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Family farms in the tropics mainly rely on rainfed agriculture. Water availability is limited to rainfall and is one of the main constraints to crop productivity. Conservation agriculture (CA) is promoted as an alternative that, among other functions, enhances water infiltration and limits evaporation from the soil thanks to a mulch of crop residues left on the soil surface. These functions are assumed to reduce the water availability constraint by limiting water stress during crop growth. But the variability of rainfall distribution combined with the wide range of agroecological conditions and the variety of crop husbandries in the tropics makes it difficult to evaluate the efficiency of mulching. The aim of this study was to capture the variability through a simple modeling approach using the crop growth model PYE-CA, which requires a limited set of parameters and a virtual experiment (VE). We applied our approach to a case study of upland rice in the Lake Alaotra region in Madagascar, where rainfall distribution is highly variable. The VE used a 17-year series of weather data with a range of soil water conditions, sowing dates, and growth and yield limitations that cover the variability of agroecological conditions and management systems in the study area. The VE revealed that variable successions of wet and dry episodes during the rainy season resulted in both water stress and an increase in deep drainage. In the majority of conditions simulated, enhancing water infiltration through CA mainly increased water loss through drainage. However, better water infiltration may also reduce the production risks involved in early sowing or crop intensification, thereby offering new opportunities to farmers. As an alternative to time consuming and labor intensive experimentation, we propose a suitable modeling approach to identify the main drivers of rainfall × crop interactions that could be extrapolated to other regions in the tropics.

Since the early 2000s conservation agriculture (CA) has been promoted in the Lake Alaotra region of Madagascar for a more sustainable and profitable agriculture. There is, however, little known about its performance in low-input rainfed rice-based cropping systems. We conducted a study during two growing seasons (2013/14 and 2014/15) on an experiment that was established in 2009 at the agricultural research station of the National Center for Applied Research and Rural Development (FOFIFA) in the Lake Alaotra region of Madagascar. The experimental setup was a randomized block design with four replications. Two soil/residue management treatments were studied, conventional tillage without residue retention (CT) and no-tillage with residue retention (NT). These two treatments were tested for a 2-year rotation of maize + Dolichos lablab followed by rice (MD//R) and a 3-year rotation of maize + Stylosanthes guianensis, followed by S. guianensis in the second year, and rice in the third year (MS//S//R). During the 2013/14 and 2014/15 seasons, two levels of weed pressure: 'high' and 'low' were introduced as a split-plot design on the rice plots. The main determining factors of rice yield in the study region were studied: radiation interception, weed infestation, soil moisture and soil mineral nitrogen (N). Our results showed that five to six years of continuous practice of NT with retention of high amounts of crop residues (more than 5 Mg DM ha−1) on the soil surface had a significant (p = 0.02) positive effect on rice yield, irrespective of the level of weed pressure and type of crop rotation. CA systems significantly (p

SUMMARYFamily farming in the tropics suffers from low crop productivity mainly due to a combination of poor soil fertility, low investment capacity, and a variable climate. The Lake Alaotra region of Madagascar is no exception and rainfed production is particularly hard hit. To evaluate the agronomic benefits of conservation agriculture (CA) in a region of erratic rainfall, we analysed four years of yield, management and climatic data from 3803 upland rice fields cultivated by farmers and monitored by researchers. Fields located on rainfed lowlands and hillsides were cultivated with sole rice using conventional tillage (Cv) or rice sown with no-tillage on dead organic mulch and rotated with other cereal/legume combinations (CA) from 2006 to 2011. A first global comparison across seasons, locations and years of adoption showed significantly higher average yields under CA, with no change in variance (on lowland: 2.6 ± 0.9 t ha–1 Cv, 2.8 ± 0.9 t ha–1 CA; on hillside: 2.1 ± 0.8 t ha–1 Cv, 2.4 ± 0.8 t ha–1 CA). Grouping fields according to the number of years under CA (first to fourth) revealed that CA gradually increased average yields and reduced the coefficient of variation in the short and mid-term (on lowland: +0.2 t ha–1 and –6% coefficient of variation; on hillside: +0.7 t ha–1 and –13% coefficient of variation, over four to six years of successive CA cropping). The average yield increase under CA was not associated with an increase in mineral fertiliser use, as farmers used the same amounts of fertilisers (or none) under Cv and CA. The comparison Cv versus CA also highlighted a major benefit of CA regarding climate: it widened the window of possible sowing dates. A classification and regression tree analysis of the entire dataset revealed that rice yield was more affected by agro-environmental factors than management factors (fertilisation, Cv or CA), and extreme climate variability such as the severe drought of 2007–2008 could not be offset by CA. The hypothesis of yield penalties during the first years of implementation of CA cannot be verified with the evidence presented in this study.

International audience; AbstractConservation agriculture, which is based on minimum tillage, permanent soil cover and crop rotations, has widely been promoted as a practice to maintain or improve soil quality and enhance crop productivity. To a large extent, the beneficial effects of conservation agriculture are expected to be provided by permanent soil cover with crop residues. Surface crop residues play an important role for crop growth through their benefits on soil-related structural components and processes in the agro-ecosystem, referred to in this study as agro-ecological functions. Through a meta-analysis of the literature, we have studied the relative effects of surface crop residue levels on the performance of a set of agro-ecological functions compared with a no-till bare soil, i.e., without surface residues. The selected agro-ecological functions were soil water evaporation control, soil water infiltration, soil water runoff control, soil loss control, soil nutrient availability, soil organic carbon (SOC) stocks and gains, weed control and soil meso- and macrofauna abundance. The potential effects of crop residue cover were quantified using boundary line models. Our main findings were (1) 8 t ha−1 of residues were needed to decrease soil water evaporation by about 30% compared to no-till bare soil. (2) To achieve the maximum effect on soil water infiltration, water runoff and soil loss control, residue amounts of at least 2 t ha−1 were required. (3) The effect of increasing the amounts of surface crop residues on soil nutrient supply (N, P and K) was relatively low; the boundary line models were not significant. (4) The average annual SOC gain increased with increasing amounts of residues, with a mean of 0.38 t C ha−1 year−1 with 4 to 5 t ha−1 of residues. (5) Weed emergence and biomass can be reduced by 50% compared to a no-till bare soil with residue amounts of 1 t ha−1 or more. (6) There was a weak response in soil meso- and macrofauna abundance to increasing amounts of surface crop residues. The maximum effect corresponded to an increase of 45% compared to a no-till bare soil and was reached from 10 t ha−1 of residues. Our findings suggest that optimal amounts of surface residues in the practice of conservation agriculture will largely depend on the type of constraints to crop production which can be addressed with mulching.

Vu la degradation de la productivite des sols laboures, il fallait tester l'effet du semis direct sous litiere vegetale pour controler l'erosion hydrique et ameliorer le rendement des cultures pluviales sur les Hautes-Terres centrales malgaches. L'essai a ete entrepris sur un dispositif de parcelles d'erosion a Andranomanelatra, sous pluies naturelles. Le dispositif compare plusieurs traitements en semis direct au labour conventionnel (bechage a l'angady). Des mesures du ruissellement, de l'erosion et du rendement des cultures principales ont ete effectuees durant quatre campagnes culturales. Les resultats ont confirme l'effet positif des systemes de semis direct pour controler le ruissellement et l'erosion. Par contre, les systemes en semis direct n'ameliorent pas automatiquement les productions. En effet, les rendements en grain de haricot sont significativement plus eleves sur les parcelles en semis direct que labourees. Alors que pour les rendements en mais et en riz pluvial, d'une maniere generale, ils sont significativement plus faibles en semis direct avec couverture vivante a base de Brachiaria ruziziensis que pour les parcelles labourees. Au fil du temps, il y a une nette augmentation des rendements en grain de riz et de mais pour l'ensemble des traitements etudies, avec une hausse encore plus visible sous systemes en semis direct. (Resume d'auteur)

International audience; The isotopic exchange kinetic (IEK) method allows assessing the rate of orthophosphate ions (Pi) exchange between the solid phase and the solution of the soil with time. However, two challenges have to be tackled when using this technique in soils with a very low water extractable Pi concentration (C-P) and with a high sorbing capacity for Pi. The first is that current colorimetric methods do not allow quantifying Pi concentrations lower than 10 mu g P L-1. While the second challenge is that a significant fractionation between P isotopes may occur in the soil solution system in the presence of soils with high Pi sorption capacity. We assessed here: i) whether concentrating the blue phosphomolybdate complex (BPMC) in hexanol prior to its measurement would allow to lower the detection and quantification limits of Pi, ii) whether a significant isotopic fractionation between (32)Pi and (33)Pi could occur during IEK experiments conducted in the presence of high Pi sorbing substrates (e.g., pure goethite or Malagasy soils) and iii) whether the IEK method when used in conjunction with the hexanol concentration of the BPMC to measure C-P would detect changes in Pi isotopic exchangeability in a ferralsol cropped with upland rice following the input of manure or water soluble fertilizer. The detection and quantification limits of the BPMC concentrated by hexanol were 0.3 and 0.8 mu g P L-1, respectively, using a cell of 10 cm length for the colorimetric measurement. The IEK conducted on Pi amended goethite and on Malagasy soils with (32)Pi and (33)Pi did not show any systematic isotopic fractionation between both isotopes, suggesting that in these soils (31)Pi and (32)Pi or (33)Pi have a similar behavior during isotopic exchange. The analysis of the soils sampled in the field experiment showed a significant increase in the amount of Pi isotopically exchangeable after 1 min only after the application of water soluble P. This increase was paralleled by increases in rice yield and P export by grains demonstrating an increased P availability in this treatment. In conclusion, the IEK method can be used in low P and high Pi sorbing soils as the hexanol concentration method allows measuring very low C-P and as the different P isotopes have a similar behavior in the soil/solution system. The IEK experiments conducted in the presence of goethite, however, point out to the necessity of taking into account the dispersion of particles for a proper interpretation of the isotopic data.

Urban planners are increasingly interested in agriculture around cities and have to decide whether to maintain or not areas of agricultural land use within and close to growing cities. There is therefore a need for researchers to design tools to guide public decision-making on land use. Various approaches, originating from different disciplines, may be adopted in this respect. We designed an interdisciplinary research program in order to test two related concepts: the "sustainability" and the "multi-functionality" of agriculture. We show that these concepts provide a useful framework for obtaining appropriate knowledge about urban agriculture, which urban planners could apply in real situations. In close collaboration with urban planners, we applied an interdisciplinary research methodology, based on common farm surveys and territorial approaches, to the Antananarivo area (Madagascar). The main functions analyzed were the food production and environmental roles of urban agriculture. Two aspects of sustainability were assessed: the farm sustainability and the territorial sustainability, with expert scores. This approach identified a wide diversity of farming systems that performed differently, depending on their intra- or suburban location. The food supply function appeared to be important not only for fresh produce but also for rice consumption. The function of protection against flooding is now important and this importance will increase with climate change. A diagnosis of sustainability was made and discussed with urban planners: several farming systems and zones were identified in which agriculture was considered important as a means of maintaining or developing the food supply, employment and incomes, and even landscape or environmental quality. We also identified other areas in which poor production conditions and/or the negative effects of urbanization on agriculture jeopardized its sustainability. This methodology appeared to be useful for determining the most appropriate role of urban agriculture in the land-use planning of this city. Our study raises new questions on the subject and should lead to more focused research programmes. We discuss several points of interest and the limitations and possible extension of this method.

La durabilité de l'agriculture urbaine d'Antananarivo (Madagascar) a été questionnée dans le cadre d'un projet de recherche pluridisciplinaire à visée opérationnelle. Six thèmes de recherche articulent quatre disciplines (agronomie, économie, géographie, chimie) autour de sites, de systèmes de production ou de produits communs. On montre que la diversité des systèmes d'activités et de production, dont on propose une typologie, est principalement liée à la distance à la ville et à l'accès quantitatif et qualitatif à l'eau. On quantifie la fonction alimentaire de cette agriculture, forte pour les produits maraîchers et non négligeable pour le riz. La lutte contre les inondations en plaine et l'érosion dans les collines bâties justifient aussi certaines décisions de maintien local de l'activité agricole, alors que les performances agricoles, quantitatives et qualitatives (risques sanitaires) sont diminuées par les effluents industriels et urbains. La durabilité intrinsèque et exogène de l'agriculture est approchée en première analyse en proposant une hiérarchie entre les systèmes de production et entre les sites. Cependant, les questions liées à la qualité sanitaire variable de certains produits agricoles issus de l'agriculture urbaine, à la segmentation croissante de certaines filières ou aux risques liés à de nouvelles fonctions de l'agriculture ont émergé dans ce projet, mais ouvrent surtout de nouveaux champs de recherche., An interdisciplinary program (ADURAA) analyzes the sustainability and the urban functions of agriculture inside and around Antananarivo. Its aim is to develop knowledge and to help public decisions about the interest, limits and conditions of the local maintenance of agriculture, compared with other land uses. Six research themes were identified based on a typology of urban farms; they contribute knowledge on three main functions of this agriculture, viz. food supply, land use and risk prevention, and the emerging role of urban waste reuse. A wide diversity of activities systems and agricultural production systems has been identified in relation to distance to the town and water availability. This urban agriculture significantly contributes to the vegetables supply of the district as well as to its rice supply. Flooding in the lowland and erosion in the hills are also e ciently contained by agricultural land use, but anarchic building and the use of agricultural soil for brick making largely counteract this role. Uncontrolled discharge of industrial liquid wastes into the irrigation network greatly reduces rice production and has even led to the desertion of some intra-urban rice-growing areas, while domestic pollution strongly a ects the sanitary quality of intra-urban vegetables. However a positive externality could be developed by using some urban solid waste as fertilisers for the growing horticultural sector. Emerging questions such as the quality of urban produce and potential new risks open up new avenues for future research on this agriculture.