Iron (Fe) toxicity in lowland rice is caused by high Fe levels and mineral imbalances within plants. Thus, maintaining whole-plant mineral balance may be crucial for tolerance. However, there is limited evidence from field studies on elemental profiling that elucidates specific tolerance mechanisms. Here, we tested whether contrasting Fe tolerance mechanisms are associated with distinct patterns of nutrient uptake and distribution. We conducted multi-season experiments in Fe-toxic fields in Madagascar using rice genotypes with contrasting tolerance mechanisms. Elemental profiling was conducted for different parts of shoot tissues. Belowground Fe dynamics were investigated by measuring Fe plaque formation on roots and rhizosphere Fe2⁺ levels. During the vegetative stages, tolerant genotypes that maintain low shoot Fe concentrations (excluders) showed a 48

The integration of pigeon pea as a leguminous shrub into agroforestry systems (AFS) is expected to enhance soil nitrogen and its availability for associated crops. This study aimed to estimate, over a three-month period, the atmospheric nitrogen fixation rate (

Agricultural intensification by smallholder farmers in Madagascar often combines conventional and agroecological practices. Assessing the performance of these practices through a multicriteria approach is essential to capture their economic, environmental and social performance. This study aimed to assess how the adoption of different cropping systems based on agroecological principles performs across sustainability indicators. We used survey data from 455 farmers to characterize and select 9 cropping systems across upland and lowland areas. A set of 11 indicators were chosen to represent systems' performances and perform field-scale evaluation, while addressing issues at farm and territory levels. Results revealed that most cropping systems exhibited trade-offs among indicators, suggesting that agroecology may not perform optimally across all indicators. Crop rotations had better environmental performance but were less effective in terms of social indicators, particularly labor. Monocropping systems performed better in terms of labor minimization, but not economically. Results also showed that crop choice within the cropping system had a greater influence on sustainability than fertilization type for a given crop. The study revealed key constraints associated with current practices, and which indicators should be used at farm and territory scale to assess ex ante the agroecological practices scaling out potential.

In Madagascar, rice serves as a primary dietary staple, making the nation the second largest producer of rice on the African continent. However, rice production remains limited in fulfilling the needs of the entire population due to low soil nutrient availability. Conventional organic amendments and mineral fertilizers can enhance soil nutrients and rice growth but can also affect methane (CH4) emissions from paddy rice soils. Potential CH4 emissions from fertilizer management in tropical contexts are not well known. This study conducted two experiments to assess the impact of fertilizer management on CH4 emissions and rice production: a pot experiment involving farmyard manure (FYM; 10 t ha−1), NPK fertilizer (60 kg N: 60 kg P2O5: 60 kg K2O kg ha−1), and a control (no fertilizer); and a field experiment involving FYM (10 t ha−1) and NPK fertilizer (60 kg N, 66 kg P2O5, and 48 kg K2O ha−1). In the pot experiment, the application of both FYM and NPK fertilizers resulted in significant increases in grain yield of 105.1 % and 187.1 %, alongside increases CH4 emissions of 84.8 % and 71.0 %, respectively, compared to those in the control. The field experiment results partially aligned with those of the pot experiment, showing similar CH4 emissions from NPK and FYM fertilizers. However, the yield increase for NPK fertilizer was lower in the field (5.7 %) than that for FYM fertilizer. These findings underscore the need for balanced nutrient management strategies to enhance rice productivity while mitigating greenhouse gas emissions, which are crucial for sustainable rice cultivation in Madagascar.

Smallholder Eucalyptus plantations (EP) in Madagascar Central Highlands (MCH) address substantial fuelwood demand and reduce pressure on natural forests. However, their sustainability is challenged by low soil fertility and inadequate management. While fertilization increases tree growth, high mineral fertilizer costs limit its use by smallholder farmers. Both biomass estimating equation and impact of fertilization on smallholder EP in MCH remain poorly documented. This study aimed to evaluate the impact of low starter mineral fertilization (15 kg.ha-1 N, 12 kg.ha-1 P, and 17 kg.ha-1 K) and weed competition on smallholder Eucalyptus robusta growth and aboveground biomass (AGB) while providing allometric equations for AGB estimation. Dendrometric data were collected from six stands aged 2 to 6 years, with fertilized and non-fertilized parts. AGB of 16 trees per stand (8 fertilized, 8 non-fertilized) were destructively measured. AGB of trees were estimated by regression based on tree circumference, height and their combinations. The results indicated circumference as the best single variable predictor (R2 > 0.90) for all tree compartments and stands Fertilization significantly improved global tree survival by 7% and increased height by 3.1 m and circumference by 8.3 cm in 6-year-old stands AGB per hectare globally doubled with fertilization, reaching 55.3 Mg.ha-1at 6 yo compared to 29.2 Mg.ha-1 in non-fertilized plots. Weed cover and biomass had significant negative linear relationships with AGB and survival rate. This study provides robust allometric equations for biomass estimation and highlights that even low fertilizer application combined with effective weed control can significantly enhance AGB production in smallholder EP.

In semi-arid regions where nitrogen deficiency limits crop productivity, the integration of leguminous tree species into agroforestry systems (AFS) is likely to enhance nitrogen availability for associated crops. We tested this hypothesis in southern Madagascar, a region characterized by erratic and low rainfall, and nutrient-poor soils where the pigeon pea grown as a pure crop or mixed with cassava. Over a 3-month period, we estimated the atmospheric nitrogen fixation rate (%Ndfa) by two-year-old pigeon pea (Cajanus cajan) grown as a pure crop and in association with cassava (Manihot esculenta) using N dilution method, and the transfer of N from pigeon pea to cassava (%Ndft) in the mixed pigeon pea-cassava plots. Six-month-old pure cassava plots were used as reference. Each system was replicated three times. 15N soil labeling (98 atom% 15N) was used to estimate %Ndfa and %Ndft. %Ndfa was 100% in both pigeon pea systems. Considering %Ndfa of 100% from planting date, the quantity of N fixed was two times higher in pure pigeon pea plot than in mixed pigeon pea-cassava plots, with values of 17.1 kg N ha-¹ and 8.5 kg N ha-¹, respectively. This finding was consistent with the 2.8 times higher planting density in pure pigeon pea plots compared to mixed pigeon pea-cassava plots. Nitrogen transfer from pigeon pea to cassava was greater (72.9%) when the cassava plant was positioned farther from pigeon pea rows (in the middle) compared to when it was planted next to pigeon pea row (42.4%). Our results suggest that integrating pigeon pea into semiarid agroforestry systems could enhance nitrogen status of the associated crop, particularly in unfertilized AFS.

Background and Aims Root dipping (i.e. attaching fertilizer-soil slurry to the seedling roots) with N and P fertilizers is a method to enhance nutrient uptake efficiency (NUE) in paddy rice (Oryza Sativa L.), yet NH4+ toxicity may be the limiting factor. This study was set up to test waste-derived struvite (ST) for root dipping relative to a soluble N-P source (MAP). In addition, root dipping with ST was compared to using a non-toxic P source (TSP). Methods Short-term NH4+ toxicity effects on transplanted seedlings were studied for MAP and ST applied at equal N dose by root dipping in two soils with contrasting CEC, i.e. different risk for toxicity due to NH4+. In a 44 DAT greenhouse trial, long-term P availability from ST was compared to TSP at equal P dose in a P deficient soil. Results A diffusion test showed slow NH4+ release from ST compared to MAP in both soils. Root dipping with ST showed 2 times higher biomass than MAP at equal N dose and no signs of NH4+ toxicity, the high CEC soil buffered better against that toxicity. Shoot biomass at 44 DAT was two times higher for ST than for TSP, likely because of a pH increase with ST. Conclusion Root dipping with ST is a safe method to enhance NUE in paddy rice. With its liming effect and its lower solubility, ST offers a safer, more efficient and sustainable alternative to MAP and TSP.

Description of the subject. In Madagascar, afforestation plays a significant role in providing wood energy for the local population. However, reforestation projects often face low success rates due to biophysical constraints. Objectives. The aim of this study was to evaluate the impact of topographic position on the physico-chemical properties of potential reforestation sites in the central highlands of Madagascar. Method. The study covered 16 sites and analyzed 192 soil samples. The results were compared with deficiency thresholds established for Ferralsols. Results. The soil pHwater varied slightly, from 5.3 to 5.5. The 0-10 cm depth showed significantly higher levels of organic carbon (Corg), total nitrogen (Ntot), available phosphorus (Pdispo), and exchangeable potassium (Kéch) than the 10-40 cm depth. Soils on the slope were significantly poorer in Ntot and Kéch than those on the bottom of the slope, with mean values of 1,3 g N·kg-1 ± 0,07 and 0,09 cmolc·kg-1 ± 0,01 compared to 1,5 g N·kg-1 ± 0,08 and 0,12 cmolc·kg-1 ±0,01, respectively. Of the 16 sites, eight had to be discarded for future reforestation. Of the remaining eight, only two showed soils with little or no Ntot, Pdispo, and Kéch deficiency over 0-10 cm, but both were deficient in Pdispo. Conclusions. This study highlighted the importance of prior soil characterization, choice of topographic position and recommends starter fertilization for successful reforestation. The results of this study are a contribution to the choice of the most appropriate afforestation sites in Madagascar.

Description of the subject. In Madagascar, afforestation plays a significant role in providing wood energy for the local population. However, reforestation projects often face low success rates due to biophysical constraints. Objectives. The aim of this study was to evaluate the impact of topographic position on the physico-chemical properties of potential reforestation sites in the central highlands of Madagascar. Method. The study covered 16 sites and analyzed 192 soil samples. The results were compared with deficiency thresholds established for Ferralsols. Results. The soil pHwater varied slightly, from 5.3 to 5.5. The 0-10 cm depth showed significantly higher levels of organic carbon (C-org), total nitrogen (N-tot), available phosphorus (P-dispo), and exchangeable potassium (K-ech) than the 10-40 cm depth. Soils on the slope were significantly poorer in N-tot and K-ech than those on the bottom of the slope, with mean values of 1,3 g N.kg-1 +/- 0,07 and 0,09 cmolc.kg-1 +/- 0,01 compared to 1,5 g N.kg(-1) +/- 0,08 and 0,12 cmolc.kg(-1) +/- 0,01, respectively. Of the 16 sites, eight had to be discarded for future reforestation. Of the remaining eight, only two showed soils with little or no N-tot, P-dispo, and K-ech deficiency over 0-10 cm, but both were deficient in P-dispo. Conclusions. This study highlighted the importance of prior soil characterization, choice of topographic position and recommends starter fertilization for successful reforestation. The results of this study are a contribution to the choice of the most appropriate afforestation sites in Madagascar.

Understanding the controlling factors of soil organic carbon (SOC) content in tropical rice paddy fields is crucial for improving soil fertility and carbon sequestration. In tropical soil, SOC is primarily stabilized by interactions with active aluminum (Al) and iron (Fe), yet the relative importance of these elements remains unclear. This study aimed to (1) determine key factors of SOC content in paddy fields, and (2) examine the relationship between SOC and Al and Fe, specifically oxalate-extractable Al (Alox) and Fe (Feox). We analyzed 306 rice paddy soils (0-15 cm) in six communes in the central highlands of Madagascar, three influenced by volcanic activity. Structural equation modeling revealed that SOC content was significantly and positively correlated with Alox + Feox content across volcanic and non-volcanic soils. Alox was the primary SOC-controlling factor across all soils, while Feox played a significant role when its relative abundance to Alox increased, particularly in non-volcanic soils. This suggests that SOC stabilization depends on the relative proportions of Alox and Feox. Additionally, SOC content decreased with increasing soil pH. Overall, these findings underscore the importance of incorporating site-specific geochemical characteristics-particularly the balance between active Al and Fe-into the development of accurate SOC sequestration models for tropical paddy soils.

Understanding how land cover and seasonal variations influence soil microbial communities and nutrient cycling is crucial for sustainable land management in tropical forests. However, such investigations are limited in Madagascar’s tropical ecosystems. This study investigated the impacts of land cover types and seasonal variations on soil properties and microbial communities in the tropical forest region of Andasibe, Madagascar. Soil samples were collected from four land cover types—tree fallow (TSA), shrub fallow (SSA), eucalyptus forest (EUC), and degraded land (TM)—across three seasonal periods: the dry season, the start of the rainy season, and the end of the rainy season. Both land cover and sampling season affected soil pH and available P, whereas total nitrogen, soil organic carbon, and the C/N ratio were affected only by land cover. The soil organic carbon and total nitrogen concentrations were greater in TM. NextSeq sequencing of the 16S rRNA gene and ITS regions of the nuclear rRNA operon revealed distinct microbial community compositions across land covers, with greater diversity in the TSA and SSA. Bacteria are more sensitive to seasonal changes than are fungi, with phosphate-solubilizing (gcd) and phosphate-mineralizing (phoD) genes being more abundant in the rainy season, emphasizing the role of microbes in nutrient availability under different climatic conditions. Principal component analysis highlighted SSA as a hotspot for microbial activity, reinforcing the potential of shrub ecosystems in soil restoration. These findings reveal strong land cover and seasonal effects on soil microbial functions, with implications for nutrient cycling, ecosystem resilience, and sustainable land management in tropical forest landscapes.

Our understanding of the role of tropical lakes in regional carbon budgets remains hampered by a lack of data covering the vast diversity of lake types and settings. Here, we provide a first comprehensive survey of the carbon (C) biogeochemistry of the Lake Alaotra system, a large shallow lake (surface of 200 km2 and maximum depth of 2 m) surrounded by an extensive floodplain and rice fields located in the highlands of Madagascar. The current landscape in the region is grassland-dominated and dotted by major gullies called “lavaka”, which have historically been claimed to lead to high erosion rates and would, thus, also mobilize large amounts of soil C. We investigated the seasonal variability in the concentrations and stable isotope ratios of inorganic and organic C pools; moreover, we examined a range of other relevant proxies, including physicochemical parameters, dissolved CO2 and CH4 concentrations, total alkalinity, and chlorophyll a (Chl a) from spatially distributed sampling and seasonal monitoring of several rivers. While rivers were found to carry high total suspended matter (TSM) loads with a modest particulate organic C (POC) content, the lake itself and its outflow were characterized by much lower TSM values and a high relative contribution of POC to TSM (% POC). The POC concentration of the outflow (13.0±7.7 mg L−1) was substantially higher than in the inflowing water (1.9±2.1 mg L−1), and δ13C values were also distinct between inflowing water (-24.6±1.8 ‰) and the lake (-26.5±2.1 ‰) or its outflow (-25.2±1.4 ‰). Similarly, the lake outflow was surprisingly rich in dissolved organic carbon (DOC) (9.5±1.4 mg L−1) compared to inflowing water (2.6±1.1 mg L−1). This indicates that the lake and its surrounding wetlands act as a substantial source of additional organic C which is exported downstream. The CO2 and CH4 concentrations in inflowing and outflowing rivers were substantially higher than in lake waters, and they peaked during the rainy season due to lateral inputs from wetlands. However, sources of POC and DOC were uncoupled: δ13C data indicated that marsh vegetation was the main source of net DOC inputs, while phytoplankton contributed substantially to POC in the lacustrine waters, at least during parts of the sampling period. Indeed, lake suspended matter has relatively low POC / Chl a ratios (143–564, particularly during the May sampling period), high % POC (10 % to 29 %), and δ13C values (-26.5±2.1 ‰) distinct from those in marsh-derived organic matter. Despite the evidence for phytoplankton production as a contributor to the lake POC pool, the lake acted as a net source of CO2 to the atmosphere, likely due to the high C inputs from the surrounding marshes and to sediment respiration (considering the shallow water depth). Nevertheless, the partial pressure of CO2 (pCO2) levels in the surface waters of the lake were lower than those in the inflowing and outflowing rivers. This reduction is likely due to the combined effects of phytoplankton production, which assimilates CO2 during photosynthesis, and degassing processes. When CO2-supersaturated riverine water enters the open lake, increased turbulence caused by wind fetch enhances gas exchange with the atmosphere, allowing CO2 to escape more readily from the water column. The biogeochemical functioning of Lake Alaotra differs substantially from the large and deeper East African (sub)tropical lakes and is more similar to lakes surrounded by flooded forests in the Congo River basin, likely due to a combination of its large surface area and shallow water depth and the large extent of surrounding wetlands and floodplains. It acts as an abrupt element in the land–ocean continuum of the catchment, whereby the biogeochemical characteristics of the Maningory River (i.e. the lake outflow) are strongly determined by processes taking place in Lake Alaotra and its wetlands, rather than being reflective of characteristics and processes further upstream in the catchment.

Soil organic carbon (SOC) stock assessment is important regarding soil fertility, crop production, land degradation, and climate change mitigation particularly for Sub Saharan Africa (SSA) region. SSA countries face several methodological and operational challenges for producing SOC stock data. This paper analyzed the methods commonly recommended and used, and the barriers encountered in SOC measurement and monitoring in SSA. A literature review was complemented with a survey within eight research partner laboratories of the CaSA (Soil carbon for sustainable agriculture in Africa) network. Results showed that SOC stock assessment varied according to (i) studied depth (8 identified depths), (ii) SOC content analysis (5 identified methods), (iii) bulk density assessment (6 identified methods), (iv) coarse fragment percentage computation (4 identified methods); but also in monitoring (3 identified approaches). The main barriers in SOC stock studies are the lack of financial and expert human resources. Therefore, SSA laboratories adapt methods according existing equipment for producing SOC stock data. Challenges facing SSA are to raise funding to increase the number of soil analyses and to empower its technical and scientific teams. The challenges are also to be able to establish a common and specific guideline and database for SOC stock studies in SSA.

Summary The lack of affordable mineral fertilizers and scarcity of organic materials cause decline in soil fertility for smallholder farmers and producers in the highlands of Madagascar, challenging crop productivity. To fulfill plant growth and nutrition, we explored the effect of 132 combinations of 17 different fertilizing resources, both organic and mineral, on rice growth and nutrition using a greenhouse experiment. Two clustering approaches were used to evaluate the effects of fertilizing resources: elemental clustering and functional clustering. Elemental clustering grouped resources based on their elemental intrinsic composition, while functional clustering grouped resources based on their effect in improving plant growth and nutrition when combined in soil. We found that some resources closely grouped based on their elemental composition exhibited different effects on plant growth and nutrition when combined in soil. Zebu horn emerged as a particular organic resource in elemental clustering, and a key resource in functional clustering by promoting plant growth and nutrition when combined with other resources in soil. Its unique elemental composition played a significant role in driving positive interactions with other resources. We proposed to extend the concept of ‘assembly motif’ within soil fertilization strategy, suggesting that the combination of functional groups of resources determines better their fertilizing effect than their elemental composition. Resources inducing high interaction effects should be combined with those having high elemental composition to optimize crop productivity.

Abstract. The catchment of Lake Alaotra, a large shallow lake (surface is 200 km2, maximum depth 2 m) in the Malagasy highlands, is a region where the grassland dominated landscape is dotted by major gullies called “lavaka”, which has historically been claimed to lead to high erosion rates. Sedimentary archives in lakes such as Lake Alaotra could be of great help to resolve questions about the natural versus anthropogenic influences on the changing landscape, provided that we understand carbon sources and sinks within the lake, as well as the connection with the surrounding landscape through the input of material via inflowing water. Here, we provide a first comprehensive survey of the carbon (C) biogeochemistry of the Lake Alaotra system. We investigated the seasonal variability of the concentrations and stable isotope C ratios of inorganic and organic C pools, as well as a range of other relevant proxies, including physico-chemical parameters, dissolved CO2 and CH4 concentrations, total alkalinity, and Chl-a (chlorophyll a) from spatially distributed sampling and seasonal monitoring of several rivers. While rivers were found to carry high total suspended matter (TSM) loads with a modest particulate organic C (POC) content, the lake itself and its outflow were characterised by much lower TSM values and high %POC (relative contribution of POC to TSM). The POC concentration of the outflow (13.0 ± 7.7 mg L-1) was substantially higher than in the inflowing water (1.9 ± 2.1 mg L-1), and δ13C values were also distinct between inflowing water (-24.6 ± 1.8 ‰) and the lake (-26.5 ± 2.1 ‰) or its outflow (-25.2 ± 1.4 ‰). Similarly, the lake outflow was surprisingly rich in DOC (9.5 ± 1.4 mg L-1) compared to inflowing water (2.6 ± 1.1 mg L-1). This indicates that the lake and its surrounding wetlands act as a substantial source of additional organic C which is exported downstream. The CO2 and CH4 concentrations in inflowing and outflowing rivers were substantially higher than in lake waters, and peaked during the rainy season due to lateral inputs from wetlands. However, sources of POC and DOC were uncoupled: δ13C data were consistent with marsh vegetation being the main source of net DOC inputs, while phytoplankton was expected to be an important source of POC in the lacustrine waters. Lake suspended matter has low POC/Chl-a ratios (143–564), high %POC (10 to 29 %), and δ13C values around 20 ‰ lower than the dissolved inorganic C (DIC) pool (-26.5 ± 2.1 ‰ versus -6.7 ± 1.6 ‰). Despite the importance of phytoplankton production to the lake POC pool, the lake acted as a net source of CO2 to the atmosphere, likely due to the high C inputs from the surrounding marshes, and sediment respiration considering the shallow water depth. Nevertheless, the pCO2 in the surface waters of the lake was lower than in the inflowing and outflowing rivers, possibly reflecting the impact of phytoplankton production (CO2 assimilation), although also reflecting degassing to the atmosphere. The biogeochemical functioning of Lake Alaotra differs substantially from the large and deeper East African (sub)tropical lakes and was similar to lakes surrounded by flooded forest in the Congo River basin, likely due to a combination of its large surface area and shallow water depth, and the large extent of surrounding wetlands and floodplains. It acts as an abrupt element in the land-ocean gradient of the catchment, whereby the biogeochemical characteristics of the Maningory River (i.e., the lake outflow) are strongly determined by processes taking place in Lake Alaotra and its wetlands, rather than being reflective of characteristics and processes higher up in the catchment.

Although involved in key functions of the terrestrial ecosystems, the activity and the diversity of soil microorganisms can be severely limited by energy and nutrients in weathered tropical soils. To optimize nutrient cycling for crop nutrition, assessing which nutrients limit the activity of the microbial food web is thus essential. This was our aim in this study carried out on a tropical ferrallitic soil from a natural grassland in Madagascar. For this purpose, we employed an innovative nutrient-omission approach in microcosms to test the distinct effects of omitting C, six nutrients (N, P, K, S, Ca, Mg) and a cocktail of micronutrients (B, Mn, Cu, Na and Mo) on the composition, biomass and activities of the microbial community and on the abundance and biomass of their nematode grazers. C and P were identified as primary limitations, but other nutrients (N, Mg and S) played significant roles as co-limiting factors. Some bacterial and fungal taxa were significantly associated to specific nutrient deficiencies and can act as biological indicators. Additionally, the abundance and biomass of microbial-feeding nematodes provided valuable insights into the responses of the soil microbial community to nutrient deficiencies. These findings contribute to our understanding of nutrient dynamics and microbial community growth in tropical grassland ecosystems and have implications for sustainable crop fertility management and ecosystem ecology in similar environments.

In the Highlands of Madagascar, providing organic amendments (OAs) is the main farmer fertilization practice in upland rainfed agrosystems dominated by nutrient-depleted Ferralsols. Therefore, enhancing the performance of OAs on plant functions is particularly important for improving both plant productivity and agrosystem sustainability. Earthworms are well-known ecosystem engineers involved in the decomposition of organic matter and play a critical role in nutrient turnover. Inoculating earthworms could be a promising approach to improve the fertilizer performance of OAs. However, there is limited knowledge regarding the interaction between earthworms and OAs of varying biochemical qualities on plant growth and nutrition. In this microcosm experiment, we assessed the effect of different types of OAs (i.e., unamended control, conventional farmyard manure, kraal manure, improved farmyard manure, compost, vermicompost, urban waste, taroka, poultry droppings, pig manure) and earthworm inoculation (with and without earthworms) on the growth and nutrition of rice. The endogeic species Pontoscolex corethrurus (Rhinodrilidae) was used in the experiment. After eight weeks of growth, the poultry droppings had the best fertilizer performance, followed by improved farmyard manure and pig manure in treatments without earthworms. The main driver of fertilizer performance appeared to be the quality of OAs. However, other mechanisms, such as an increase in soil pH, may also play a significant role. The inoculation of earthworms significantly enhanced the fertilizer performance of OAs but the magnitude of this enhancement depends on the OAs. Overall, OAs with low fertilizer performance showed a more pronounced response to earthworms. Additionally, earthworms improved the performance of high-quality OAs likely by enhancing the availability of P and Mg. Our findings emphasize the synergistic effects of earthworms and OAs of various qualities on plant growth and nutrition, offering valuable insights for improving fertilization practices in nutrient-poor Ferralsols of Madagascar.

IntroductionOne-third of the human population consumes insufficient zinc (Zn) to sustain a healthy life. Zn deficiency can be relieved by increasing the Zn concentration ([Zn]) in staple food crops through biofortification breeding. Rice is a poor source of Zn, and in countries predominantly relying on rice without sufficient dietary diversification, such as Madagascar, Zn biofortification is a priority.MethodsMulti-environmental trials were performed in Madagascar over two years, 2019 and 2020, to screen a total of 28 genotypes including local and imported germplasm. The trials were conducted in the highlands of Ankazomiriotra, Anjiro, and Behenji and in Morovoay, a location representative of the coastal ecosystem. Contributions of genotype (G), environment (E), and G by E interactions (GEIs) were investigated.ResultThe grain [Zn] of local Malagasy rice varieties was similar to the internationally established grain [Zn] baseline of 18–20 μg/g for brown rice. While several imported breeding lines reached 50% of our breeding target set at +12 μg/g, only few met farmers’ appreciation criteria. Levels of grain [Zn] were stable across E. The G effects accounted for a main fraction of the variation, 76% to 83% of the variation for year 1 and year 2 trials, respectively, while GEI effects were comparatively small, contributing 23% to 9%. This contrasted with dominant E and GEI effects for grain yield. Our results indicate that local varieties tested contained insufficient Zn to alleviate Zn malnutrition, and developing new Zn-biofortified varieties should therefore be a priority. GGE analysis did not distinguish mega-environments for grain [Zn], whereas at least three mega-environments existed for grain yield, differentiated by the presence of limiting environmental conditions and responsiveness to improved soil fertility.DiscussionOur main conclusion reveals that grain [Zn] seems to be under strong genetic control in the agro-climatic conditions of Madagascar. We could identify several interesting genotypes as potential donors for the breeding program, among those BF156, with a relatively stable grain [Zn] (AMMI stability value (ASV) = 0.89) reaching our target (>26 μg/g). While selection for grain yield, general adaptation, and farmers’ appreciation would have to rely on multi-environment testing, selection for grain [Zn] could be centralized in earlier generations.

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.