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.

Soil fertility degradation is a major stake in sub-Saharan Africa. Strong population growth, local demand for food and land scarcity are likely to increase this phenomenon. While mineral fertilizers are little used, particularly in Madagascar, soil fertility management is therefore largely dependent on organic fertilizers produced on the farm. In order to increase the efficiency of nutrient recycling at farm level, it is necessary to understand the factors that contribute to improved manure management practices. This study proposes to analyze the relationships between the structure of farms integrating crop and livestock in the Vakinankaratra region (Highlands of Madagascar) and their manure management practices. Semi-structured interviews were carried out on 300 farms. Then, a farm typology was built through a Principal Component Analysis (PCA). Seven farm types were identified, distinguishing dairy farms from other agri-livestock farms (zebus, pigs). A Chi(2) test indicates a significant effect of farm structure on effluent management methods. In particular, the presence of dairy and pig farming within farms seems to favor improved manure management. The cluster analysis contributes to understanding the adoption of improved biomass management practices by crop-livestock farming systems. The adoption of such practices is a prerequisite for improving nutrients recycling from animals to agricultural plots, and thus improving the sustainability of farms in the Vakinankaratra Highlands.

Carbon sequestration in agricultural soils, through the accumulation of high-quality organic matter, represents great potential to mitigate climate change and simultaneously improve soil fertility. Such a challenge is particularly important and relevant in developing tropical countries like Madagascar, where soil carbon storage is vulnerable to climatic variations and where fertilization is generally applied through amendments in organic matter of various origins. The priming effect (PE) is considered here as the stimulation of the mineralization of soil organic matter (SOM) by a supply of fresh organic matter (FOM). PE results from different microbial processes driven by specific biotic and abiotic parameters. Depending on the processes involved, it has been suggested that PE could either counteract SOM accumulation or promote it. The objective of the present study was to explore the relationships between certain agricultural practices (type of crop, quality of fertilization, association with trees), the potential intensity of PE, as well as several abiotic (texture, quantity and quality SOM, nutrient enrichment) a1nd biotic (biomass and phylogenetic composition of microbial communities) factors which have been proposed in the literature as specific determinants of the different PE generation mechanisms. The soils for this study come from a network of farms in a commune in the Highlands of Madagascar. The PE, generated by a supply of 13C-enriched wheat straw, could not directly correlate with agricultural treatments. However, several indirect correlations could be found via several specific abiotic and microbial determinants that are discussed in terms of soil fertility restoration.

La dégradation de la fertilité des sols est un problème majeur en Afrique subsaharienne. La forte croissance démographique, la demande locale de nourriture et la réduction des terres arables disponibles risquent d’accroître ce phénomène. Alors que les engrais minéraux sont peu utilisés, notamment à Madagascar, la gestion de la fertilité des sols est largement dépendante des matières organiques produites à la ferme. Afin d’augmenter l’efficacité du recyclage des nutriments à l’échelle de la ferme, il est nécessaire de comprendre les facteurs favorables à l’amélioration des pratiques de gestion des effluents d’élevage. Cette étude propose d’analyser les relations entre la structure d’exploitations agricoles intégrant agriculture et élevage dans la région de Vakinankaratra (Hautes Terres de Madagascar) et leurs pratiques de gestion des effluents. Des entretiens semi-directifs ont été réalisés auprès de 300 exploitations. Une typologie a été réalisée à l’aide d’une analyse en composantes principales (ACP). Sept types d’exploitation ont été retenus, permettant de distinguer des exploitations laitières d’autres exploitations d’agro-élevage (bovins de trait, porcs). Un test de Khi2 indique un effet significatif de la structure des exploitations sur le mode de gestion des effluents. En particulier, la présence d’un élevage laitier et de porcins semble favoriser des modes de gestion améliorés. L’analyse typologique contribue à la compréhension de l’adoption de pratiques améliorées de gestion des biomasses par les agro-éleveurs. L’adoption de telles pratiques est un prérequis pour améliorer le recyclage des nutriments vers les parcelles agricoles, et donc améliorer la durabilité des exploitations agricoles des Hautes Terres du Vakinankaratra.

The development of techniques for the rapid, inexpensive, and accurate determination of the phosphorus (P) availability and sorption index (PSI) in soils is important for P management in highly weathered tropical soils. The applicability of near- and mid-infrared reflectance spectroscopy (NIR and MIR) as tools for estimating P availability and PSI was assessed over a wide range of highly weathered soils in Madagascar. The predictions were based on chemometric methods using multivariate calibration models with partial least squares (PLS) regressions, and pedotransfer functions (PTFs). Chemometric methods failed to predict available P (Presin). However, a P sorption index, determined as the P remaining in solution (Prem), was estimated with acceptable accuracy with both NIR and MIR (R2cv = 0.70 − 0.73; R2v = 0.65 − 0.77; SEP(c) = 5.5 − 4.6 mg kg−1). The PTFs showed that the PSI was well explained by iron oxide, gibbsite, and sand contents, all of these compounds being well predicted by NIR or MIR (R2v > 0.70). These results indicate that NIR and MIR can be helpful for a rapid estimate of PSI of highly weathered ferralitic soils.

Mixed farming systems are still prevalent in sub-Saharan Africa. In these systems, the recycling of nutrients through crop-livestock integration (CLI) practices is crucial for the sustainability of soil fertility and crop production. The objective of this study was to analyze nutrient (N, P, K) flows and balances of mixed farming systems to assess CLI contribution to the performance of those systems. We hypothesized that more intensive farms had a better nutrient balance at the farm level, and that improved biomass management methods improved their nutrient balance. Nine farms in the Madagascar highlands were selected, some corresponding to poor traditional farms with only draft cattle; some small or medium-sized, more intensive farms with a dairy herd; and some of the latter with some improvement to management methods of livestock effluents (manure composting, liquid manure collection). The nutrient balance of the farming systems was determined, and performance indicators were calculated at both farming, livestock, and CLI levels. Results showed that nutrient recycling through CLI is significant in the functioning of the systems studied, contributing primarily to circulating nutrient flows (up to 76%) and leading to greater efficiency and productivity. Nutrient flows resulting from these practices mainly concerned animal feeding (higher than 60% of nutrient flows), even if manure management was central for crop fertilization and that manure remained a desired animal product of these types of farms (up to 100% of animal products). Large negative balances of N and K (up to 80% of inputs) were observed in traditional livestock systems with draft cattle. They were smaller (39–68%) in more intensive dairy farms. Composting of manure did not decrease negative balances, whereas their magnitude was significantly reduced by the collection of liquid manure (19% for N; 42% for K). Better management of biomass at the farm level, in particular the collection of liquid manure, seemed to substantially reduce nutrient losses in MFS.

The cultivation of grain legumes (e.g., common bean) in sub-Saharan Africa contributes to the provision of food for a growing population and delivers environmental benefits such as inputs of nitrogen (N) to crops and soil via symbiotic nitrogen fixation (SNF). However, the success of SNF is constrained by several factors such as the poor efficiency of native rhizobial strains to fix N, the low availability of phosphorus (P) and the acidity of soils. Two trials have been conducted in low-fertility tropical soils at the smallholder farm scale in Madagascar to assess the effects of Rhizobium inoculation together with inputs of P and lime on the growth of the common bean. We showed that inoculation with native strains of Rhizobium had significant effects on bean root nodulation, which was increased by up to 15-fold on plant growth, which increased by 78% and on bean yield, which increased by 126%. Moreover, we observed positive and significant relationships between inoculation with Rhizobium and P fertilization on nodulation, plant growth and yield. However, the addition of dolomite lime did not show any effect in our study. The addition of P decreased the mycorrhization rate of roots. Additional research is still needed to improve our understanding of soil fertility conditions (mainly on nutrient availability, including micronutrients) allowing better efficiency of legume symbionts (rhizobium and mycorrhiza) in such low-fertility soils.

Plant diversification through crop rotation or agroforestry is a promising way to improve sustainability of agroecosystems. Nonetheless, criteria to select the most suitable plant communities for agroecosystems diversification facing contrasting environmental constraints need to be refined. Here, we compared the impacts of 24 different plant communities on soil fertility across six tropical agroecosystems: either on highly weathered Ferralsols, with strong P limitation, or on partially weathered soils derived from volcanic material, with major N limitation. In each agroecosystem, we tested several plant communities for diversification, as compared to a matching low diversity management for their cropping system. Plant residue restitution, N, P and lignin contents were measured for each plant community. In parallel, the soil under each community was analyzed for organic C and N, inorganic N, Olsen P, soil pH and nematode community composition. Soil potential fertility was assessed with plant bioassays under greenhouse controlled climatic conditions. Overall, plant diversification had a positive effect on soil fertility across all sites, with contrasting effects depending on soil type and legumes presence in the community. Communities with legumes improved soil fertility indicators of volcanic soils, which was demonstrated through significantly higher plant biomass production in the bioassays (+18%) and soil inorganic N (+26%) compared to the low diversity management. Contrastingly, communities without legumes were the most beneficial in Ferralsols, with increases in plant biomass production in the bioassays (+39%), soil Olsen P (+46%), soil C (+26%), and pH (+5%). Piecewise structural equation models with Shipley's test revealed that plant diversification impacts on volcanic soil fertility were related to soil N availability, driven by litter N. Meanwhile, Ferralsols fertility was related to soil P availability, driven by litter P. These findings underline the importance of multifactorial and multi-sites experiments to inform trait-based frameworks used in designing optimal plant diversification in agroecological systems.

Cropping systems with legumes play key roles in farming systems in sub-Saharan Africa. However, how commonly legume associations perform in low input-systems is not well-known. Here, we studied four legume species used in three systems in rotation with upland rice, i.e., groundnut monocropping, sorghum–cowpea intercropping, and velvet bean–crotalaria intercropping, in two fertilization managements on the previous rice, i.e., manure alone or complemented with mineral fertilization. Legume suitability was assessed using rhizobial and mycorrhizal colonization rates, plant biomass production, shoot N and P content, and biological N2 fixation based on their δ15N natural abundance. Shoot and root biomasses varied significantly between legume species and were positively correlated with nodule number (r = 0.49 and 0.74, p-value < 0.05 and <0.001, respectively) and the amount of fixed N (r = 0.73 and 0.50, p-value < 0.001 and <0.05, respectively). The proportion of plant N derived from N2 fixation also varied significantly between species, with a higher percentage for velvet bean (66%), compared to the other three species (50 to 60%). Legume roots were weakly colonized by AM fungi, with similar levels between species. Overall, fertilization management did not significantly impact legume biomass, symbioses, or N2 fixation, yet the organo-mineral fertilization significantly increased legume shoot P content. The lack of effect of mineral fertilization on N2 fixation and biomass could be due to other nutrient deficiencies (Ca, Mg, micronutrients), which can hamper symbioses with rhizobia and mycorrhizae.

Plant diversification is one of the main ways to ecologically intensify agroecosystems to improve their sustainability and resilience. Rotations and/or a mixture of crops can mitigate pest and weed infestation, reduce diseases, and improve soil fertility and crop productivity. However, rainfed rice yields in the Malagasy highlands remain low despite the frequent use of cropping systems including crop rotations and mixtures. In this study, we compared three rainfed rice based short rotations with rainfed rice monocropping to quantify the benefits of plant diversification on different ecosystem functions such as weed and nematode control, soil fertility, soil macrofauna abundance and diversity, and rice yield over four cropping seasons. The three rotations were based on rice in rotation with one legume, groundnut (RG), a cereal-legume mixture, sorghum and cowpea (RSC), or a mixture of legumes, velvet bean and crotalaria (RVC). Rice growth, N content and yield, soil N content, weed biomass, nematofauna and macrofauna were assessed and a profitability analysis was performed at rotation scale. The legume mixture had a significant and positive effect on rice growth, N content and yield, soil N content, and weed and nematode control due to high biomass production in the cropping cycle including legume mixture, by limiting weed growth and leaving a large quantity of N-rich residues to enrich the soil for the following rice crop. The nematicide properties of the legume mixture may reduce the infestation of plant-feeding nematodes. The RG and RSC rotations produced intermediate results. While rice yields were higher in these rotations than when rainfed rice was grown alone, weed biomass remained high due to minimal competition with weeds during the crop rotation cycle especially with groundnut. For RSC, nematode control was limited as both sorghum and cowpea are host plants for nematodes. Despite a year with no crop income with the RVC rotation, profitability was higher mainly due to the increased rice yield and reduced field management costs. The choice of species is thus crucial to optimise ecosystem functions adapted to farmers' context and objectives.

The effects of earthworm inoculation and cropping systems on upland rice systems were examined over a four-year period in the Highlands of Madagascar. Each year, endogeic earthworms Pontoscolex corethrurus (Rhinodrilidae) were inoculated (EW+) at a density of 75 ind m−2 or were not inoculated (EW0). Inoculation was tested in three cropping systems: conservation agriculture (CA) and traditional tillage with or without residues restitution. Soil and plant properties were measured during the first three years while soil biological properties were assessed at the fourth year. At the end of the experiment, earthworm density was three-fold higher in EW+ than in EW0, demonstrating the success of the inoculation. Earthworm density was more important in CA than in tillage systems. Earthworm inoculation had higher significant effects on soil and plant properties than cropping systems. Earthworm inoculation had positive effects on soil macroaggregation (+43%), aboveground biomass (+27%), rice grain yield (+45%), and N grain amount (+43%). Intensifying earthworm activity in field conditions to meet the challenge of ecological transition is supported by our study.

Soil acidification and declining fertility are widespread in sub‐Saharan Africa. Nutrient depletion is mainly related to nutrient mining driven by biomass removal without replenishment of nutrients through use of fertilisers. Concomitant acidification is due to the high ash alkalinity of harvested biomass. We determined the nutrient content and ash alkalinity of biomass of the main crops produced in smallholder mixed crop–livestock farming systems in the Malagasy Highlands of Madagascar and calculated the soil acidification/alkalinisation occurring through biomass transfer. Samples of rice and forage were collected from 70 rice plots and 91 cultivated forage plots, and 70 manure samples were collected from farms. Nutrient exports induced by crop harvesting resulted in annual losses of 57 kg nitrogen (N), 6 kg (phosphorus) P and 33 kg potassium (K) ha–1 for rice (grain + straw), and 31–51 kg N, 8–9 kg P and 29–57 kg K ha–1 for each forage cut (with three cuts per year on average). The ash alkalinity of samples, calculated as the difference between cation and anion contents, was 49–100 cmolc kg–1 for forage crops, 31 cmolc kg–1 for rice straw, and only 4 cmolc kg–1 for rice grains. Biomass removal caused a loss of nutrients and an increase in soil acidity. Owing to low nutrient retention efficiency during the handling and storage of manure, the traditional input of manure at 5 t fresh matter ha–1 is insufficient to balance nutrient and alkalinity losses in Malagasy mixed crop–livestock farming systems. Maintaining productive and sustainable mixed crop–livestock farming systems requires greater attention to ensuring a nutrient balance at both plot and farm levels.

We conducted a greenhouse experiment in mesocosms for 28 days to assess the effects of the endogeic earthworm Pontoscolex corethrurus on plant-available soil phosphorus (P) and rice (Oryza sativa) P nutrition in a Malagasy Ferralsol. To assess plant-available soil P, we determined the L-value by measuring the specific activity of P taken up from the soil by rice and by applying the isotopic dilution principle. Despite earthworm mortality, P. corethrurus significantly increased rice shoot biomass (+26%) and P nutrition (+65%), confirming that the soil used for the experiment was P-deficient. The L-value also markedly increased from 6.8 +/- 0.9 to 14.2 +/- 1.3 mg P mesocosm(-1) in the presence of P. corethrurus. We estimated that the orthophosphate ions released due to earthworm mortality contributed to 30% of the L-value increase. We attributed the remaining 70% increase to the solubilization of native soil P during its transit through the digestive tract. Thereafter, we discussed the sources of uncertainty associated with the L-value calculation and their utilization to assess earthworm effects on P availability in further studies.

Ultramafic (UM) rocks are defined as igneous rocks that contain more than 90% of mafic minerals. Soils derived from ultramafic rock are generally nutrient-deficient and have concomitant high concentrations of potentially phytotoxic trace elements (Ni, Cr, Co, Mn). Consequently, to assess the dynamics of nutrients and metals in the ultramafic complex of Niquelândia (Brazil), soil solutions have been sampled in soils characterized by high Cr(VI) availability. The metal contents in surficial water have also been analyzed to investigate the metals’ leaching and mobility. Soil solutions featured low nutrient contents, a large Ca:Mg imbalance, and high Ni and Cr concentrations. Chromium was present in its toxic dissolved form (Cr(VI)) in the soil and surficial solutions. Metals concentrations were often above the toxic limit for biota and were therefore able to affect soil functioning. Ni behavior in the topsoil appeared to be primarily controlled by organic matter, while Cr was more likely to be released from Fe-oxides by anionic exchange. This result agreed with the Cr(VI) lability assessed using isotopic exchange kinetics in a companion study. In these serpentinic tropical soils, the highly leached Fe-oxide horizons appear to play a large role in the sequestration and diffuse leaching of labile Cr(VI) and Ni, respectively, in the deeper part of soil profiles and the topsoil. At the catchment scale, surficial solutions results suggest that metals may be exported to surrounding ecosystems that are not adapted to these metals.

ABSTRACT Tropical Savannas cover an area of approximately 1.9 billion hectares around the word and are subject to regular fires every 1 to 4 years. This study aimed to evaluate the influence of burning windrow wood from Cerrado (Brazilian Savanna) deforestation on the spatial variability of soil chemical properties, in the field. The data were analysed by using geostatistical methods. The semivariograms for pH(H2O), pH(CaCl2), Ca, Mg and K were calculated according to spherical models, whereas the phosphorus showed a nugget effect. The cross semi-variograms showed correlations between pH(H2O) and pH(CaCl2) with other variables with spatial dependence (exchangeable Ca and Mg and available K). The spatial variability maps for the pH(H2O), pH(CaCl2), Ca, Mg and K concentrations also showed similar patterns of spatial variability, indicating that burning the vegetation after deforestation caused a well-defined spatial arrangement. Even after 20 years of use with agriculture, the spatial distribution of pH(H2O), pH(CaCl2), Ca, Mg and available K was affected by the wood windrow burning that took place during the initial deforestation.

Free-living nematodes have beneficial effects on plant growth and nutrition. Exploring how agricultural practices modulate these beneficial effects is still challenging. A study was conducted in Ferralsols from Madagascar from one unmanaged grassland and 16 upland rainfed rice fields, representative of different agricultural practices: rotation, agroforestry and monoculture. Intact soil cores in plastic cylinders were sampled in the field to assess the effects of agricultural practices on changes in plant growth and nutrition induced by the presence of bacterial-feeding nematodes. The soil cores were fumigated to kill the nematodes and moistened with a filtered fresh soil suspension containing only microbial cells. A rice seed was introduced in the core, which was then incubated under natural climatic conditions for 40 days with or without inoculation of the bacterial-feeding nematode Acrobeloides sp. The inoculation of the nematodes induced lower, similar or higher plant biomass and nutrient content in comparison to the control according to the agricultural practices. Positive effects of Acrobeloides sp. on plant functions were frequent in soil cores sampled from fields with high plant diversity, especially from agroforestry systems. The intact soil core technique appears to be a robust means of mimicking field conditions and constitutes a promising tool to assess effects on soil processes of the ecological intensification of agricultural practices.

Phosphorus (P) is often the main limiting factor for plant growth in highly weathered tropical soils. Phosphate use efficiency and crop yields could be increased in low-input agroecosystems through cropping systems that favor organic matter accumulation or regular additions of composts and manure. Our objective was to determine the amounts and forms of P according to companion crops (Brachiaria ruziziensis–a grass species–or Cajanus cajan–a leguminous species) and tillage systems (conventional or no-tillage). Soil P pools were determined (Presin, POlsen, PNaOH-EDTA, and Ptotal) and P species were characterized by 31P NMR spectroscopy. The concentrations of available and labile inorganic P (Presin, PiOlsen, and PiNaOH-EDTA) were greater in soil samples where companion crops (Brachiaria ruziziensis or Cajanus cajan) and maize were planted on the same row, with a more significant effect with the legume species. According to the 31P NMR, it is mostly the proportion and amount of POrthophosphate that was increased, with a slight increase of the content of PMonoester. Overall, there was a decrease of the proportion of organic P (PoNMR/PNMR) from 32 to 16% when the legume companion crop was associated with maize. The tillage systems did not lead to any change in the amounts and forms of P. Companion crops are thought to increase available P through mineralization of Po from plant residues and soil organic matter, leading to an increase of fluxes between active P pools.