Chapitre 2. Spatialiser les stocks de carbone

Introduction Dans les laboratoires d’analyse des sols à Madagascar, la mesure du carbone organique du sol (COS) sert à calculer la teneur en matière organique (MO), une information utile pour la gestion de la fertilité des sols. Outre son évaluation quantitative, diverses études sur le COS ont été menées sur (1) sa dynamique, en interaction avec les autres constituants du sol selon les pratiques et modes d’usage des terres ou (2) sur sa variabilité spatio-temporelle. Ces études ont été effect..

A global spectral library to characterize the world's soil

Soil provides ecosystem services, supports human health and habitation, stores carbon and regulates emissions of greenhouse gases. Unprecedented pressures on soil from degradation and urbanization are threatening agro-ecological balances and food security. It is important that we learn more about soil to sustainably manage and preserve it for future generations. To this end, we developed and analyzed a global soil visible-near infrared (vis-NIR) spectral library. It is currently the largest and most diverse database of its kind. We show that the information encoded in the spectra can describe soil composition and be associated to land cover and its global geographic distribution, which acts as a surrogate for global climate variability. We also show the usefulness of the global spectra for predicting soil attributes such as soil organic and inorganic carbon, clay, silt, sand and iron contents, cation exchange capacity, and pH. Using wavelets to treat the spectra, which were recorded in different laboratories using different spectrometers and methods, helped to improve the spectroscopic modelling. We found that modelling a diverse set of spectra with a machine learning algorithm can find the local relationships in the data to produce accurate predictions of soil properties. The spectroscopic models that we derived are parsimonious and robust, and using them we derived a harmonized global soil attribute dataset, which might serve to facilitate research on soil at the global scale. This spectroscopic approach should help to deal with the shortage of data on soil to better understand it and to meet the growing demand for information to assess and monitor soil at scales ranging from regional to global. New contributions to the library are encouraged so that this work and our collaboration might progress to develop a dynamic and easily updatable database with better global coverage. We hope that this work will reinvigorate our community's discussion towards larger, more coordinated collaborations. We also hope that use of the database will deepen our understanding of soil so that we might sustainably manage it and extend the research outcomes of the soil, earth and environmental sciences towards applications that we have not yet dreamed of

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

Emissions of CO2 and N2O from a pasture soil from Madagascar-Simulating conversion to direct-seeding mulch-based cropping in incubations with organic and inorganic inputs

In the highlands of Madagascar, agricultural expansion gained on grasslands and cropping systems based on direct seeding with permanent vegetation cover are emerging as a means to sustain upland crop production. The objective of this study was to examine how such agricultural practices affect greenhouse-gas emissions from a loamy Ferralsol previously used as a pasture. We conducted an experiment under controlled laboratory conditions combining cattle manure, crop residues (rice straw), and mineral fertilizers (urea plus NPK or di-NH4-phosphate) to mimic on-field inputs and examined soil CO2 and N2O emissions during a 28-d incubation at low and high water-filled pore space (40% and 90% WFPS). Emissions of N2O from the control soil, i.e., soil receiving no input, were extremely small (< 5 ng N2O-N (g soil)(-1) h(-1)) even under anaerobic conditions. Soil moisture did not affect the order of magnitude of CO2 emissions while N2O fluxes were up to 46 times larger at high soil WFPS, indicating the potential influence of denitrification under these conditions. Both CO2 and N2O emissions were affected by treatments, incubation time, and their interactions. Crop-residue application resulted in larger fluxes of CO2 but reduced N2O emissions probably due to N immobilization. The use of di-NH4-phosphate was a better option than NPK to reduce N2O emissions without increasing CO2 fluxes when soil received mineral fertilizers. Further studies are needed to translate the findings to field conditions and relate greenhouse-gas budgets to crop production

Comparing near and mid-infrared reflectance spectroscopy for determining properties of Malagasy soils, using global or LOCAL calibration

Nowadays, near infrared (NIR) and mid-infrared (mid-IR) reflectance spectroscopy are recognised useful approaches for quantifying soil properties, cost and time effectively. The aim of this work was to compare predictions of soil carbon (C) and nitrogen (N) content, C/N ratio, substrate-induced respiration (SIR) and denitrifying enzyme activity (DEA) using NIR and mid-IR spectroscopy over a diverse set of 360 Malagasy topsoils. Partial least square regression was used for fitting NIR and mid-IR spectra to conventional data through procedures of calibration either global (one prediction model for all samples) or LOCAL (one prediction model per sample). Prediction accuracy was assessed according to validation (r(2)), standard error of prediction (SEP) in proportion to the mean and ratio of standard deviation to SEP (RPD). Using both NIR and mid-IR spectroscopy, global calibration over the whole sample set yielded predictions that were excellent for C and N (r(2) > 0.9, SEP= 3), good for C/N, acceptable for SIR, but poor for DEA. LOCAL calibration improved C/N and SIR predictions with both NIR and mid-IR spectroscopy, while DEA prediction became acceptable with NIR spectroscopy only. Additional improvement was achieved when LOCAL calibration was carried out over the fine-textured sub-set, especially for SIR (r(2)>0.9, SEP3). In contrast, LOCAL calibration over the coarse-textured sub-set was clearly not useful for improving prediction accuracy. NIR outperformed mid-IR spectroscopy whatever the variable, the calibration procedure and the sample set (except for SIR over the coarse-textured sub-set, where both similar), suggesting its possible superiority for tropical soils

Nitrous oxide fluxes from Malagasy agricultural soils

E-mail address: [email protected] audienceIn Madagascar, no-tillage practices were developed since the early 90s to prevent soil erosion and improve soil fertility. Although such practices have helped to restore soil carbon in most cases, the impact on N2O emissions has not been investigated yet. The soil N2O fluxes and concentrations were measured during the growing season of an intercropping maize-soybean on a clayey soil of the Malagasy Highlands. Management treatments consisted of direct seeding mulch based cropping system (DMC) and traditional hand-ploughing after the preceding crop residues were harvested (HP), both with low N inputs (55-57 kg N ha− 1). No significant difference in N2O emissions was observed between treatments (DMC vs. HP). The N2O fluxes were weakly correlated to soil mineral N contents (R2 = 0.13; P = 0.03) while no relationship was emphasized with soil water filled pore space (WFPS). N2O concentrations in the soil atmosphere were correlated to fluxes at the soil surface and to soil WFPS. N2O emissions at the soil surface were low ranging from 0 to 8.84 g N-N2O ha− 1 d− 1, probably due to the low mineral N content of soil. The cumulative annual N2O emission was 0.26 kg N ha− 1 for both systems. The corresponding N loss as N2O-N was around 0.5% of applied N. This is in the uncertainty range of IPCC N2O emission factor (EF), but the IPCC EF mean estimate (1%) would overestimate true N2O emissions for the soil under evaluation