Search

L’émergence de dispositifs analytiques portables tels que la spectrométrie de fluorescence des rayons X et la spectrométrie sur plasma induit par laser, permet de disposer d’outils pratiques et rapides pour réaliser des mesures élémentaires directement sur les sols, sans préparation préalable des échantillons. Leur utilisation peut être envisagée pour cartographier une parcelle, pour fournir des indicateurs en temps réel des potentialités d’un sol mais aussi pour suivre ces performances dans le temps en fonction d’un changement de contexte.

International audience

International audience

International audience

International audience; Introduction Agronomic research has a growing interest in collecting faster, cheaper and still efficient information about agricultural matrices in order to achieve a better monitoring of soil quality and fertility1. Laser-induced breakdown spectroscopy (LIBS) and X-Ray fluorescence spectrometry (XRF) have been identified as high potential techniques for on-site multi-elemental analysis of agronomic matrices such as plants2 and soils3. Materials and Methods The study of agricultural soils and plants, more precisely the multi-elemental quantification of several chemical elements of interest (P, Mg, Fe, Al, Ca, K…), was realized using portable LIBS (Z300 – SCIAPS) and portable XRF (S1 TITAN 800 – Bruker) systems. 62 soil samples and 40 plant samples have been prepared as pressed pellets. Multivariate calibrations have been achieved for quantitative analysis of the LIBS spectra. Both XRF and LIBS quantitative analysis were compared to the concentration values determinate by ICP OES. Results and Discussion Good practices related to sample preparation and experimental conditions to achieve relevant LIBS/XRF measurements in laboratory will be discussed. For both LIBS and XRF measurements, the influence of environmental parameters has been examined in order to assess the robustness of on-site analysis. Finally, a first campaign of on-site LIBS and XRF analyses on agricultural soils and plants has been performed. Conclusion The evaluation in the laboratory of two portable instruments, LIBS and XRF, on soils and plant samples, have revealed that these two techniques are very sensitive to moisture. The results of the field measurements on wheat leaves and soil will be presented and discussed.

International audience; As it is the case for any spectroscopic technique, laser-induced breakdown spectroscopy (LIBS) is strongly influenced by the signal fluctuations, and the LIBS spectra need to be normalized to obtain enhanced analytical performance. Nowadays, normalization in LIBS remains an open question and, in the present review, the normalization methods commonly applied to LIBS are presented and discussed, in particular those based on background, total area, internal standard, and Standard Normal Variate. We emphasize that the figures of merit, namely the coefficient of determination, the root-mean square error of prediction and the limit of quantification used to assess the advantages of processing normalized instead of non-normalized LIBS spectra, in a context of quantification, must be calculated in a rigorous way to be able to draw conclusions. We thus propose advices and good practices to achieve a rigorous comparison between quantitative models involving various normalization approaches, the final choice of the best normalization being ultimately driven by the analytical context. In order to take the best advantage from normalization in LIBS and thus increase the analytical performance of this technique, we encourage the analyst to thoroughly compare different normalization methods.

International audience

National audience

National audience

National audience

National audience

National audience

In this paper, we demonstrate the importance of variable selection on the prediction ability of LIBS quantitative partial least squares (PLS) models. The spectral lines of potassium at 766.49 nm and 769.90 nm were considered in the framework of an agricultural soils analysis. Univariate models demonstrating very poor correlation between the peak areas of the potassium lines and the related concentration values, a series of PLS models allowed to significantly improve the prediction ability compared to the univariate approach. This improvement was due to advanced variable selection, achieved through the use of two output data provided after PLS calculation, namely the Variable Importance in Projection (VIP) and the Coefficients graph. In this demonstration, the gain was significant because the two spectral lines of potassium at 766.49 nm and 769.90 nm exhibited unusual profiles. Indeed, including in a PLS model only the variables related to the edges of these lines allowed a significant improvement of its predictive ability (Q 2 = 0.84, RMSE = 1.49 g/kg) compared to another PLS model only including the variables related to the central parts of these lines (Q 2 = 0.78, RMSE = 1.59 g/kg).

We analysed the responses of 11 ecosystem models to elevated atmospheric [CO2] (eCO(2)) at two temperate forest ecosystems (Duke and Oak Ridge National Laboratory (ORNL) Free-Air CO2 Enrichment (FACE) experiments) to test alternative representations of carbon (C)-nitrogen (N) cycle processes. We decomposed the model responses into component processes affecting the response to eCO(2) and confronted these with observations from the FACE experiments. Most of the models reproduced the observed initial enhancement of net primary production (NPP) at both sites, but none was able to simulate both the sustained 10-yr enhancement at Duke and the declining response at ORNL: models generally showed signs of progressive N limitation as a result of lower than observed plant N uptake. Nonetheless, many models showed qualitative agreement with observed component processes. The results suggest that improved representation of above-ground-below-ground interactions and better constraints on plant stoichiometry are important for a predictive understanding of eCO(2) effects. Improved accuracy of soil organic matter inventories is pivotal to reduce uncertainty in the observed C-N budgets. The two FACE experiments are insufficient to fully constrain terrestrial responses to eCO(2), given the complexity of factors leading to the observed diverging trends, and the consequential inability of the models to explain these trends. Nevertheless, the ecosystem models were able to capture important features of the experiments, lending some support to their projections. (Less)

National audience

1

International audience

International audience

International audience

International audience; Climate change has consequences for terrestrial functioning, but predictions of plant responses remain uncertain because of the gaps in the representation of nutrient cycles and C–N–P interactions in ecosystem models. Here, we review the processes that are included in ecosystem models, but focus on coupled C–N–P cycle models. We highlight important plant adjustments to climate change, elevated atmospheric CO2, and/or nutrient limitations that are currently not—or only partially—incorporated in ecosystem models by reviewing experimental studies and compiling data. Plant adjustments concern C:N:P stoichiometry, photosynthetic capacity, nutrient resorption rates, allocation patterns, symbiotic N2 fixation and root exudation (phosphatases, carboxylates) and the effect of root exudation on nutrient mobilization in the soil rhizosphere (P solubilization, biochemical mineralization of organic P and priming effect). We showed that several plant adjustments could be formulated and calibrated using existing experimental data in the literature. Finally, we proposed a roadmap for future research because improving ecosystem models necessitate specific data and collaborations between modelers and empiricists.

International audience; Phosphorus (P) fractions are commonly assessed on dried soils in environmental and long-term field research, because this is the most convenient method of storing large quantities of samples before analysis. However sample pre-treatment may seriously affect the results, especially those regarding the organic P fractions. The objective of the present study was to assess the effect of soil drying under laboratory conditions on the P status of soils with contrasting physico-chemical and microbial properties. We used soils from a ( Pinus pinaster) forest in southwest France sampled from different soil depths and different site classes featuring a gradient of soil organic matter (SOM), and aluminium (Al) and iron (Fe) oxides. Total P, total organic and inorganic P, water soluble phosphate ions (iP), microbial P and the potentially mineralizable P in 'dead' organic matter were determined on fresh, moist soils maintained at 4°C and on the same soils, oven-dried at 60°C. Laboratory-drying resulted in a general decrease in total organic P, which was positively correlated with microbial P and SOM content. These drying-induced changes in total organic P were associated with a concomitant increase in total inorganic P and water soluble iP, which were dramatic in the organic forest floor samples (on average +2965% (*31 increase) in total inorganic P, and +11880% (*123 increase) in water soluble iP). The changes in water soluble iP were correlated with the SOM: Al and Fe oxides ratio and showed little effect of soil drying in deeper soil layers with a low SOM and high oxide contents. Our results indicated that soil drying mainly resulted in the lysis of the microbial pool, and to a much lesser extent, in the physical disruption and mineralization of the SOM. We conclude that soil drying before analyses can generate a significant bias in the evaluation of soil P stocks and fluxes, especially in SOM-rich layers. We recommend that fresh soil samples should be used as far as possible to determine soil P status.

International audience; Because carbon dioxide (CO2) concentration is rising, increases in plant biomass and productivity of terrestrial ecosystems are expected. However, phosphorus (P) unavailability may disable any potential enhanced growth of plants in forest ecosystems. In response to P scarcity under elevated CO2, trees may mine deeper the soil to take up more nutrients. In this scope, the ability of deep horizons of forest soils to supply available P to the trees has to be evaluated. The main objective of the present study was to quantify the relative contribution of topsoil horizons and deep horizons to P availability through processes governed by the activity of soil micro-organisms. Since soil properties vary with soil depth, one can therefore assume that the role of microbial processes governing P availability differs between soil layers. More specifically, our initial hypothesis was that deeper soil horizons could substantially contribute to total plant available P in forested ecosystems and that such contribution of deep horizons differs among sites (due to contrasting soil properties). To test this hypothesis, we quantified microbial P and mineralization of P in ‘dead’ soil organic matter to a depth of 120 cm in forest soils contrasting in soil organic matter, soil moisture and aluminum (Al) and iron (Fe) oxides. We also quantified microbiological activity and acid phosphomonoesterase activity. Results showed that the role of microbial processes generally decreases with increasing soil depth. However, the relative contribution of surface (litter and 0–30 cm) and deep (30–120 cm) soil layers to the stocks of available P through microbial processes (51–62 kg P ha−1) are affected by several soil properties, and the contribution of deep soil layers to these stocks vary between sites (from 29 to 59%). This shows that subsoils should be taken into account when studying the microbial processes governing P availability in forest ecosystems. For the studied soils, microbial P and mineralization of P in ‘dead’ soil organic matter particularly depended on soil organic matter content, soil moisture and, to a minor extent, Al oxides. High Al oxide contents in some sites or in deep soil layers probably result in the stabilization of soil organic compounds thus reducing microbiological activity and mineralization rates. The mineralization process in the litter also appeared to be P-limited and depended on the C:P ratio of soil organic matter. Thus, this study highlighted the effects of soil depth and soil properties on the microbial processes governing P availability in the forest spodosols.

The earth’s future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO2. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO2 stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO2 as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.

International audience; Microbial biomass phosphorus (P) can play an important role in P cycling and availability to plants by acting as a source (remineralization) or sink (immobilization) of phosphate ions (iP). To assess the role of the microbial P pools, both the dynamics (i.e. the turnover) and the size of the microbial P pools were studied in forest soils. Combining an isotopic dilution method with a modelling approach, we showed the existence of two pools of microbial P with different dynamics and therefore of different importance in soil P availability and cycling. In particular, we showed that the largest pool of microbial P (80%) had a fast turnover (nine days). Microbial P increased with an increase in soil organic matter and represented up to 53% of total P in contrasting forest soils. By combining these results with the turnover times of microbial P obtained in the modelling study, we evaluated that 8.5–17.3 kg P ha−1 of microbial P could turn over in a few days. This suggests that microbial biomass P is a potentially significant source of available iP, and that micro-organisms can play a major role in P cycling in the forest studied here. However, microbial biomass can also be in competition with the trees since most of the remineralized P could be immobilized again in the microbial turnover.

Forest ecosystems are important sinks for rising concentrations of atmospheric CO 2 . In previous research, we showed that net primary production (NPP) increased by 23 ± 2% when four experimental forests were grown under atmospheric concentrations of CO 2 predicted for the latter half of this century. Because nitrogen (N) availability commonly limits forest productivity, some combination of increased N uptake from the soil and more efficient use of the N already assimilated by trees is necessary to sustain the high rates of forest NPP under free-air CO 2 enrichment (FACE). In this study, experimental evidence demonstrates that the uptake of N increased under elevated CO 2 at the Rhinelander, Duke, and Oak Ridge National Laboratory FACE sites, yet fertilization studies at the Duke and Oak Ridge National Laboratory FACE sites showed that tree growth and forest NPP were strongly limited by N availability. By contrast, nitrogen-use efficiency increased under elevated CO 2 at the POP-EUROFACE site, where fertilization studies showed that N was not limiting to tree growth. Some combination of increasing fine root production, increased rates of soil organic matter decomposition, and increased allocation of carbon (C) to mycorrhizal fungi is likely to account for greater N uptake under elevated CO 2 . Regardless of the specific mechanism, this analysis shows that the larger quantities of C entering the below-ground system under elevated CO 2 result in greater N uptake, even in N-limited ecosystems. Biogeochemical models must be reformulated to allow C transfers below ground that result in additional N uptake under elevated CO 2 .

International audience

International audience

International audience; We analysed the responses of 11 ecosystem models to elevated atmospheric [CO2] (eCO2) at two temperate forest ecosystems (Duke and Oak Ridge National Laboratory (ORNL) Free-Air CO2 Enrichment (FACE) experiments) to test alternative representations of carbon (C)–nitrogen (N) cycle processes. * We decomposed the model responses into component processes affecting the response to eCO2 and confronted these with observations from the FACE experiments. * Most of the models reproduced the observed initial enhancement of net primary production (NPP) at both sites, but none was able to simulate both the sustained 10-yr enhancement at Duke and the declining response at ORNL: models generally showed signs of progressive N limitation as a result of lower than observed plant N uptake. Nonetheless, many models showed qualitative agreement with observed component processes. The results suggest that improved representation of above-ground–below-ground interactions and better constraints on plant stoichiometry are important for a predictive understanding of eCO2 effects. Improved accuracy of soil organic matter inventories is pivotal to reduce uncertainty in the observed C–N budgets. * The two FACE experiments are insufficient to fully constrain terrestrial responses to eCO2, given the complexity of factors leading to the observed diverging trends, and the consequential inability of the models to explain these trends. Nevertheless, the ecosystem models were able to capture important features of the experiments, lending some support to their projections.

National audience

International audience; Symbiotic nitrogen fixation (SNF) is the main natural source of nitrogen (N) in terrestrial ecosystems worldwide. Previous studies have shown that fixation of N by plants can be limited by the availability of phosphorus (P) in soils. We used global meta-analysis to investigate how P availability controls SNF. In experiments in which plants were grown in an artificial medium, severe P deficiencies in the nutritive solution ([PO4]

By overemphasizing the role of overstory trees in forest studies, researchers, managers and policy makers could well ignore an important functional component of the forest ecosystem: the understory. This could be particularly true in forests with relatively open canopies. In maritime pine (Pinus pinaster Ait.) plantation forests in the southwest of France, a considerable amount of the direct radiation is transmitted through the canopy and is available for the growth of understory species. This study focused on five dominant understory species, including three woody species (Calluna vulgaris L., Erica cinerea L. and Cytisus scoparius L.) and two herbaceous perennials species (Molinia caerulea L. (Moench) and Pteridium aquilinum L. Khun). For each species five pine stands with varying amounts of understory were selected to predict biomass for a wide range of values. The number of pine trees and their dimensions were recorded enabling us to estimate the aboveground and root biomass of the pines using allometric relationships. Biometrical measurements of the understory and destructive harvests were carried out in each stand to measure aboveground, coarse and fine root biomass. The results showed that the understory represented an average of 6.8% of total ecosystem aboveground biomass and 27.4% of total ecosystem root biomass, underlining the need for a broader view of this component of the forest ecosystem. Allometric relationships derived from the biometrical data enabled satisfactory predictions of aboveground biomass for all five understory species and for belowground biomass of three species (M. caerulea, P. aquilinum, C. scoparius); allometric relationships failed to predict root biomass for two woody understory species (E. cinerea and C. vulgaris).

The earth's future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO₂. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO₂ stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO₂ as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.

International audience

International audience; Nitrogen availability in terrestrial ecosystems strongly influences plant productivity and nutrient cycling in response to increasing atmospheric carbon dioxide (CO2). Elevated CO2 has consistently stimulated forest productivity at the Duke Forest free-air CO2 enrichment experiment throughout the decade-long experiment. It remains unclear how the N cycle has changed with elevated CO2 to support this increased productivity. Using natural-abundance measures of N isotopes together with an ecosystem-scale 15N tracer experiment, we quantified the cycling of 15N in plant and soil pools under ambient and elevated CO2 over three growing seasons to determine how elevated CO2 changed N cycling between plants, soil, and microorganisms. After measuring natural-abundance 15N differences in ambient and CO2-fumigated plots, we applied inorganic 15N tracers and quantified the redistribution of 15N for three subsequent growing seasons. The natural abundance of leaf litter was enriched under elevated compared to ambient CO2, consistent with deeper rooting and enhanced N mineralization. After tracer application, 15N was initially retained in the organic and mineral soil horizons. Recovery of 15N in plant biomass was 3.5 ± 0.5% in the canopy, 1.7 ± 0.2% in roots and 1.7 ± 0.2% in branches. After two growing seasons, 15N recoveries in biomass and soil pools were not significantly different between CO2 treatments, despite greater total N uptake under elevated CO2. After the third growing season, 15N recovery in trees was significantly higher in elevated compared to ambient CO2. Natural-abundance 15N and tracer results, taken together, suggest that trees growing under elevated CO2 acquired additional soil N resources to support increased plant growth. Our study provides an integrated understanding of elevated CO2 effects on N cycling in the Duke Forest and provides a basis for inferring how C and N cycling in this forest may respond to elevated CO2 beyond the decadal time scale.

International audience; The earth’s future climate state is highly dependent upon changes in terrestrial C storage in response to rising concentrations of atmospheric CO2. Here we show that consistently enhanced rates of net primary production (NPP) are sustained by a C-cascade through the root-microbe-soil system; increases in the flux of C belowground under elevated CO2 stimulated microbial activity, accelerated the rate of soil organic matter decomposition and stimulated tree uptake of N bound to this SOM. This process set into motion a positive feedback maintaining greater C gain under elevated CO2 as a result of increases in canopy N content and higher photosynthetic N-use efficiency. The ecosystem-level consequence of the enhanced requirement for N and the exchange of plant C for N belowground is the dominance of C storage in tree biomass but the preclusion of a large C sink in the soil.

Symbiotic nitrogen fixation is the main natural source ofnitrogen in ecosystems. Previous studies have shown that this process can be limited by the availability of phosphorus (P) in soils. However, it is still unclear whether or not phosphorus controls symbiotic nitrogen fixation. Through a compilation of a global dataset, our analyses showed that worldwide in the case of nutritive solution experiments, the specific nitrogen fixation rate increased until a very low P concentration threshold value but remained constant above this value. Conversely, in field experiments, the specific fixation rate remained constant at any P supply, highlighting that plants can obtain sufficient P from the soil to maintain an optimal nitrogen fixation rate. Those results wereeconfirmed by an extensive analysis of soil P status throughout the world, showing always adequate quantities of bioavailable soil P, despite common low soil solution phosphorus values. Our results demonstrate that the in situ symbiotic nitrogen fixation rate is not dependent on P availability and that the fixation flux is only proportional to plant biomass increment. By identifying the mechanistic basis for the phosphorus control of terrestrial nitrogen fixation, this study provides a theoretical framework easily implementable into further models that couple nutrient cycling and global changes.

International audience

International audience; Phosphorus (P) is a critical limiting factor of plant growth and production in many ecosystems, which often require to be fertilized. However, there is an increasing concern regarding appropriate local and global management of phosphorus resources, since the existing finite phosphate reserves are rapidly being depleted. This implies to understand what processes (biological, physico-chemical) are governing soil P availability in agroecosystems, and in particular in forests, which will be increasingly managed for their C-sink potential in the future. We characterized the P status in forest soils of the largest managed pine forest in Europe (Landes of Gascogne, southwest of France) using isotopic and extraction methods, as well as modelling approaches. Total P concentration in topsoils were extremely low, ranging from 7 to 195 mg Pkg-1. The concentration of phosphate ions in solution decreased with depth and was related to the Al and Fe oxide content, which controlled the diffusion of P from the soil solid phase to the solution. The gross amount of diffusive P in one week as determined by 32P isotopic dilution in batch experiments was low, ranging from 0.2 to 52 mg P kg-1 in the topsoil layer, and could be predicted by pedotransfer functions built on the Al and Fe oxide and soil organic matter contents. Organic P represented 80% of total P in litter and 60% in the surface mineral soil layer, suggesting a higher contribution of biological processes to soil P cycling. Biological mineralization of organic P was quantified using a long-term incubation study (154 days) of a low-sorbing soil labelled with 33P, associated with a batch experiment with 32P labelled soil: gross mineralization of dead soil organic matter and diffusive phosphate P were low (