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Madagascar is known for its high erosion rates in the central highlands, yet the role of human disturbance versus natural processes is not well understood and is a topic of ongoing debate. At present the necessary quantitative data to couple vegetation dynamics and sediment fluxes over time in Madagascar is scarce. This study aims to provide more insight in vegetation changes and sediment transfers over the last millennia in the Lake Alaotra region, and specifically on the role of human disturbances and natural processes. Our vegetation reconstruction is based on pollen records from two lake sediment cores, covering the last 2600 years. Sediment accumulation rates were calculated from cores obtained from the floodplains, from wetlands surrounding the lake, and from Lake Alaotra itself. Our data show an early opening in the landscape, between 2050 and 1700 cal a BP, with a transition from a wooded grassland or woodland/grassland mosaic towards open grassland and an increase in charcoal accumulation rates. (Indirect) human impact is suggested as the main driver for these vegetation changes. Floodplain and wetland sediment accumulation rates only increase in the last 1000 years and peak in the last 400 years. This increased accumulation can mainly be linked to the increased anthropogenic pressure (grazing and farming activities) that triggered increased lavaka (gullies) activity. No changes in accumulation rate were observed in Lake Alaotra, indicating that most sediments are buffered in the floodplains and wetlands. Overall, our pollen and charcoal data suggest an indirect effect of human disturbance on vegetation shifts whilst strong evidence was found for a direct effect of human disturbance on sediment accumulation through intensified use of the grasslands.

Cosmogenic nuclide analysis of river sediment provides insight into erosion and catchment dynamics. Studies on factors controlling spatial variations in long-term erosion rates have often focussed on tectonically active mountainous areas, where strong linkages with topographic variables like catchment gradient and normalized river steepness have been found. Less is known about rates and controls in tropical areas with deeper soils in tectonically less active regions which are often intensively used by people. Information on long-term erosion rates is crucial if human impact on landscape dynamics is to be understood in these areas.Here, we investigate spatial patterns and controls on 10Be-inferred erosion rates in Madagascar, a moderately seismically active island surrounded by passive margins, with considerable relief and a climate that varies from humid tropical to semi-arid. We use a dataset of 99 detrital in-situ 10Be measurements from a wide range of catchments (combining new measurements with data from the literature), covering more than 30% of the country and including a wide range of topographic, bioclimatic, and geological characteristics. Overall, 10Be erosion rates are very low (2.4 - 51.1 mm kyr-1) but clear differences were found between different geomorphic regions with some of the highest rates on the eastern escarpment while most catchments in the central highlands had extremely low erosion rates. The latter shows that, under (sub-) tropical climax vegetation catchments can be very stable, despite a pronounced topography with convex slopes exceeding 30° and the presence of a thick and erodible regolith mantle covered by a protective laterite. Statistical tests indicate that 27% of the observed variation in 10Be erosion rates is associated with elevation (lower rates for higher catchments) and an additional 18% of the variation is associated with river concavity, seismicity, and lithological erodibility (higher rates for more convex, more seismically active and more erodible catchments). An additional test using random within-between (REWB) analysis, in which different geomorphic regions are also considered as independent variables, shows that the main variations in 10Be-inferred rates between the different regions are linked to river concavity, seismicity and gully abundance, where additional variation within geomorphic regions is linked with seismicity only. The random within-between model explained 73% of the observed variation, suggesting that differences between regions are indeed important, yet are only partly explained by the environmental controls we considered in our analysis. The fact that the association between topographical controls and 10Be-inferred erosion rates is weaker in Madagascar in comparison to tectonically (very) active areas is likely to be related to the long time scale considered and the fact that sediment buffering as well as individual random events may have a more important impact on 10Be-inferred erosion in relatively stable environments such as the ones we studied in Madagascar.

Upland rainfed rice cropping in the highlands of Madagascar is strongly limited by poor Ferralsol mineral fertility. There is an urgent need to identify efficient and sustainable fertilization practices that improve soil fertility without inducing pest proliferation. For that purpose, using a field trial for 2 successive years, we tested the effect of 16 fertilization practices on the abundance and taxonomic diversity of soil active nematodes, which are known to be biological indicators of soil fertility. We tested both fertilization practices traditionally used by farmers and innovative ones based on the assemblage of organic, mineral and biological (earthworms and mycorrhiza) fertilizers. We identified eight types of practices: (1) no fertilization; (2) fertilization with NPK and urea; (3) low input rates (3 t dry matter ha−1) of organic fertilizers without NPK; (4) low input rates of organic fertilizers with NPK; (5) high input rates (6 t dry matter ha−1) of organic fertilizers; (6) high input rates of organic fertilizers with mineral fertilizers; (7) high input rates of a mixture of organic fertilizers and (8) high input rates of a mixture of organic fertilizer with mineral fertilizers. After 2 years, we identified 41 soil nematode taxa. The taxonomic composition of the nematode communities revealed that Ferralsols are a stressful environment for the soil biota. The low abundance of opportunistic bacterivores indicated that the different fertilization practices did not significantly and deeply increase the amount of plant-available nutrients, and thus soil fertility. However, organic fertilizers significantly increased the abundance of omni-predators, indicating a moderately mature food web. Some practices induced worrying increases in the endoparasitic Meloidogyne and Pratylenchus, which requires monitoring of root symptoms to avoid the establishment of their populations. Monitor soil nematode communities in upland rice growing on Ferralsols could be used as agronomic indicators to promote sustainable rice production systems in Madagascar.

Description of the subject. Farmers’ knowledge is a significant source of information about cultivated soil knowledge. Objectives. This paper focuses on farmers in the Alaotra Lake Region in Madagascar to identify their knowledge of the soil and soil fertility and to propose a conceptual framework for developing participatory approaches. Method. The methodology is based on surveys conducted with 100 farmers randomly selected within five Communes (villages) to (i) study their knowledge on soil, (ii) design local soil map, and (iii) identify local soil fertility assessment. For each task, an exploratory survey with non-directive individual interviews followed by more in-depth surveys with semi-directive individual interviews and restitution workshops were conducted. Results. The results revealed two types of local soil classifications according to toposequence. Twelve and nine local soil types were identified respectively in the southern zone and in the northern zone. Moreover, farmers used 14 main local indicators to assess the fertility of these soils. Farmers’ soil classification was found to comply with previous pedological studies and even provided complementary details to the reference French Soil Classification (CPCS). Finally, the local soil fertility indicators have been proved to be identical to those used by other farmers in other countries and are scientifically validated to be effective. Conclusions. This paper offers a new conceptual framework that may be effective for agricultural development service and scientific research in other contexts. It provides a foundation for supporting a common language between farmers and researchers, one that may prove instrumental in future projects including the co-conception of sustainable, innovative agricultural practices.

Phosphorus (P) deficiency is a major yield constraint for lowland rice production in the tropics. As P-fertilizer resources are finite, alternative fertilizer management is needed for sustainable rice production. We examined whether farmyard manure (FYM), a major nutrient source for smallholder farms, can overcome issue in typical P-deficient lowlands in the central highlands of Madagascar. A multi-location trial in sites varying in altitude and soil P availability, clarified that the effect of both FYM and mineral P fertilizer application on grain yield greatly increased at higher elevation and when the soil oxalate-extractable P content was

The extent to which the central highlands of Madagascar were once covered by forests is still a matter of debate: while reconstructing past environments is inherently difficult, the debate is further hampered by the fact that the evidence documenting land cover changes and their effects on carbon and sediment dynamics in Madagascar has hitherto mainly been derived from lake coring studies. Such studies provide an integrated view over relatively large areas but do not provide information on how land-use change affects hillslopes in terms of carbon and sediment dynamics. Such information would not only be complementary to lake inventories but may also help to correctly interpret lake sediment data. Carbon stable isotope ratios (δ13C) are particularly useful tracers to study the past dynamics of soil carbon over time spans ranging from years to millennia and thus to understand the consequences of land-use change over such time spans. We analysed soil profiles down to a depth of 2 m from pristine forests and grasslands in the Lake Alaotra region in central Madagascar. Along grassland hillslopes, soil organic carbon (SOC) content was low, from 0.4 % to 1.7 % in the top layer, and decreased rapidly to ca. 0.2 % below 100 cm depth. The current vegetation predominantly consists of C4 grasses (δ13C ∼ −13 ‰), yet topsoil δ13C-OC ranges between −23.0 ‰ and −15.8 ‰, and most profiles show a decrease in δ13C-OC with depth. This contrasts with our observations in the C3-dominated forest profiles, which show a typical profile whereby δ13C values increase slightly with depth. Moreover, the SOC stock of grasslands was ∼ 55.6 % lower than along the forested hillslopes for the upper 0–30 cm layer. δ13C values in grassland and forest profiles converge to similar values (within 2.0 ± 1.8 ‰) at depths below ∼ 80 cm, suggesting that the grasslands in the Lake Alaotra region have indeed developed on soils formerly covered by a tree vegetation dominated by C3 plants. We also observed that the percent of modern carbon (pMC) of the bulk OC in the top, middle and lower middle positions of grasslands was less than 85 % near the surface. This could reflect a combination of (i) the long residence time of forest OC in the soil, (ii) the slow replacement rate of grassland-derived OC (iii) and the substantial erosion of the top positions towards the valley position of grasslands. At the valley positions under grassland, the upper 80 cm contains higher amounts of recent grass-derived OC in comparison to the hillslope positions. This is likely to be related to the higher productivity of the grassland valleys (due to higher moisture and nutrient availability), and the deposition of OC that was eroded further upslope may also have contributed. The method we applied, which is based on the large difference in δ13C values between the two major photosynthetic pathways (C3 and C4) in (sub-)tropical terrestrial environments, provides a relatively straightforward approach to quantitatively determine changing vegetation cover, and we advocate for its broader application across Madagascar to better understand the island's vegetation history.

Laboratory visible and near-infrared (Vis-NIR) spectroscopy with partial least squares (PLS) regression can be used to determine the soil carbon (C) content, and the waveband selection procedures can refine the predictive ability. However, individually selected wavebands are not always the same depending on the location, scale, and approach. To simplify the variable selection issue, some methods for selecting wavelength regions instead of individual wavebands have been proposed. In this study, we explore relevant wavelength regions for predicting the total carbon (TC) content of lowland and upland soils in Madagascar from Vis-NIR spectroscopy using a dynamic version of backward interval PLS (biPLS) regression. The predictive ability of dynamic biPLS was compared with that of standard full-spectrum PLS (FS-PLS) using the cross-validated coefficient of determination (R2), root mean squared error (RMSE), and ratio of performance to interquartile distance (RPIQ). The biPLS models using reflectance (R2 = 0.877, RMSE = 0.690) and first derivative reflectance (FDR) (R2 = 0.940, RMSE = 0.494) data sets showed better predictive accuracy than the FS-PLS models using reflectance (R2 = 0.826, RMSE = 0.809) and FDR (R2 = 0.933, RMSE = 0.518) data sets, the spectral efficiency was improved. By using biPLS to predict soil TC, the model was simplified by using only four selected wavelength regions in the reflectance (400–490, 1402–1440, 1846–1980 and 2151–2283 nm) and FDR (652–687, 1322–1443, 1856–1985, and 2290–2400 nm) data sets, which yielded reliable (RPIQ > 2.5) predictions.

The soil map of Madagascar has not been substantially updated since the 1960s. The number of reported soil profiles that meet the criteria of the Reference Pedon Description – the highest level of ...

Over the past decades advanced technology has become available, revolutionizing the assessment of surface topography. At smaller scales (up to a few km²) structure from motion (SfM) algorithms applied to uncrewed aerial vehicle (UAV) imagery now allow sub-meter resolution. On the other hand, spaceborne digital elevation models (DEMs) are becoming increasingly accurate and are available at a global scale. Two recent spaceborne developments are the 12 m TanDEM-X and 30 m Copernicus DEMs. While sub-meter resolution UAV-SfM DEMs generally serve as a reference, their acquisition remains time-consuming and spatially constrained. However, some applications in geomorphology, such as the estimation of regional or national erosion quantities of specific landforms, require data over large areas. TanDEM-X and Copernicus data can be applied at such scales, but this raises the question of how much accuracy is lost because of the lower spatial resolution.Here, we evaluate the performance of the 12 m TanDEM-X DEM and the 30 m Copernicus DEM to i) estimate gully volumes, ii) establish an area-volume relationship, and iii) determine sediment mobilization rates, through comparison with a higher resolution (0.2 m) UAV-SfM DEM. We did this for six study areas in central Madagascar where lavaka (large gullies) are omnipresent and surface area changes over the period 1949-2010s are available. Copernicus derived lavaka volume estimates were systematically too low, indicating that the Copernicus DEM is not suitable to estimate erosion volumes for geomorphic features at the lavaka scale (100 – 105 m²). The relatively coarser resolution of the DEM prevents to accurately capture complex topography and smaller geomorphic features. Lavaka volumes obtained from the TanDEM-X DEM were similar to UAV-SfM volumes for the largest features, while smaller features were generally underestimated. To deal with this bias we introduce a breakpoint analysis to eliminate volume reconstructions that suffered from processing errors as evidenced by significant fractions of negative volumes. This elimination allowed the establishment of an area-volume relationship for the TanDEM-X data with fitted coefficients within the 95% confidence interval of the UAV-SfM relationship. Combined with surface area changes over the period 1949-2010s, our calibrated area-volume relationship enabled us to obtain lavaka mobilization rates ranging between 18 ± 3 and 311 ± 82 t ha-1 yr-1 for the six study areas, with an average of 108 ± 26 t ha-1 yr-1. This does not only show that the Malagasy highlands are currently rapidly eroding by lavaka, but also that lavaka erosion is spatially variable, requiring the assessment of a large area in order to obtain a meaningful estimate of the average erosion rate.With this study we demonstrate that medium-resolution global DEMs can be used to accurately estimate the volumes of gullies exceeding 800 m² in size, where the proposed breakpoint-method can be applied without requiring the availability of a higher resolution DEM. This might aid geomorphologists to quantify sediment mobilisation rates by highly variable processes such as gully erosion or landsliding at the regional scale, as illustrated by our first assessment of regional lavaka mobilization rates in the central highlands of Madagascar.

Over the past few decades, developments in remote sensing have resulted in an ever-growing availability of topographic information on a global scale. A recent development is TanDEM-X (TerraSAR-X add-on for digital elevation measurements), an interferometric synthetic aperture radar (SAR) mission of the Deutsche Zentrum für Luft- und Raumfahrt, providing near-global coverage and 12 m resolution digital elevation models (DEMs). Moreover, ongoing developments in uncrewed aerial vehicle (UAV) technology have enabled acquisitions of topographic information at a sub-meter resolution. Although UAV products are generally preferred for volume assessments of geomorphic features, their acquisition remains a time-consuming task and is spatially constrained. However, some applications in geomorphology, such as the estimation of regional or national erosion quantities of specific landforms, require data over large areas. TanDEM-X data can be applied at such scales, but this raises the question of how much accuracy is lost because of the lower spatial resolution. Here, we evaluated the performance of the 12 m TanDEM-X DEM to (i) estimate gully volume, (ii) establish an area–volume relationship, and (iii) determine mobilization rates through comparison with a higher-resolution (0.2 m) UAV structure-from-motion (SfM) DEM and a lower resolution (30 m) Copernicus DEM. We did this for six study areas in the Lake Alaotra region (central Madagascar), where lavaka (gullies) are omnipresent and surface area changes over the period 1949–2010s are available for 699 lavaka. Copernicus-derived lavaka volume estimates were systematically too low, indicating that the Copernicus DEM is too coarse to accurately estimate volumes of geomorphic features at the lavaka scale (100–105 m2). Lavaka volumes obtained from TanDEM-X were similar to UAV-SfM volumes for the largest features, whereas the volumes of smaller features were generally underestimated. To deal with this bias we introduce a breakpoint analysis to eliminate volume reconstructions that suffer from processing errors as evidenced by significant fractions of negative volumes. This elimination allowed the establishment of an area–volume relationship for the TanDEM-X data with fitted coefficients within the 95 % confidence interval of the UAV-SfM relationship. Our calibrated area–volume relationship enabled us to obtain large-scale lavaka mobilization rates ranging between 18 ± 3 and 311 ± 82 tha-1yr-1 for the six different study areas, with an average of 108 ± 26 tha-1yr-1 for the full dataset. These results indicate that current lavaka mobilization rates are 2 orders of magnitude higher than long-term erosion rates. With this study we demonstrate that the global TanDEM-X 12 m DEM can be used to accurately estimate volumes of gully-shaped features at the lavaka scale (100–105 m2), where the proposed breakpoint method can be applied without requiring the availability of a higher-resolution DEM. Furthermore, we use this information to make a first assessment of regional lavaka erosion rates in the central highlands of Madagascar.

Topical Collection on Regional management practices with positive effects on soil carbon to meet the goals of the 4p1000 initiative 1000; International audience; Agroforestry systems (AFS) are recognized as one of the practices with high potential to store carbon in soils. In the Itasy region, AFS were introduced to improve farmers' livelihoods by diversifying income sources and to address problems related to soil degradation. Previous studies in the region have shown the potential of AFS to store organic carbon in the soil. In the present work, we carried out further studies to assess the main factors affecting SOC stocks in AFS. In 2014, we performed a soil sampling on 137 AFS farmers'plots to assess SOC stocks in different AFS. In 2018, a second sampling was carried out to calculate SOC storage rates using the diachronic approach on 30 most representative AFS. The results revealed that the factors "age of the system" and "type of organic inputs" significantly affected SOC stocks in AFS. SOC stocks increased significantly over time in AFS plots, benefiting from regular organic inputs such as manure and/or compost. In contrast, SOC stocks remained unchanged over time in AFS plots where no organic fertilization was used. Our study showed a substantial SOC storage up to 47 parts per thousand year(-1), mainly explained by regular additions of organic inputs to maintain soil fertility and crop production. However, to fully understand the process of SOC storage in this context, further works, such as the analysis of the link between organic matter quality and the SOC storage process, and the quantification of the share of soil carbon inputs derived from tree biomass should be undertaken.

Accurate assessments of soil properties are required to improve fertilizer management practices for crop production. Conventional chemical analysis in the laboratory is costly and time-consuming. Soil color is related to different soil compositions, while soil magnetic susceptibility (MS) has been found to reflect the abundance of magnetic minerals relevant to soil properties. Improving proximal sensing techniques for the analysis of soil color and MS provides opportunities for affordable and rapid assessments of soil properties. The aim of this study was to evaluate the potential use of soil color parameters and MS values to predict soil properties using stepwise multiple linear regression (SMLR), random forest (RF), and nonlinear regression approaches in lowland and upland fields in the central highlands of Madagascar. The target properties included the contents of soil organic carbon (SOC), total nitrogen (TN), oxalate-extractable phosphorus and iron (Feox), and the soil texture. The model prediction accuracy was assessed using the coefficient of determination (R2), root-mean-square error (RMSE), and the ratio of performance to interquartile distance (RPIQ). The use of soil color parameters yielded an acceptable prediction accuracy of the Feox content (loge Feox) for all rice fields (R2 = 0.54, RMSE = 0.55, RPIQ = 1.70) using the RF algorithm, while the SMLR approach gave the most accurate prediction for upland fields with acceptable reliabilities for SOC, Feox, and clay and sand content prediction, with R2 ranging from 0.43 to 0.67 and RPIQ from 1.63 to 1.77. In lowland fields, TN content was predicted with acceptable accuracy (R2 = 0.34, RMSE = 0.49, RPIQ = 1.71) using SMLR with the color parameter. The combination of the soil color parameters with the MS value as predictor variables increased SOC prediction for lowland fields using the RF approach (R2 = 0.57, RMSE = 6.37, RPIQ = 1.96). Use of the soil color and MS parameters was revealed to be a promising way to simplify the assessment of soil properties in upland and lowland ecosystems by using RF and SMLR approaches. A combined use of the soil color and MS parameters improved the prediction accuracy for the SOC content.

Lavaka (gullies) are often considered as the prime indication of a currently ongoing human-induced environmental crisis in Madagascar's highlands. Yet, lavaka are known to have existed long before human arrival and account for the majority of the long-term sediment input into the highland rivers and floodplains. The role of anthropogenic disturbances in their formation therefore remains highly debated and it is unclear whether lavaka erosion has recently increased. Here,we address these questions by evaluating the dynamics of lavaka in the Lake Alaotra region (central Madagascar). An overall birth to stabilization ratio of 6.1 indicates a rapid lavaka population growth over the period 1949–2010s. Using data on lavaka development we calculated a mean lavaka population age of 410 ± 40 years and estimate that the disequilibrium started at 870 ± 430 cal. BP. Floodplain sedimentation starts to increase around 1000 cal. BP and peaks over the last 400 years, thereby independently confirming this time frame of increased lavaka activity. Lavaka population dynamics modelling shows that a strong increase in environmental pressure over the last centuries is needed to attain current disequilibrium levels. A general drying of the climate since 950 cal. BP in combination with the introduction of cattle and growing human presence around 1000 cal. BP will likely have triggered the increase in lavaka erosion. However, the recent acceleration cannot be explained by climatic changes alone and seems to be linked to increased anthropogenic pressure on the environment. As such, we offer a fresh and quantitatively supported perspective on lavaka dynamics and human impact in central Madagascar, where our methodology can be used in other locations where similar questions on geomorphic equilibrium need to be answered. ispartof: Science Of The Total Environment vol:806 issue:Pt 2 ispartof: location:Netherlands status: Published online

Soil color has been conventionally measured using a Munsell soil color chart. Recently launched colorimeters can also measure the color of an object at a reasonable cost. This study was undertaken ...

Over the past decades, developments in remote sensing have resulted in an ever growing availability of topographic information on a global scale. A recent development is TanDEM-X, an interferometric SAR mission of the Deutsche Zentrum für Luft- und Raumfahrt providing near-global coverage and high resolution DEMs. Moreover, ongoing developments in unmanned aerial vehicle (UAV) technology has enabled acquisitions of topographic information at a sub-meter resolution. Although UAV products are generally preferred for volume assessments of geomorphic features, their acquisition remains time-consuming and is spatially constrained. However, some applications in geomorphology such as the estimation of regional or national erosion quantities of specific landforms, require data over large areas. TanDEM-X data can be applied at such scales, but this raises the question of how much accuracy is lost because of the lower spatial resolution. Here, we evaluated the performance of the 12 m TanDEM-X DEM to i) estimate gully volumes, ii) establish an area-volume (A-V) relationship, and iii) determine mobilization rates, through comparison with a high resolution (0.2 m) UAV-SfM DEM and lower resolution (30 m) SRTM DEM. We did this for six study areas in the Lake Alaotra region (central Madagascar) where lavaka (gullies) are omnipresent and lavaka surface area changes over the period 1949–2010s are available for 699 lavaka. SRTM derived lavaka volume estimates were systematically too low, indicating that the SRTM DEM is too coarse to accurately estimate volumes of geomorphic features at the lavaka-scale (100–100 000 m2). Lavaka volumes obtained from TanDEM-X were similar to UAV-SfM volumes for the largest features, whereas the volumes of smaller features were generally underestimated. To deal with this bias we introduce a breakpoint analysis to eliminate volume reconstructions that suffer from processing errors as evidenced by significant fractions of negative volumes. This elimination allowed the establishment of an area-volume relationship for the TanDEM-X data that is within the 95 % confidence interval of the UAV-SfM A-V relationship. Our calibrated area-volume relationship enabled us to obtain large-scale lavaka mobilization rates ranging between 18 ± 6 and 289 ± 125 ton ha−1 yr−1 with an average of 102 ± 41 ton ha−1 yr−1. These results indicate that current lavaka mobilization rates are two orders of magnitude higher than long-term erosion rates. With this study we demonstrate that the global TanDEM-X 12 m DEM can be used to accurately estimate volumes of gully-shaped features at the lavaka-scale (100–100 000 m2), where the proposed breakpoint-method can be applied without requiring the availability of a higher resolution DEM. Furthermore, we use this information to make a first assessment of regional lavaka erosion rates in the central highlands of Madagascar.

Madagascar is an island characterized by a sharp vegetation gradients in landscape and the extent to which the central highlands were once covered by forest is still under debate. Stable carbon isotope ratios have been an important tracer to understand vegetation shifts in a landscape over time because forest plants (following the C3 photosynthetic pathway) and grasses (following the C4 pathway for the majority of tropical and subtropical species) show a different degree of isotope fractionation. The main aim of this study is to provide insight into the past vegetation history of the grasslands in the Lake Alaotra region (central Madagascar). This region is under high anthropogenic pressure as it is the most important rice-producing region in Madagascar. While much of the Lake Aloatra catchment consists of grassland-covered hills, with a high density of lavaka (large erosional features), pristine forests are located just east of the lake and these form the most western part of a larger rainforest-covered region extending up to the east coast of the island. Soil profiles were sampled along the hillslope gradient at both forested and grassland sites, whereby carbon stocks were quantified and δ13C values of soil organic carbon (OC) were measured to find evidence for past forest vegetation in the grassland sites. The soil organic carbon content of grassland soil profile was extremely low, from 0.4 to 1.8% in the top layer and rapidly decreasing to 0.2 % below 100 cm depth. The current vegetation predominantly consists of C4 grasses (δ13C ~-13 ‰), yet soil δ13C-OC ranges between -25.9 and -16.6‰, and most profiles show a decrease in δ13C-OC with depth, in contrast to observations in the (C3-dominated) forest profiles, which show a typical enrichment in 13C with depth. δ13C values in grassland and forest profiles converge to similar values (within 2.1 ±1.8 ‰) at depths below ~80cm, suggesting that the grasslands in the Lake Alaotra region have developed on formerly forested soils. Moreover, the soil OC stock of grasslands was ~55.6% lower than along the forested hillslopes for the upper 30 cm layer. Our results are consistent with the hypothesis that a vegetation change has occurred in the lake Alaotra region, and offer a promising avenue to expand this approach on a wider scale to help understand the vegetation cover change in the central highland of Madagascar.

To understand the driving processes for changes in African ecosystems and related problems such as soil degradation, it is crucial to gain insight in the relative importance of human disturbance and climate change. Madagascar is known for its particularly high erosion rates in the central highlands, yet the role of human disturbance versus natural processes are not well understood and is a topic of ongoing debate. Recent studies have challenged the traditional view that the currently observed intense erosion processes and sediment fluxes in Madagascar are mainly driven by recent large-scale deforestation. However, at present almost no quantitative data is available to couple vegetation dynamics and sediment fluxes over time in Madagascar. This study aims to provide more insight in landscape changes (vegetation changes, sediment mobilization and deposition) in central Madagascar, and in the specific role of man and climate. The study focuses on the 1800 km² catchment of Lake Alaotra, located ca 200 km northeast of Antananarivo. Lake Alaotra is formed in a graben system in the highlands of Madagascar, and is the largest freshwater lake of the country (400 km²). A pollen record from the lake was used to reconstruct regional vegetation changes. Radiocarbon dates of extracted pollen provide a detailed chronostratigraphic framework. Augerings and radiocarbon dates from floodplains and marshes in the catchment were used to reconstruct the sediment deposition history. The pollen record and charcoal data shows the vegetation changes over the last 3000 years. The main observed shift in vegetation is a transition from a woodland/grassland mosaic towards an open grassland, starting ca 1850 years ago, which coincides with the onset of human activities. Data on floodplain sedimentation show an increase in accumulation rates in the last 600 years, from ca 1 mm yr-1 to ca 30 mm yr-1, which can be linked to increased hillslope erosion rates during that time period. The sedimentation wave, however, does not reach Lake Alaotra nor the surrounding marshes as floodplains act as a buffer. Overall, this study provides a spatial and temporal integrated reconstruction of vegetation changes in the Lake Alaotra catchment and the link with sediment mobilization and deposition, thereby providing a better understanding of environmental changes in central Madagascar and its driving forces.

The Malagasy highlands are scattered with large inverse teardrop-shaped gullies called lavaka, which are by many considered as the prime indication of a currently ongoing human-induced environmental crisis. Yet, these gullies are known to have existed long before human arrival on the island, resulting in the highly debated role of anthropogenic disturbances on their formation. Here, we assess the dynamics of 700 lavaka in the lake Alaotra region from 1949 to the 2010s by using historical aerial pictures and present day satellite imagery. An overall birth to stabilization ratio of 6.1 indicates a currently rapid growing lavaka population. Observed growth-, birth- and stabilization rates allowed us to calculate a mean lavaka population age of 410 ± 40 years, and estimate that the current crisis started at 943 ± 430 cal. yr BP. This timeframe corresponds well with the “subsistence-shift”, where people move from hunting and foraging to farming and herding practices upon the introduction of cattle in the region. These findings were integrated into a novel, temporally explicit lavaka population model - building upon the observed lavaka growth-, birth- and stabilization rates and lavaka size distributions - where different environmental pressure scenarios were tested. Modelling outcomes show that the currently observed lavaka crisis largely results from a rapid increase in environmental pressure over the last centuries, likely caused by the combined effects of deforestation and overgrazing related to human population growth and the introduction of cattle. With this study we show the potential of an integrated data-modeling approach, where demographic concepts are applied to geomorphological features, allowing to link their evolution with past anthropogenically driven environmental changes.

SummaryDecision-makers need readily accessible tools to understand the potential impacts of alternative policies on forest cover and greenhouse gas (GHG) emissions and to develop effective policies to meet national and international targets for biodiversity conservation, sustainable development and climate change mitigation. Land change modelling can support policy decisions by demonstrating potential impacts of policies on future deforestation and GHG emissions. We modelled land change to explore the potential impacts of expert-informed scenarios on deforestation and GHG emissions, specifically CO2 emissions, in the Ankeniheny–Zahamena Corridor in eastern Madagascar. We considered four scenarios: business as usual; effective conservation of protected areas; investment in infrastructure; and agricultural intensification. Our results highlight that effective forest conservation could deliver substantial emissions reductions, while infrastructure development will likely cause forest loss in new areas. Agricultural intensification could prevent additional forest loss if it reduced the need to clear more land while improving food security. Our study demonstrates how available land change modelling tools and scenario analyses can inform land-use policies, helping countries reconcile economic development with forest conservation and climate change mitigation commitments.

Phosphorus (P) deficiency is a major constraint for rice production in the tropics. Field-specific P management is key for resource-limited farmers to increase yields with minimal inputs. We used soil P fractionation analysis to identify the relevant factors controlling P uptake and the responses to P fertilization of rice in flooded and highly weathered soils. Phytometric pot-based experiments and a modified Hedley fractionation analysis were repeated for soils from extensive regions and from geographically adjacent fields in Madagascar. Large field-to-field variations in indigenous P supply from soils (total P uptake of rice when P is omitted) and fertilizer-P recovery efficiencies (increased P uptake when P is applied) were observed not only for soils with various geological backgrounds but also for soils from adjacent fields. Regression models indicated that the indigenous P supply in soils was largely controlled by readily available inorganic and organic P pools (r2 = 0.72), whereas fertilizer-P recovery efficiencies were controlled by the abundance of oxalate-extractable aluminum and iron in soils (r2 = 0.81). Spatial heterogeneity even within adjacent fields leads to benefits from field-specific fertilizer management based on indigenous P supply from soils and fertilizer-P recovery efficiencies evaluated by different soil properties.

International audience; The priming effect (PE) in soil, when induced by a fresh carbon supply, is believed to result from two different mechanisms, “stoichiometric decomposition” and “nutrient mining”, and contributes to either long-term SOM stabilization or depletion. Understanding how to affect the balance between both mechanisms can provide valuable insight into agroecology, especially in Southern countries where organic matter management is the primary method of fertilizing cultivated plots. Therefore, the objective of this study was to investigate the bacterial actors in each mechanism and the influence of three drivers in Malagasy Ferralitic soils: the quality of the inducing substrate, the quality of the SOM, and the soil nutrient status. Three different agricultural Ferralitic soils, characterized by different levels of NH4 and PO4 availability, different SOM size fractionation profiles, and different bacterial communities, were amended with three types of 13C-labeled organic substrate (glucose, wheat residue, and rice residue) and the PE was measured at two incubation times (7 and 42 days). The results showed that a PE generated by stoichiometric decomposition was mainly induced by the polymerized fraction of crop residues, especially in soil enriched by decaying plant tissues, mineral N, and its associated decomposers guild such as Verrucomicrobia, α- and δ-Proteobacteria and Actinomycetes. Conversely, when this young SOM pool was reduced, the PE was mainly generated by N mining, especially when induced by soluble compounds and when N was limiting but P was available.

International audience; Organic carbon saturation in soils refers to the theoretical maximum soil organic carbon (SOC) that can be associated with and stabilized on fine silt plus clay particles (F < 20 μm). We reviewed the literature dealing with SOC distribution between soil fractions to evaluate carbon saturation for tropical soils and estimate the C storage potential of cropland. We collected 258 data points on SOC distribution between soil fractions in tropical soils from 84 sites in 27 countries. We used boundary line analysis to estimate the maximum stabilized SOC depending on soil group, clay type and land use. SOC storage potential was calculated as the SOC saturation deficit, the difference between the actual SOC content and the maximum stabilized SOC content. We found that the maximum SOC in the fine fraction of tropical soils (53 g C kg− 1 fine fraction) was lower than previous assessments of global SOC storage scale based mainly on temperate soils. The F < 20 μm fractions were closer to SOC saturation in forest soils than in croplands. The cropland had a higher soil C storage potential, but changing agricultural management practices did not fill the deficit that is calculated using the whole dataset. The deficit was much lower when it was estimated with grassland or cropland data only: this provides a more realistic estimate for SOC storage potential for croplands. The SOC content in the coarser fraction (F > 50 μm) did not depend on soil texture and significantly contributed to the total SOC, especially in sandy soils (41.3%). This is affected by changes in agricultural management practices. We concluded that, although the aim of increasing SOC stabilization originally arose from climate change mitigation strategies, it must now be more viewed as being more relevant to food security and local adaptation to climate change.

Efficient phosphorus (P) fertilizer management is highly required for resource-limited farmers to achieve higher yields in typical P-deficient farmlands in sub-Saharan Africa. Although soil P retention has been conventionally used to estimate the possible responses to P fertilizer applied to soils, its applicability has not been clearly confirmed at the field level. In this study, the applicability of P retention to predict the response of rice plants to P application was investigated using pot experiments with soils collected from various locations (N = 62) and on-farm experiments at two nutrient-poor sites (N = 38) in the central highlands of Madagascar. Further, we explored a simple prediction method of P retention for multiple location assessment in local farmers’ fields using an alternative property of soils collected from rice fields in the same region (N = 213). P retention was negatively correlated with the increase in rice P uptake (ΔPuptake, r = − 0.550) in the pot experiment and with the increase in yield (ΔYield, r = − 0.697) in the on-farm field experiment as a response to P application. Path analysis revealed that oxalate-extractable aluminum (Alox) content was the most important factor of P retention across all the soils, indicating that rice plants grown on soils with higher Alox and P retention are less sensitive to P application. Given its high correlation with P retention (r = 0.642) and its simplicity in measurement, we proposed the use of moisture content of air-dried soils (ωair) as a parameter to predict soil P retention. It was further confirmed that ωair had a significant negative correlation with ΔPuptake based on the pot experiment across a wide range of soil statuses (r = − 0.518). However, ωair could not clearly explain ΔYield within a small range of soil properties in the field experiment. Overall, soil P retention can predict the responses of rice plants to P application in the typical P-deficient and low-yielding lowlands in the central highlands of Madagascar, and this simple evaluation technique using air-dried soil moisture content will be helpful for the assessment of multiple rice fields on a broad scale.

Summary Soil organic carbon (SOC) is an important carbon pool in terrestrial ecosystems. Prediction of SOC based on soil properties and environmental factors helps to describe the spatial and vertical distribution in SOC; however, the effectiveness and accuracy of various prediction methods, including classical and recently developed model approaches, need to be tested for tropical soil and environments. In this study, random forest (RF) and linear mixed effects model (LMM) approaches were tested to predict the spatial and vertical variation of SOC stocks in Eastern Madagascar. Topography, climate, soil types and vegetation-based variables were used as predictor variables for modelling SOC stocks at different soil depths to 1 m. The LMM was the most accurate method for predicting SOC stocks for different depth ranges; altitude, soil clay content, land use and precipitation were identified as the most relevant factors for prediction. The accuracy of prediction in SOC modelling decreased with increasing soil depth, resulting in a root mean square prediction error (RMSE) that ranged from 1.98 Mg ha−1 (90–100-cm depth) to 5.54 Mg ha−1 (10–20-cm depth) for LMM, which resolved 43–68% of the variation in SOC stocks. Explanatory variables, which contributed to the fixed effect of the model, explained from 2.6 to 28.2% of the total variance, whereas the random effect contributed from 21.7 to 35.0%. This study emphasizes the strength of LMM for predicting SOC stocks in tropical soil taking into account the random effect related to sampling. These results could be used to improve SOC mapping in Madagascar. Highlights Prediction accuracy of vertical variation in SOC stocks was tested with RF and LMM approaches. Linear mixed effects model provides the most accurate predictions of SOC stocks. Altitude, clay content, climate and land use were identified as relevant predictor variables of SOC. The LMM can be used to improve SOC mapping of tropical soil.

Assessment of soil organic carbon stocks (SOCs) is highly relevant considering that SOCs is the central driver in climate change mitigation and soil fertility. In Madagascar, a first attempt at mapping SOCs on a national scale was undertaken in 2009 with previous soil data. Advanced research on soil carbon mapping on a global scale is required to enable better land use decisions. This study aims to (i) evaluate the drivers of soil organic carbon (SOC) storage in the first 30 cm soil layer on a national scale from spatially explicit explanatory environmental variables and a recent soil database and (ii) update the spatial distribution of SOCs at this scale through digital mapping. A spatial model was established using randomForest, a decision tree algorithm and based on 10 pedoclimatic, topographic, and vegetation variables. The model was developed with 1993 available soil plot data (collected from 2010 to 2015) and their environmental information (“VALSOL-Madagascar” database). These data were divided into two sets: a first set (n = 835) used for model calibration and a second set (n = 358) for external validation. Results showed that mean annual temperature (MAT, °C), mean annual precipitation (MAP, mm), elevation (m) and Normalized Difference Vegetation Index (NDVI) were the most important predictors of SOCs. The retained prediction model had a Root Mean Squared Error (RMSE) equal to 25.8 MgC·ha− 1. The predicted SOCs from fitted models ranged from 28 to 198 MgC·ha− 1 with total SOCs to 4137 TgC. Depending on soil type, Ferralsols (35 to 165 MgC·ha− 1) and Andosols (48 to 198 MgC·ha− 1) had relevant results related to the number of soil profiles (n = 856 and 171 respectively). Despite the need for in-depth analysis, the model and map produced in the present study has greatly improved our knowledge of the spatial distribution of SOCs in Madagascar and reduced uncertainty compared to the former map. This map provides new perspectives in sustainable land management in Madagascar.

Madagascar shows very high erosion rates and, in some regions, the landscape is dotted by major gullies called “lavaka”. Lavaka are also very frequent in the surroundings of Lake Alaotra, a large, shallow lake in the Malagasy highlands. A central question with respect to landscape evolution in Madagascar is to what extent human impact has triggered environmental change in terms of vegetation, erosion rates (lavaka formation) and sediment dynamics. Sedimentary archives in lakes such as lake Alaotra can be of great help to resolve this question provided that we understand how sediment and carbon are mobilised and transported through the landscape. In this study, we traced pathways of sediment and carbon fluxes through this eroding landscape, from the eroded hillslopes over various sediment deposition zones (floodplains, reservoirs, marshes..) to Lake Alaotra. Detailed profiles taken along convex hillslope transects (grasslands and primary forest), in the marsh peat, floodplains and lake were analyzed for carbon and nitrogen content, texture, and stable carbon isotope ratios (δ13C). Along the grassland hillslopes, soil OC content is extremely low, from 0.4 to 1.8% in the top layer and rapidly decreasing to 13C ~-13 ‰), yet soil δ13C ranges between -24 and -18‰, and most profiles show a decrease in δ13C with the depth – in contrast to observations in most C3-dominated systems. Contrary to our expectations, Lake Alaotra was found not to be a major sink of hillslope-derived sediments and/or carbon. Sediment cores from different parts of the lake have high OC contents (5 to 18%) and contain only minor amounts of sand, the dominant grain size class on the hillslopes. The high OC content of the lake sediments, in combination with data on C/N ratios and δ13C indicate that the OC in the lake sediments is mainly derived from the surrounding marshes and in situ primary production rather than from terrestrial C eroded from the catchment. Floodplains are likely a key sink for soil-derived sediments: similar to hillslope soils, sediment profiles in the floodplains show a low %OC and relatively high δ13C values ranging between -21 and -14‰.We conclude that most of the detritic sediments and carbon mobilised on the hillslopes through erosion do not reach lake Alaotra, even though erosion rates in the landscape are extremely high. Studying sedimentary profiles in the lake may provide information on environmental change (e.g. through changes in carbon contents and/or characteristics) but is insufficient to understand the entire sediment and carbon cascades in the Malagasy landscape.

Les sols sont une ressource essentielle a preserver pour la production d'aliments, de fibres, de biomasse, pour la filtration de l'eau, la preservation de la biodiversite et le stockage du carbone. En tant que reservoirs de carbone, les sols sont par ailleurs appeles a jouer un role primordial dans la lutte contre l'augmentation de la concentration de gaz a effet de serre. Ils sont ainsi au centre des objectifs de developpement durable (ODD) des Nations unies, notamment les ODD 2 « Faim zero », 13 « Lutte contre le changement climatique », 15 « Vie terrestre », 12 « Consommation et production responsables » ou encore 1 « Pas de pauvrete ». Cet ouvrage presente un etat des lieux des sols africains dans toute leur diversite, mais au-dela, il documente les capacites de stockage de carbone selon les types de sols et leurs usages en Afrique. Il propose egalement des recommandations autour de l'acquisition et de l'interpretation des donnees, ainsi que des options pour preserver, voire augmenter les stocks de carbone dans les sols. Tous les chercheurs et acteurs du developpement impliques dans les recherches sur le role du carbone des sols sont concernes par cette synthese collective. Fruit d'une collaboration entre chercheurs africains et europeens, ce livre insiste sur la necessite de prendre en compte la grande variete des contextes agricoles et forestiers africains pour ameliorer nos connaissances sur les capacites de stockage de carbone des sols et lutter contre le changement climatique. Cet ouvrage est une contribution du reseau Carbone des sols pour une agriculture durable en Afrique (Casa). Les objectifs du reseau Casa s'inscrivent dans le cadre de l'Action commune de Koronivia pour l'Agriculture (Koronivia Joint Work on Agriculture, KJWA). Adoptee en 2017, lors de la 23e conference des Parties (COP 23), la KJWA fournit un cadre pour renforcer la place de l'agriculture dans les discussions internationales sur le climat, a la fois pour son role dans la reduction de la concentration en gaz a effet de serre dans l'atmosphere et sa necessaire adaptation face au changement climatique.