Carbone des sols en Afrique

Les sols sont une ressource essentielle à préserver pour la production d’aliments, de fibres, de biomasse, pour la filtration de l’eau, la préservation de la biodiversité et le stockage du carbone. En tant que réservoirs de carbone, les sols sont par ailleurs appelés à jouer un rôle primordial dans la lutte contre l’augmentation de la concentration de gaz à effet de serre. Ils sont ainsi au centre des objectifs de développement durable (ODD) des Nations unies, notamment les ODD 2 « Faim zéro », 13 « Lutte contre le changement climatique », 15 « Vie terrestre », 12 « Consommation et production responsables » ou encore 1 « Pas de pauvreté ». Cet ouvrage présente un état des lieux des sols africains dans toute leur diversité, mais au-delà, il documente les capacités de stockage de carbone selon les types de sols et leurs usages en Afrique. Il propose également des recommandations autour de l’acquisition et de l’interprétation des données, ainsi que des options pour préserver, voire augmenter les stocks de carbone dans les sols. Tous les chercheurs et acteurs du développement impliqués dans les recherches sur le rôle du carbone des sols sont concernés par cette synthèse collective. Fruit d’une collaboration entre chercheurs africains et européens, ce livre insiste sur la nécessité de prendre en compte la grande variété des contextes agricoles et forestiers africains pour améliorer nos connaissances sur les capacités de stockage de carbone des sols et lutter contre le changement climatique

Agroforesterie et services écosystémiques en zone tropicale

Respectueux de l’environnement et garantissant une sécurité alimentaire soutenue par la diversification des productions et des revenus qu’ils procurent, les systèmes agroforestiers apparaissent comme un modèle prometteur d’agriculture durable dans les pays du Sud les plus vulnérables aux changements globaux. Cependant, ces systèmes agroforestiers ne peuvent être optimisés qu’à condition de mieux comprendre et de mieux maîtriser les facteurs de leurs productions. L’ouvrage présente un ensemble de connaissances récentes sur les mécanismes biophysiques et socio-économiques qui sous-tendent le fonctionnement et la dynamique des systèmes agroforestiers. Il concerne, d’une part les systèmes agroforestiers à base de cultures pérennes, telles que cacaoyers et caféiers, de régions tropicales humides en Amérique du Sud, en Afrique de l’Est et du Centre, d’autre part les parcs arborés et arbustifs à base de cultures vivrières, principalement de céréales, de la région semi-aride subsaharienne d’Afrique de l’Ouest. Il synthétise les dernières avancées acquises grâce à plusieurs projets associant le Cirad, l’IRD et leurs partenaires du Sud qui ont été conduits entre 2012 et 2016 dans ces régions. L’ensemble de ces projets s’articulent autour des dynamiques des systèmes agroforestiers et des compromis entre les services de production et les autres services socio-écosystémiques que ces systèmes fournissent

Comparison between predictions of C and N contents in tropical soils using a Vis–NIR spectrometer including a fibre-optic probe versus a NIR spectrometer including a sample transport module

International audienceIncreasing attention is being paid to near-infrared reflectance (NIR) spectroscopy for the rapid and cost-effective determination of soil carbon (C) and nitrogen (N) contents. The objective of the present paper was to compare the performances of two spectrometers: one covered the visible and NIR ranges (Vis-NIR, 350-2500 nm) and included a fibre-optic probe and the other covered the NIR range only (1100-2500 nm) and included a sample transport module. The comparison was carried out on two sets of clayey (n = 97) and sandy (n = 72) soil samples from tropical Africa and America.On the whole, both technologies provided good calibrations (R²>0.74) and predictions (R²>0.62) of soil C and N contents. The most accurate calibrations were achieved with the NIR spectrometer (R²>0.86), which also yielded the most accurate predictions for the sandy soils (R² = 0.90 and standard error of prediction = 0.68 and standard error of prediction <20% of the mean). Using first or second derivatives of spectra did not affect calibration but had an impact on validation

Quantifier le carbone organique du sol par spectroscopie infrarouge

International audienceSoil organic carbon (SOC) is a pillar of soil ecosystem services. However, SOC quantification, especially in terms of stock (in kgC m-2 for a given soil layer), or SOC fraction characterization, which helps understand SOC dynamics, are often tedious and/or costly. This article proposes an overview to show the value of IR spectroscopy to quantify SOC and its fractions time- and cost-effectively.Many works have demonstrated that IR spectroscopy allows accurate determination of SOC concentration (gC kg-1). In this approach, a statistical model is built on a set of calibration samples in order to express SOC (determined conventionally) as a function of the spectrum; then the model is applied on the spectra of new samples to predict their SOC concentration. In general, the mid-IR domain (MIR, 2,500-25,000 nm) yields better predictions for soils from temperate regions when spectra are acquired on finely ground samples ( 2 mm) is high and varies between samples. Furthermore, the IR spectrum of bulk soil can be used for predicting SOC distribution among its fractions, for instance particle size fractions. So IR spectroscopy facilitates SOC characterization, and increases the value of conventional analyses carried out on some samples by using them for predictions on other samples.El carbono orgánico del suelo (COS) es un pilar de los servicios ecosistémicos del suelo; pero su cuantificación es a menudo tediosa y/o costosa, especialmente en términos de stock de COS (en kgC°m-2 para una capa de suelo), como lo es también la caracterización de las fracciones que componen el COS. Este artículo propone una visión general para mostrar el valor de la espectroscopia infrarroja (IR) para cuantificar el COS y sus fracciones de una forma rápida y rentable.Muchos trabajos han demostrado que la espectroscopia IR permite determinar con precisión la concentración de COS (gC kg-1). En este enfoque, se construye un modelo estadístico sobre un conjunto de muestras de calibración con el fin de expresar el COS en función del espectro; a continuación, el modelo se aplica sobre los espectros de nuevas muestras para predecir su concentración de COS. En general, el dominio del IR medio (MIR, 2500-25 000 nm) arroja mejores predicciones para los suelos de regiones templadas cuando las muestras son finamente molidas ( 2 mm) es elevado y varía entre las muestras. Por último, la distribución del COS entre sus diferentes fracciones, por ejemplo la granulométrica, puede predecirse a partir del espectro IR del suelo total. Así pues, la espectroscopia IR facilita la caracterización del COS y aumenta el valor de los análisis convencionales realizados en algunas muestras al utilizarlos para hacer predicciones en otras.Le carbone organique du sol (COS) est un pilier des services écosystémiques rendus par les sols. Cependant, quantifier le COS, notamment en termes de stock (kgC.m-2 pour une couche de sol), ou caractériser les fractions qui le composent, pour mieux comprendre sa dynamique, sont souvent fastidieux et/ou coûteux. Cet article propose une vue d’ensemble présentant l’intérêt de la spectroscopie infrarouge (IR) pour quantifier à haut débit et faible coût le COS et ses fractions.De nombreux travaux montrent que la spectroscopie IR permet de quantifier précisément la concentration en COS (gC.kg-1). Dans cette approche, un modèle statistique est construit sur un jeu d’échantillons d’étalonnage pour exprimer la concentration en COS (dosée de manière conventionnelle) en fonction du spectre ; puis ce modèle est appliqué au spectre de nouveaux échantillons pour prédire leur concentration en COS. En général le domaine moyen IR (MIR, 2500-25 000 nm) permet de meilleures prédictions pour les sols des régions tempérées lorsque les spectres sont acquis sur matériau finement broyé ( 2 mm) est élevée et varie entre échantillons. Enfin, la distribution du COS entre ses différentes fractions, par exemple granulométriques, peut être prédite à partir du spectre IR du sol total. La spectroscopie IR permet ainsi de faciliter la caractérisation du COS, et de valoriser les analyses conventionnelles réalisées sur certains échantillons en les utilisant pour des prédictions sur d’autres échantillons

Soil organic carbon content and stock in Martinique – relations to near infrared spectra

International audienceThis data paper presents observations on soil organic carbon (SOC) gravimetric and volumetric contents (SOCg, in g kg −1 , and SOCv, in g dm −3 , respectively) for 98 profiles at least 30 cm deep (1 m deep for 59 of them), in rural areas of the Martinique island, in relation to soil types and land uses and management. The paper also presents particle size distribution down to 30 cm, and near infrared reflectance (NIR) spectra for the main soil types. This dataset allows evaluating the effects of land use, soil type and texture on SOC content and stock, at regional scale. It also allows inferring SOCg, SOCv and particle size distribution from NIR spectra. Such information is useful for studying and managing soils in the Martinique island and in other tropical volcanic regions

Use of near infrared reflectance spectroscopy (NIRS) for predicting soil fertility and historical management

International audienceThis study tests the potential of near infrared reflectance spectroscopy (NIRS) for predicting soil fertility and management history from topsoil (0-10 cm deep) spectra. Soil fertility was assessed by measuring the growth of a test plant, and soil management history was determined through inquiries with farmers. Moreover, NIRS predictive value was compared with that of a group of topsoil parameters: total carbon and nitrogen, nitrate, potential respiration and denitrification, and microbial biomass. Modelling used partial and modified partial least square regressions to ensure comparisons between predictions by NIRS versus by soil parameters. Soil fertility and management history were well predicted by NIRS (Q² = 0.78 and R² = 0.89 both; Q² and R² are cross-validation and calibration coefficients of determination, respectively), as were the soil parameters (Q² = 0.79-0.92 and R² = 0.86-0.98). Soil fertility and management history were more accurately predicted by NIRS than by the set of soil parameters

A Congo Basin ethnographic analogue of pre-Columbian Amazonian raised fields shows the ephemeral legacy of organic matter management

The functioning and productivity of pre-Columbian raised fields (RFs) and their role in the development of complex societies in Amazonian savannas remain debated. RF agriculture is conducted today in the Congo Basin, offering an instructive analogue to pre-Columbian RFs in Amazonia. Our study of construction of present-day RFs documents periodic addition of organic matter (OM) during repeated field/fallow cycles. Field investigations of RF profiles supported by spectrophotometry reveal a characteristic stratigraphy. Soil geochemistry indicates that the management of Congo RFs improves soil fertility for a limited time when they are under cultivation, but nutrient availability in fallow RFs differs little from that in uncultivated reference topsoils. Furthermore, examination of soil micromorphology shows that within less than 40 years, bioturbation almost completely removes stratigraphic evidence of repeated OM amendments. If Amazonian RFs were similarly managed, their vestiges would thus be unlikely to show traces of such management centuries after abandonment. These results call into question the hypothesis that the sole purpose of constructing RFs in pre-Columbian Amazonia was drainage

Prediction of soil organic and inorganic carbon contents at a national scale (France) using mid-infrared reflectance spectroscopy (MIRS)

This work aimed to evaluate the potential of mid-infrared reflectance spectroscopy (MIRS) to predict soil organic and inorganic carbon contents with a 2086-sample set representative of French topsoils (030 cm). Ground air-dried samples collected regularly using a 16 x 16-km grid were analysed for total (dry combustion) and inorganic (calcimeter) carbon; organic carbon was calculated by difference. Calibrations of MIR spectra with partial least square regressions were developed with 1080% of the set and five random selections of samples. Comparisons between samples with contrasting organic or inorganic carbon content and regression coefficients of calibration equations both showed that organic carbon was firstly associated with a wide spectral region around 2500-3500 cm-1 (which was a reflection of its complex nature), and inorganic carbon with narrow spectral bands, especially around 2520 cm-1. Optimal calibrations for both organic and inorganic carbon were achieved by using 20% of the total set: predictions were not improved much by including more of the set and were less stable, probably because of atypical samples. At the 20% rate, organic carbon predictions over the validation set (80% of the total) yielded mean R2, standard error of prediction (SEP) and RPD (ratio of standard deviation to SEP) of 0.89, 6.7 g kg-1 and 3.0, respectively; inorganic carbon predictions yielded 0.97, 2.8 g kg-1 and 5.6, respectively. This seemed appropriate for large-scale soil inventories and mapping studies but not for accurate carbon monitoring, possibly because carbonate soils were included. More work is needed on organic carbon calibrations for large-scale soil libraries

Restauration agroécologique des sols de savanes d’Afrique de l’Ouest avec des rameaux d’arbres

International audiencepas de résum

Effect of ramial wood amendment on sorghum production and topsoil quality in a Sudano-Sahelian ecosystem (central Burkina Faso)

International audienceIn Sudano-Sahelian agriculture, organic amendments are often limited by resource availability. Small branches (ramial wood, RW) represent an organic resource found in many landscapes but little is known about their effects. This field trial (2007-2009) studied the effects of RW or straw at low application rate (0.69 Mg C ha(-1) year(-1)) on topsoil carbon (C), nitrogen (N) and available phosphorus (P, P-av), termite cast abundance, and sorghum yield. Straw and RW were chopped and either buried (StBu, WoBu) or mulched (StMu, WoMu). Mineral fertilizers were added to straw so that RW- and straw-amended plots received similar applications of C, N, P, and potassium. Another treatment had RW buried with additional N (WoBuN), and there was a control (Ctrl). Branches came from Piliostigma reticulatum, very common in the area. The treatments had little significant effect on topsoil and crop, owing to the low application rate and spatial variability. However, P-av was significantly lower with buried than mulched amendments in 2009, and decreased significantly over time in Ctrl and with buried amendments. Topsoil C also decreased significantly with time in WoMu. Significantly more termite casts were observed with RW. The sorghum yield averaged 0.87 Mg DM ha(-1) in 2007 and then decreased. The treatments affected yield significantly in 2008 only: it was higher in WoBuN and StBu than in Ctrl. In 2009, the yield was mainly affected by initial topsoil P-av. These results suggest that RW stimulated biological activity, leading to P immobilization and C mineralization, but had little effect on yields