed on the same particle. Finally, by showing how (15)N enrichment at one microsite could be linked to the presence of microbial metabolites, we emphasize the potential of this combined imaging and spectroscopic approach to link microenvironment with geochemical process and/or location with ecological function.

Increasing soil carbon storage: mechanisms, effects of agricultural practices and proxies. A review

The international 4 per 1000 initiative aims at supporting states and non-governmental stakeholders in their efforts towards a better management of soil carbon (C) stocks. These stocks depend on soil C inputs and outputs. They are the result of fine spatial scale interconnected mechanisms, which stabilise/destabilise organic matter-borne C. Since 2016, the CarboSMS consortium federates French researchers working on these mechanisms and their effects on C stocks in a local and global change setting (land use, agricultural practices, climatic and soil conditions, etc.). This article is a synthesis of this consortium’s first seminar. In the first part, we present recent advances in the understanding of soil C stabilisation mechanisms comprising biotic and abiotic processes, which occur concomitantly and interact. Soil organic C stocks are altered by biotic activities of plants (the main source of C through litter and root systems), microorganisms (fungi and bacteria) and ‘ecosystem engineers’ (earthworms, termites, ants). In the meantime, abiotic processes related to the soil-physical structure, porosity and mineral fraction also modify these stocks. In the second part, we show how agricultural practices affect soil C stocks. By acting on both biotic and abiotic mechanisms, land use and management practices (choice of plant species and density, plant residue exports, amendments, fertilisation, tillage, etc.) drive soil spatiotemporal organic inputs and organic matter sensitivity to mineralisation. Interaction between the different mechanisms and their effects on C stocks are revealed by meta-analyses and long-term field studies. The third part addresses upscaling issues. This is a cause for major concern since soil organic C stabilisation mechanisms are most often studied at fine spatial scales (mm–μm) under controlled conditions, while agricultural practices are implemented at the plot scale. We discuss some proxies and models describing specific mechanisms and their action in different soil and climatic contexts and show how they should be taken into account in large scale models, to improve change predictions in soil C stocks. Finally, this literature review highlights some future research prospects geared towards preserving or even increasing C stocks, our focus being put on the mechanisms, the effects of agricultural practices on them and C stock prediction models.

A multi-scale approach to determine accurate elemental and isotopic ratios by nano-scale secondary ion mass spectrometry imaging

RATIONALE Nano-scale secondary ion mass spectrometry (NanoSIMS) is still hampered by a lack of appropriate calibration method for the quantification of elemental and isotopic ratios in heterogeneous materials such as soil samples. The potential of (13)C-(15)N-labeled density fractions of soil to calibrate the C/N, (13)C/(12)C and (15)N/(14)N ratios provided by NanoSIMS was evaluated. METHODS The spatial organization of soil particles found at the macro- and micro-scales were compared. The C/N, (13)C/(12)C and (15)N/(14)N ratios measured at the macroscopic scale from different density fractions using an elemental analyzer coupled to an isotope ratio mass spectrometer (EA/IRMS) were compared with the corresponding micro-scale NanoSIMS measurements. When the macro- and micro-scales patterns were similar, macroscopic scale measurements obtained by EA/IRMS and the corresponding NanoSIMS C/N and (15)N/(14)N ratios averaged per fraction were used to obtain correction equations. The correction method using the internal calibration procedure was compared with the traditional one using a single organic standard. RESULTS It was demonstrated that the correction method using an internal calibration procedure was applicable for NanoSIMS images acquired on more than 500 µm(2) per fraction and provided more accurate C/N and (15)N/(14)N ratios than the traditional correction method. CONCLUSIONS As long as the NanoSIMS sampling was representative of the macroscopic properties, the correction method using an internal calibration procedure allowed better quantification of the isotope tracers and characterization of the C/N ratios. This method not only produced qualitative images, but also accurate quantitative parameters from which ecological interpretations can be derived.

Phosphorus status of soils from contrasting forested ecosystems in southwestern Siberia: effects of microbiological and physicochemical properties

The Siberian forest is a tremendous repository of terrestrial organic carbon (C), which may increase owing to climate change, potential increases in ecosystem productivity and hence C sequestration. Phosphorus (P) availability could limit the C sequestration potential, but tree roots may mine the soil deep to increase access to mineral P. Improved understanding and quantification of the processes controlling P availability in surface and deep soil layers of Siberian forest ecosystems are thus required. The objectives of the present study were to (1) evaluate P status of surface and deep soil horizons from different forest plots in southwestern Siberia and (2) assess the effects of physicochemical soil properties, microbiological activity and decomposition processes on soil P fractions and availability. Results revealed high concentrations of total P (879–1042 mg kg −1 in the surface mineral soils) and plant-available phosphate ions. In addition, plant-available phosphate ions accumulated in the subsoil, suggesting that deeper root systems may mine sufficient available P for the trees and the potentially enhanced growth and C sequestration, may not be P-limited. Because the proportions of total organic P were large in the surface soil layers (47–56% of total P), we concluded that decomposition processes may play a significant role in P availability. However, microbiological activity and decomposition processes varied between the study plots and higher microbiological activity resulted in smaller organic P fractions and consequently larger available inorganic P fractions. In the studied Siberian soils, P availability was also controlled by the physicochemical soil properties, namely Al and Fe oxides and soil pH.

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.

Comparison of gas chromatography/isotope ratio mass spectrometry and liquid chromatography/isotope ratio mass spectrometry for carbon stable-isotope analysis of carbohydrates

RATIONALE We compared gas chromatography/isotope ratio mass spectrometry (GC/IRMS) and liquid chromatography/isotope ratio mass spectrometry (LC/IRMS) for the measurement of δ(13)C values in carbohydrates. Contrary to GC/IRMS, no derivatisation is needed for LC/IRMS analysis of carbohydrates. Hence, although LC/IRMS is expected to be more accurate and precise, no direct comparison has been reported. METHODS GC/IRMS with the aldonitrile penta-acetate (ANPA) derivatisation method was compared with LC/IRMS without derivatisation. A large number of glucose standards and a variety of natural samples were analysed for five neutral carbohydrates at natural abundance as well as at (13)C-enriched levels. Gas chromatography/chemical ionisation mass spectrometry (GC/CIMS) was applied to check for incomplete derivatisation of the carbohydrate, which would impair the accuracy of the GC/IRMS method. RESULTS The LC/IRMS technique provided excellent precision (±0.08‰ and ±3.1‰ at natural abundance and enrichment levels, respectively) for the glucose standards and this technique proved to be superior to GC/IRMS (±0.62‰ and ±19.8‰ at natural abundance and enrichment levels, respectively). For GC/IRMS measurements the derivatisation correction and the conversion of carbohydrates into CO2 had a considerable effect on the measured δ(13)C values. However, we did not find any significant differences in the accuracy of the two techniques over the full range of natural δ(13)C abundances and (13)C-labelled glucose. The difference in the performance of GC/IRMS and LC/IRMS diminished when the δ(13)C values were measured in natural samples, because the chromatographic performance and background correction became critical factors, particularly for LC/IRMS. The derivatisation of carbohydrates for the GC/IRMS method was complete. CONCLUSIONS Although both LC/IRMS and GC/IRMS are reliable techniques for compound-specific stable carbon isotope analysis of carbohydrates (provided that derivatisation is complete and the calibration requirements are met), LC/IRMS is the technique of choice. The reasons for this are the improved precision, simpler sample preparation, and straightforward isotopic calibration.

Distributions of fine root length and mass with soil depth in natural ecosystems of southwestern Siberia

AimsForest-steppe and sub-taiga, two main biomes of southwestern Siberia, have been predicted to shift and spread northward with global change. However, ecological projections are still lacking a description of belowground processes in which fine roots play a significant role. We characterized regional fine root patterns in terms of length and mass comparing: 1) sites and 2) vegetation covers.MethodsWe assessed fine root length and mass down to one meter in aspen (Populus tremula) and in grassland stands on six sites located in the forest-steppe and sub-taiga zones and presenting contrasting climate and soil conditions. We distinguished fine roots over diameter classes and also between aspen and understorey in forest. Vertical fine root exploration, fine root densities and total length and mass were computed for all species. Morphological parameters were computed for aspen.ResultsIn both forest and grassland, exploration was deeper and total length and mass were higher in forest-steppe than in sub-taiga. Exploration tended to be deeper in forest than in grassland and for trees than for understorey vegetation within forest stands.ConclusionsThe differences in rooting strategies are related with both pedo-climatic conditions and vegetation cover. Further investigations on nutrient and water availability and on fine root dynamics should permit a better understanding of these patterns and help predicting their future with global changes.

Soil evaporation and organic matter turnover in the Sub-Taiga and Forest-Steppe of southwest Siberia

Southwest Siberia encompasses the forest-steppe and sub-taiga climatic zones and has historically been utilized for agriculture. Coinciding with predicted changes in climate for the region is the pressure of agricultural development; however, a characterization of the soil water and carbon dynamics is lacking. We assessed current soil water properties and soil organic carbon turnover in forests and grasslands for two sites that span the forest steppe and sub-taiga bioclimatic zones. Soil evaporation was 0.62 ± 0.17 mm d−1 (mean ± standard error) in grasslands and 0.45 ± 0.08 mm d−1 in the forests of the forest-steppe site. Evaporation at the sub-taiga site was 1.80 ± 1.70 mm d−1 in grasslands and 0.96 ± 0.05 mm d−1 in forest plots. Evaporation was significantly greater at the sub-taiga site than the forest-steppe site. The density of fine roots explained the soil water isotopic patterns between vegetation types and sites. We found soil organic matter turnover to be three times faster in the sub-taiga site than in the forest-steppe site. Our results show that while climate factors, in particular snow levels, between the two sites are drivers for water and carbon cycles, site level hydrology, soil characteristics, and vegetation directly interact to influence the water and carbon dynamics.