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The study had as objective to investigate the land use/land cover change from 1980 to 2019 in the Dja Biosphere Reserve using GIS and remote sensing techniques and the evolution of agroforestry practices. Semi structured questionnaires were administered to 320 farmers who were randomly selected from 35 villages of the Dja Biosphere Reserve characterized by the presence of agroforestry systems. The land use map revealed six classes which were dense humid forest, perennial crop-based agroforestry practices, river, wetland, built up and bare areas. Between 1980 and 2008, dense humid forest lost -4.9% of its area to the benefits of perennial crop-based agroforestry practices, built up and bare land. Between 2008 and 2019, dense humid forest gain 1.77% of its area due to increase in perennial crop-based agroforestry practices and a subsequent increase in vegetation cover. Perennial crop-based agroforestry practices and built up increase progressively from 1980 to 2019. Farmers perceived hunting (36.3%), slash and burn agriculture (43.3%) and harvesting of tree-based products (20.3%) as the anthropogenic activities impacting the reserve negatively. The evolution of agroforestry practices were observed on the field, but the precise area under agroforestry practices in the study area need to be estimated. As a mitigation strategy to livelihood needs as well as the rehabilitation of degraded land, the conversion of pure cultivated agricultural land into agroforestry is a major opportunity.

International audience; Conventional, intensively managed coffee plantations are currently facing environmental challenges. The use of shade trees and the organic management of coffee crops are welcome alternatives, aiming to reduce synthetic inputs and restore soil biological balance. However, little is known about the impacts of the different types of shade tree species on soil functioning and fauna. In this paper, we assess soil nutrient availability and food web structure on a 17-year old experimental coffee plantation in Turrialba in Costa Rica. Three shade types (unshaded coffee, shaded with Terminalia amazonia, and shaded with Erythrina poepiggiana) combined with two management practices (organic and conventional) were evaluated. Total C and N, inorganic N and Olsen P content, soil pH, global soil fertility, and nematode and microarthropod communities were measured in the top 10 cm soil layer, with the objective of determining how shade tree species impact the soil food web and soil C, N and P cycling under different types of management. We noted a decrease in soil inorganic N content and nematode density under conventional management (respectively -47% and -91% compared to organic management), which suggested an important biological imbalance, possibly caused by the lack of organic amendment. Under conventional management, soil nutrient availability and fauna densities were higher under shade, regardless of the shade tree species. Under organic management, only soils under E. poeppigiana, a heavily pruned. N-2 -fixing species, had increased nutrient availability and fauna density, while T amazonia shade had a null or negative impact. The effects of coffee management and shade type on soil nutrient availability were mirrored by changes in soil food web structure. Higher fertility was recorded in soil with balanced food webs. These results emphasize the importance of the choice of shade tree species for soil functions in low input systems, more so than in fertilized systems

International audience; Nutrient cycling in cocoa agroforestry systems (cAFS) is complex and poorly understood. To better understand the mass flow of carbon and nutrients into the soil sub-system under various contexts we quantified the mass flow of litterfall, its composition and seasonal variations in different agroforestry systems in Bokito (Central Cameroon). We studied litterfall dynamics and in situ leaf-litter cycling of cAFS established on past forest lands (F-cAFS) and savannah (S-cAFS). We also studied the decomposition of cocoa and associated tree leaf-litter in litterbags. Local secondary semi-deciduous forests were included as control. Annual litterfall in full-grown cAFS (> 15 years old) was high (9.4 Mg ha−1 y−1) and represented ca. 67% of litterfall in control forests. In full-grown cAFS, associated tree leaf-litter contributed to litterfall the most and ranged between 60 and 70% of the total amount recorded (6.3 Mg ha−1 y−1). The quantities and dynamics of the litter components monitored were similar in full-grown S- and F-cAFS. The microclimate was best buffered in forests and least buffered in young S-cAFS but could not be linked to leaf-litter decomposition. Forest leaf litterfall was higher and tended to cycle faster than total leaf-litter of cAFS, whose decomposition appeared limited by cocoa leaf-litter quality. Our study underlines (i) the critical contribution of associated trees to the nutrient cycle of agroecosystems established on poor soils and, (ii) the ability of farmers to channel associated tree communities towards similar functioning despite different past land-uses.

Many studies have shown that introducing N

International audience; Manipulating plant functional diversity to improve agroecosystem multifunctionality is a central challenge of agricultural systems world-wide. In cocoa agroforestry systems (cAFS), shade trees are used to supply many services to farmers, yet their impact on soil functioning and cocoa yields is likely to vary substantially among tree species. Here we compared the impact of five shade tree species (Canarium schweinfurthii (Canarium), Dacryodes edulis (Safou), Milicia excelsa (Iroko), Ceiba pentandra (Kapok tree), Albizia adianthifolia (Albizia)) and unshaded conditions on the functioning of poor sandy savanna soils within eight cocoa farms in Central Cameroon. We assessed the effects of plant functional traits, leaf litterfall and fine root biomass on a range of soil functions and on cocoa yield. Shade trees generally improved soil pH, NH4+, NO3- and Olsen P content, biomass production of bioassays and soil total C and N content, while leaving cocoa yields unchanged. However, these effects varied largely among species. Improvements of soil functions were low under the two fruit trees (Canarium and Dacryodes), medium under the legume tree Albizia and high under the two timber trees (Milicia and Ceiba). Low litter recalcitrance was most strongly associated with increases in soil fertility indicators such as N and P availability, whereas soil C and N content increased with litter Ca restitution. Synthesis and applications. We demonstrate that cocoa agroforest multifunctionality is substantially influenced by the functional traits of shade tree species. Shade tree species with the most dissimilar traits to cocoa (cocoa showing the lowest leaf litter quality) showed the largest improvement of soil functions. Therefore, selection of shade trees based on their functional traits appears as a promising practice to adequately manage soil functioning. In order to fully assess the beneficial role of shade trees in these agroecosystems. Future research will need to extend this approach to other below-ground traits and other aspects of multifunctionality such as long-term cocoa health and yield.

Rotational woodlots with N2-fixing trees may be efficient agroforestry systems, allowing farmers to alternate agricultural and wood-energy production on the same area. However, their long-term effect on soil fertility is poorly understood. The aim of this study was to investigate the influence of successive phases of Acacia auriculiformis stands growing in rotation with crops on the chemical properties of sandy and very poor tropical soils. The study was conducted 22 years after the afforestation of humid herbaceous savannah in Mampu, Democratic Republic of Congo. The chemical properties of top soil (0–20 cm) from control savannah plots were compared with those from acacia plots that had undergone one, two or three rotations of acacia during the 22 year period. We found that the soil properties under non-harvested acacia stands in a 1st rotation and under acacia stands in a 2nd or 3rd rotation following charcoal production and maize and cassava cultivation were similar. Soils under all acacia stands had higher C, N and $${\text{NO}}_{3}^{ - }$$ –N contents, but were more acidic, and had lower contents of exchangeable Ca, Mg, K and Na than the control savannah soils. Despite the increase in soil C and N, the sustainability of the acacia rotational agroforestry system after 22 years of practice is in question due to the steady decrease of soil cations, soil acidification and the risk of a decline in tree and crop productivity. To improve the nutrient balance and the sustainability of this system, different practices are recommended such as the debarking of tree stems before carbonization, the restitution of small branches and charcoal residues to the soil, and the supply of natural rock phosphate.

International audience; 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.Highlights:• We compared plant diversification impacts on soil fertility across 6 agroecosystems.• Benefits of plant diversification depended on both legume presence and soil type.• Legume-based diversification was mostly beneficial on the younger soils.• Ferralsols' fertility was improved with plant communities' litter P content.• Plant communities' traits were linked with diversification impacts on contrasting soils.

Respectueux de l'environnement et garantissant une securite alimentaire soutenue par la diversification des productions et des revenus qu'ils procurent, les systemes agroforestiers apparaissent comme un modele prometteur d'agriculture durable dans les pays du Sud les plus vulnerables aux changements globaux. Cependant, ces systemes agroforestiers ne peuvent etre optimises qu'a condition de mieux comprendre et de mieux maitriser les facteurs de leurs productions. L'ouvrage presente un ensemble de connaissances recentes sur les mecanismes biophysiques et socio-economiques qui sous-tendent le fonctionnement et la dynamique des systemes agroforestiers. Il concerne, d'une part les systemes agroforestiers a base de cultures perennes, telles que cacaoyers et cafeiers, de regions tropicales humides en Amerique du Sud, en Afrique de l'Est et du Centre, d'autre part les parcs arbores et arbustifs a base de cultures vivrieres, principalement de cereales, de la region semi-aride subsaharienne d'Afrique de l'Ouest. Il synthetise les dernieres avancees acquises grâce a plusieurs projets associant le Cirad, l'IRD et leurs partenaires du Sud qui ont ete conduits entre 2012 et 2016 dans ces regions. L'ensemble de ces projets s'articulent autour des dynamiques des systemes agroforestiers et des compromis entre les services de production et les autres services socio-ecosystemiques que ces systemes fournissent.

To produce charcoal and prevent the deforestation around Kinshasa (Gond et al., 2017), 7,700 ha of Acacia auriculiformis were planted on savannah ecosystems, in 1987 (Bateke Plateau, DR Congo). Since 1995, the plantation was managed using the rotational woodlot system alternating agricultural and charcoal production on the same area (Kimaro et al., 2007). The 7,700 ha produced a large amounts of charcoal, cassava and maize during many years (Bisiaux et al., 2009). However, farmers have observed for a while a decline in wood and crop productivity. The aim of this study was to compare chemical properties of soils in six acacia stands in two farms having undergone different agroforestry trajectories: - one 22-year-old acacia stand, never-harvested; 4 stands in their 2nd rotation after 1 cropping cycle; and 1 stand in its 3rd rotation after 2 cropping cycles - and soils in the native control savannah. Compared to the original savannah, all acacia stands showed an increase in soil C, N and N-NO3- contents, but a decline in soil pH and exchangeable cations, and an increase in exchangeable Al and CEC (Fig 1). To maintain the sustainability of the system, we recommend different practices in order to improve the nutrient balance and decrease the soil acidity. Such practices are the debarking of tree stems before carbonization, the restitution of small branches and charcoal residues to the soil, and the supply of natural rock phosphate (Dubiez et al., 2018).

Correction to Carbon dynamics in cocoa agroforestry systems in Central Cameroon: afforestation of savannah as a sequestration opportunity (vol 93, pg 851, 2019) WOS:000467900200009; International audience; Afforestation of savannah is suggested as an approach to help addressing climate change mitigation through increased carbon (C) storage. Previous studies in Central Cameroon evidenced farmers’ ability to realize afforestation by establishing cocoabased agroforestry systems (cAFS) on humid savannah. In this forest-savannah transition zone, we studied an 80 years chronosequence of cAFS to assess C dynamics. We selected cAFS established after forest or savannah, and we used local forest and savannah patches as controls. Aboveground carbon (AGC) was highest in the forests (118 Mg C ha-1) and lowest in the savannahs (8 Mg C ha-1). Systems established after forest (F-cAFS) revealed a mean AGC 40% lower than that of forests and did not evolve with time. The AGC of cAFS established after savannah (ScAFS) increased with time and reached the mean AGC of F-cAFS (72 Mg C ha-1) after ca. 75 years. Soil organic carbon (SOC) concentration depended on clay content (R2 = 0.55, P\0.001). The SOC concentration of F-cAFS did not evolve with time and revealed no difference with forest. In S-cAFS, considering a time of about 80 years after afforestation, the average annual increase in SOC concentration in the 0–15 cm layer ranged from 7.3%in soils with low clay content (10–15%) (R2 = 0.60, P\0.01) to 9.5%in soils with higher clay content (20–25%). No significant change in SOC concentration was found for the 15–30 cm layer. Overall, S-cAFS revealed to store and maintain significant levels of C both in the aboveground biomass and in the soil. Such an afforestation thus appeared as a valuable local strategy to combine cocoa and other perennial plant productions with C storage while avoiding deforestation.

Huit ans de travaux de recherche sur les services écosystémiques dans une grande ferme caféière du Costa Rica (observatoire collaboratif Coffee-Flux, en système agroforestier à base de caféiers sous de grands arbres d'Erythrina poeppigiana, surface projetée de couronne de l'ordre de 16 %) ont suggéré plusieurs applications pour les agriculteurs et les décideurs. Il est apparu que de nombreux services écosystémiques dépendaient des propriétés du sol (ici des Andisols), en particulier de l'érosion, de l'infiltration, de la capacité de stockage de l'eau et des éléments nutritifs. Nous confirmons qu'il est essentiel de lier les services hydrologiques et de conservation au type de sol en présence. Une densité adéquate d'arbres d'ombrage (plutôt faible ici) permet de réduire la sévérité des maladies foliaires avec, en perspective, une réduction de l'usage de pesticides-fongicides. Un simple inventaire de la surface basale au collet des caféiers permet d'estimer la biomasse souterraine et la moyenne d'âge d'une plantation de caféiers, ce qui permet d'évaluer sa valeur marchande ou de planifier son remplacement. Le protocole de calcul actuel pour la neutralité carbone des systèmes agroforestiers ne prend en compte que les arbres d'ombrage, pas la culture intercalaire. Dans la réalité, si on inclut les caféiers, on se rapproche très probablement de la neutralité. Des évaluations plus complètes, incluant les arbres, les caféiers, la litière, le sol et les racines dans le bilan en carbone du système agroforestier sont proposées. Les arbres d'ombrage offrent de nombreux servies écosystémiques s'ils sont gérés de manière adéquate dans le contexte local. Par rapport aux parcelles en plein soleil, nous montrons qu'ils réduisent l'érosion laminaire d'un facteur 2, augmentent la fixation de l'azote (N2) atmosphérique et le pourcentage d'azote recyclé dans le système, réduisant ainsi les besoins en engrais. Ils réduisent aussi la sévérité des maladies foliaires, augmentent la séquestration de carbone, améliorent le microclimat et atténuent substantiellement les effets des changements climatiques. Dans notre étude de cas, aucun effet négatif sur le rendement n'a été enregistré., Eight years of studying coffee ecophysiology and monitoring ecosystem services (ES) in a large coffee farm in Costa Rica revealed several practical recommendations for farmers and policy makers. The cropping system studied within our collaborative observatory (Coffee-Flux) corresponds to a coffee-based agroforestry system (AFS) under the shade of large trees of Erythrina poeppigiana (16 % of canopy cover). A lot of ES and disservices depend on local soil properties (here Andisols), especially erosion/infiltration, water/carbon and nutrient storage capacity. Therefore, for ES assessment, the type of soil is crucial. An adequate density of shade trees (rather low here) reduced the severity of leaf diseases with the prospect of reducing pesticide-fungicide use. A simple inventory of the basal area at collar of the coffee plants allowed estimating the belowground biomass and the average age of the plantation, to judge of its market value and to decide when to replace it. Coffee farms are probably much closer to C neutrality than predicted by the current C-Neutral protocol, which only considers shade trees. More comprehensive assessments, including trees, coffee, litter, soil, and roots in the C balance of the AFS are proposed. Shade trees offer many ES if they are adequately managed in the local context. As compared to full sun conditions, shade trees may (i) reduce laminar erosion by a factor of 2, (ii) increase N2 fixation and the % of N recycled into the system, thus reducing fertilizer requirements, (iii) reduce the severity of leaf diseases, (iv) increase C sequestration, (v) improve the microclimate, and (vi) substantially reduce the effects of climate change. In our case study, no negative effect on coffee yield was found.

Recent studies based on remote sensing showed a gradual expansion of tree cover over savannah and agricultural land in the forest-savannah transition zone of Cameroon1, part of this expansion is actually due to shaded cocoa. Despite unfavourable conditions in herbaceous savannah (low soil fertility, weed competition and risk of bush fire), farmers have proven that afforestation is achievable using cocoa and specific technics to build up an associated tree canopy2. Full-grown cocoa agroforestry systems created on savannah (S-cAFS) and in forest (F-cAFS) seem to exhibit comparable multi-strata structure. Nevertheless, previous land uses and related canopy structures may have contrasted impacts on production and other ecosystem services over time. We selected 1 to 70 year-old S-cAFS and F-cAFS, and we used forest and savannah patches as controls3. By combining measurements of cocoa production, litter fall and cycling, soil quality, carbon storage and tree species diversity along this age gradient, we showed that those variables in S- and F-cAFS generally tended to comparable levels after several decades. Results also emphasized the ability of S-cAFS to increase most of the ecosystem services (ES) although the time needed to reach levels found in F-cAFS varied strongly amongst variables (Fig 1). Results also showed the positive contribution of associated plants to ES, particularly C storage and nutrient cycling contributing to REDD+ 4 and sustainability of the cropping system.

International audience; Cocoa agroforestry systems (cAFS) in Central Cameroon are established on lands which were either forest or savannah. The functioning and ecosystem services (ES) delivery of an agroecosystem can be influenced by past land-use. We hypothesised that savannah-derived cocoa agroforestry systems (S-cAFS) and forest-derived cocoa agroforestry systems (F-cAFS) would (i) progressively drift away from past land-use, and (ii) eventually converge and support comparable levels of ecosystem services. We selected 25 ecosystem attributes directly related to at least one of the following six ecosystem (dis)services (ES): species conservation, carbon storage, crop production, nutrient cycling, soil quality and soil pollution. We followed their temporal evolution in S- and F-cAFS along >70-year chronosequences. Our results showed that the attributes and services studied followed typical temporal trajectories in S- and F-cAFS while generally tending to reach comparable levels on the long run. However, the time needed to do so varied strongly and ranged from 20 to 30 years for perennial species diversity to more than 70 years for C storage or some components of soil quality. The results also demonstrated that S-cAFS could sustainably improve many of the studied attributes and ES. Regarding the attributes related to the cocoa stand, both S- and F-cAFS seemed influenced by their previous land-use up until 15 and 30 years, respectively, after their establishment. With respect to soil quality, nutrient cycling and carbon storage, only S-cAFS could be significantly distinguished from their past land-use, after 15 to 30 years.

Abst ract Rubber tree plantations ( Hevea brasiliensis ) are expanding into marginal areas with low soil fertility and long dry seasons with a high risk of soil erosion and drought damage to trees. Introducing an N 2 -fixing legume cover crop in rubber plantations may reduce runoff and soil erosion as well as increasing the availability of nutrients but may also increase competition for water. This study quantified the effect of the legume cover crop Pueraria phaseoloides on N, P and K nutrition, water status and growth of young rubber trees (three years old in 2007) over a four year period (2007–2010). The plantation was located on a toposequence with a range of soil depths and water storage capacities in northeast Thailand. The legume aboveground biomass production and its nutrient content and decomposition rate were measured and the N 2 fixation was estimated using the abundance of 15 N (δ 15 N) in the legume. Measurements were taken of the tree stem girth and height and tree leaf predawn water potential, nutrient content and greenness. The transfer of N 2 fixed by the cover crop to the trees was estimated using δ 15 N in the tree leaves. The annual biomass production of the legume was 8 Mg ha −1 year −1 and the N accumulation by the legume was 250 kg N ha −1 year −1 . The natural abundance method applied to the aboveground components of the legume gave N 2 fixation rates varying from 85 to 93% depending on the year. The leaf δ 15 N was similar in the three non-legumes ( H. brasiliensis , Vetiveria zizanioides and Praxelis clematidea ) used as reference plants for estimating the N 2 fixation. The higher level of N and the much lower leaf δ 15 N values for the rubber trees intercropped with P. phaseoloides , compared to rubber trees growing without a legume cover crop, showed that there was a relatively high transfer of fixed N from the legume to the trees, varying from 39% to 46% of tree leaf N depending on the year. Neither N 2 fixation nor N transfer varied significantly along the toposequence. At the bottom of the toposequence, both the nutrient (N, P and K) and water status of trees was significantly improved with the legume cover crop, doubling the tree girth at seven years of age (tree girth: 28 cm, tree height: 700 cm). However, at the top of the toposequence with low water storage capacity, the legume cover crop improved tree nutrition and growth but reduced the trees' ability to survive intense drought. These results raise concern about the resilience to drought of the rubber tree/ P. phaseoloides system, since the positive effect of the legume on rubber tree nutrition and growth may increase the risk of water stress and tree mortality. With future changes in climate, an increasing number of areas will be concerned by the question of optimizing the tradeoff between N inputs and water availability.

Made available in DSpace on 2020-12-12T01:29:52Z (GMT). No. of bitstreams: 0 Previous issue date: 2020-11-10 Silpakorn University Ministry of Science and Technology of Thailand Agence Nationale de la Recherche Many studies have shown that introducing N2-fixing trees (e.g. Acacia mangium) in eucalypt plantations can increase soil N availability as a result of biological N2 fixation and faster N cycling. Some studies have also shown improved eucalypt P nutrition. However, the effects of N2-fixing trees on P cycling in tropical soils remain poorly understood and site-dependent. Our study aimed to assess the effects of planting A. mangium trees in areas managed over several decades with eucalypt plantations on soil organic P (Po) forms and low molecular weight organic acids (LMWOAs). Soil samples were collected from two tropical sites, one in Brazil and one in the Congo. Five different treatments were sampled at each site: monospecific acacia, monospecific eucalypt, below acacias in mixed-species, below eucalypts in mixed-species as well as native vegetation. Po forms and LMWOAs were identified in sodium hydroxide soil extracts using ion chromatography and relationships between these data and available P were determined. At both sites, the concentrations of most Po forms and LMWOAs were different between native ecosystems and monospecific eucalypt and acacia plots. Also, patterns of Po and LMWOAs were clearly separated, with glucose-6-P found mainly under acacia and phytate and oxalate mainly under eucalypt. Despite the strongest changes occurred at site with a higher N2 fixation and root development, acacia introduction was able to change the profile of organic P and LMWOAs in

International audience; Most of the C and N models published over past decades are based on parameters not always linked to the environment and underestimate the role of microorganisms. They are often over-parameterized, which can give multiple solutions for flow calculations between state variables. This work proposes a modelling method centred on the functioning of living organisms in order to calculate flow parameters using data on N stocks in decomposers, plant organs, symbiotic microorganisms, and the soil compartments. The model was settled via a complex N fixing and intercropping system of durum wheat/faba bean compared to the cropping of pure durum wheat and pure faba bean, all in the context of organic farming invaded by weeds and weeded by hand just before flowering. To avoid perturbation of natural exchanges of C and N, no fertilizer was added from 1997 to 2011. The equation system defined for the association of any number of plants, as well as parameters previously published for C-flow calculations were used, and only a few parameters specific to N flows were added, and are discussed. The results showed the strong link between N and C in the environment. The calculations converge toward an unique set of solutions that is consistent with literature data when available. The labile organic N of microbial origin was modelled as the main potentially available stock. Living microorganisms stored about 1% of total N, which was close to the N stock in faba bean and four times more than stock in durum wheat. Inorganic N was immobilized before flowering in competition with N requirement of durum wheat roots. Net N mineralization, mainly from decomposition of faba bean roots, started too late to improve wheat production. During the cropping period, weeds accounted for losses of 20 kg N ha−1, while the atmospheric N2 fixation of 90 kg N ha−1 was close to the total microbial immobilization. The model associating microbial and plant flows of C and N in complex plant covers, appears as a robust tool to quantify the exchanges of the earth organisms with the soil and atmosphere. It enables to propose essential recommendations to improve as well agro-ecology as predictions of global changes of C and N stocks.

Differentes jacheres arborees plantees d'Acacia polyacantha, de Senna siamea ou d'Eucalyptus camaldulensis et une jachere naturelle herbacee, ont ete installees en comparaison, sur un sol appauvri en matiere organique et en nutriments, dans la region de Garoua au Cameroun (1050 mm de pluviosite). L'objectif etait d'evaluer leurs effets sur la dynamique du carbone, les cycles des nutriments, les proprietes du sol et le rendement des cultures annuelles apres jachere. Par rapport a la jachere herbacee, les jacheres arborees de 7 ans ont augmente considerablement l'accumulation de matiere organique et de nutriments dans la biomasse aerienne et souterraine et aussi le prelevement des nutriments de la profondeur du sol. Ces processus ont permis, lors de l'exploitation des jacheres arborees et du brulis des residus, d'ameliorer les caracteristiques du complexe absorbant (somme des cations echangeables, en particulier Ca) dans l'horizon de surface (0-10 cm) des jacheres ligneuses par rapport a la jachere herbacee. Par la fixation de l'azote et son recyclage, l'espece fixatrice d'azote, A. polyacantha, a augmente considerablement le bilan d'azote du systeme. L'espece a accumule dans son systeme racinaire autant d'azote qu 'E. camaldulensis et S. siamea dans leur biomasse totale et leur litiere. En outre, seul A. polyacantha a montre une augmentation significative des stocks de carbone et d'azote du sol (couche 0-20 cm) apres 6 ans d'âge. Le stock d'azote plus eleve et facilement mineralisable et l'accumulation de matiere organique du sol dans la jachere a A. polyacantha, ont ete associes a un triplement du rendement du mais en premiere annee de remise en culture par rapport aux precedents, et un doublement en deuxieme annee. Cette augmentation tres importante des rendements des cultures apres jachere n'a pas ete observee apres les autres precedents arbores. Ces resultats soulignent l'importance des compartiments organiques et le potentiel des arbres fixateurs d'azote dans la restauration de la fertilite des sols degrades. (Resume d'auteur)

Pour les chercheurs en biologie, les légumineuses arborées présentent de nombreuses qualités pour l'agronomie, en particulier la capacité à fixer l'azote atmosphérique. Celle-ci permet la production de parties végétales riches en matière azotée qui, lorsqu'elles se décomposent dans le sol, sont supposées en améliorer la " fertilité ". D'autre part, on met souvent en avant leur " richesse " pour la nutrition des hommes et des animaux et la qualité de leur bois. Les spécialistes du reboisement soulignent leur capacité de multiplication sexuée et végétative ainsi que leur aptitude à se développer en milieu peu fertile et ouvert. Les agro-sylvo-pastoralistes travaillant en terrains tropicaux constatent bien ces vérités scientifiques, parfois devenues de véritables paradigmes, mais en éprouvent au quotidien les limites. Il est bien connu que toutes les légumineuses ne fixent pas l'azote et aussi que cette fixation dépend des espèces, de leur association rhizobienne et des conditions environnementales, mais ceci n'est pas toujours dit dans les messages destinés à la " vulgarisation ". Si des transferts " directs " d'azote entre plantes ont été mis en évidence, l'importance du transfert d'azote et d'autres nutriments au bénéfice de plantes associées ou en rotation dépend du recyclage des organes des plantes fixatrices. A l'inverse, la récolte de leurs bois, fruits ou feuilles, peut aboutir à un appauvrissement de la fertilité du sol qui les porte. Leur formidable capacité à se multiplier et à s'ancrer durablement dans un milieu, en particulier lorsqu'il est dégradé, a aussi son " revers de médaille ", à savoir qu'elles peuvent souvent devenir envahissantes. Par ailleurs, les écologistes dogmatiques regretteront que des légumineuses provenant d'autres continents soient maintenant présentes dans la plupart des forêts. D'autres, plus pragmatiques, y verront l'émergence de " forêts nouvelles " dont il faut étudier l'écosystème mais aussi valoriser les biens et services, comme le font souvent les paysans. Il reste cependant que ces légumineuses arborées occupent une place de choix dans la palette des possibilités disponibles pour le praticien en charge de la restauration de milieux dégradés ou de la création de systèmes agro-sylvo-pastoraux productifs et résilients face aux changements globaux, à condition de manier avec prudence la lancinante tentation de l'introduction de la " plante miracle " qui peut parfois, si on n'y prend pas garde, devenir une " plante cauchemar ".

Sustainable wood production requires appropriate management of commercial forest plantations. Establishment of industrial eucalypt plantations on poor sandy soils leads to a high loss of nutrients including nitrogen (N) after wood harvesting. An ecological intensification of eucalypt plantations was tested with the replacement of half of the Eucalyptus urophylla × E. grandis by Acacia mangium in the eucalypt monoculture to sustain soil fertility through enhancement of the N biological cycle. A randomised block design was set up on ferralitic arenosol in the Congolese coastal plains to assess differences in soil N mineralisation, N fluxes in litterfall, and N stocks in forest floor litter and soil between pure acacia (100A), pure eucalypt (100E) and mixed-species treatments (50A50E). Soil N mineralisation was enhanced under acacia, reaching on average 0.17 and 0.15 mg kg−1 soil d−1 in 100A and 50A50E, respectively, compared with 0.09 mg kg−1 soil d−1 in 100E. Higher amounts of N returning to the soil through harvest residues and litterfall were observed under acacia than under eucalypt. However, N stock in mineral soil was not increased in 100A and exhibited a limited increase only in the top soil layer of 50A50E. Our results suggest a much faster N turnover under acacia than under eucalypt. Although A. mangium is an exotic N2-fixing tree in central Africa, it appears to be well adapted to the climatic and edaphic conditions of the Congo, showing an efficient growth strategy. Eucalypt trees could benefit from the increase in soil N availability in mixed-species stands.Keywords: ammonium, arenosol, Congo, ecological intensification, forest plantation, mixed species, N2 fixation, nitrate

To investigate the effects of microbial inocula and Erythrina poeppigiana pruning residues on soil K, NO3-, and NH4? concentrations, a greenhouse trial, a field experiment in an organic farm, and three in vitro tests were conducted. Under controlled conditions, weak, temporary effects (10 %) on maize seedling growth were observed on poor soils (taken from the 10-20 cm layer) in the first 2 weeks after application. Positive effects of pruning residue applications on soil K levels (0.09 cmol kg-1, on average) were detected in both the field and greenhouse study. However, significant effects due to the addition of microbial inocula on soil K concentrations were not detected in the field; thus, microbial applications were ineffective at enhancing nutrient availability under field conditions. In contrast, in the in vitro experiments, CO2 production was 31 % greater than that of untreated soil on the 8th and 15th days of incubation. These results highlight the importance of adding tree pruning residues to support coffee-plant nutrition. Experimental outcome data could be valuable for further studies focused on microbial application dosage and timing.

Considerable amounts of nitrogen (N) and phosphorus (P) fertilizers have been mis-used in agroecosystems, with profound alteration to the biogeochemical cycles of these two major nutrients. To reduce excess fertilizer use, plant-mediated nutrient supply through N-2-fixation, transfer of fixed N and mobilization of soil P may be important processes for the nutrient economy of low-input tree-based intercropping systems. In this study, we quantified plant performance, P acquisition and belowground N transfer from the N-2-fixing tree to the cereal crop under varying root contact intensity and P supplies. We cultivated Acacia senegal var senegal in pot-culture containing 90% sand and 10% vermiculite under 3 levels of exponentially supplied P. Acacia plants were then intercropped with durum wheat (Triticum turgidum durum) in the same pots with variable levels of adsorbed P or transplanted and intercropped with durum wheat in rhizoboxes excluding direct root contact on P-poor red Mediterranean soils. In pot-culture, wheat biomass and P content increased in relation to the P gradient. Strong isotopic evidence of belowground N transfer, based on the isotopic signature (delta N-15) of tree foliage and wheat shoots, was systematically found under high Pin pot-culture, with an average N transfer value of 14.0% of wheat total N after 21 days of contact between the two species. In the rhizoboxes, we observed limitations on growth and P uptake of intercropped wheat due to competitive effects on soil resources and minimal evidence of belowground N transfer of N from acacia to wheat. In this intercrop, specifically in pot-culture, facilitation for N transfer from the legume tree to the crop showed to be effective especially when crop N uptake was increased (or stimulated) as occurred under high P conditions and when competition was low. Understanding these processes is important to the nutrient economy and appropriate management of legume-based agroforestry systems. (c) 2012 Elsevier B.V. All rights reserved.

There remains conflicting evidence on the relationship between P supply and biological N(2)-fixation rates, particularly N(2)-fixing plant adaptive strategies under P limitation. This is important, as edaphic conditions inherent to many economically and ecologically important semi-arid leguminous tree species, such as Acacia senegal, are P deficient. Our research objective was to verify N acquisition strategies under phosphorus limitations using isotopic techniques. Acacia senegal var. senegal was cultivated in sand culture with three levels of exponentially supplied phosphorus [low (200 μmol of P seedling(-1) over 12 weeks), mid (400 μmol) and high (600 μmol)] to achieve steady-state nutrition over the growth period. Uniform additions of N were also supplied. Plant growth and nutrition were evaluated. Seedlings exhibited significantly greater total biomass under high P supply compared to low P supply. Both P and N content significantly increased with increasing P supply. Similarly, N derived from solution increased with elevated P availability. However, both the number of nodules and the N derived from atmosphere, determined by the (15)N natural abundance method, did not increase along the P gradient. Phosphorus stimulated growth and increased mineral N uptake from solution without affecting the amount of N derived from the atmosphere. We conclude that, under non-limiting N conditions, A. senegal N acquisition strategies change with P supply, with less reliance on N(2)-fixation when the rhizosphere achieves a sufficient N uptake zone.

Coffee is widely grown across Central America at altitudes between 600 and 2500 m, mostly in association with trees that provide shade and other services. Research on coffee agroforestry systems has identified many environmental factors, management strategies and plant characteristics that affect growth, yield and environmental impact of the system. Much of this literature only presents qualitative estimates of the importance of the different growth determining factors, or highly site-specific estimates. Quantitative information is required to allow statistical analysis or the construction of process-based models of the system. Here, we review the available quantitative information for the latter purpose, with emphasis on the data needs for modelling agroforestry systems common in Central America. Process-based models require environmental data-weather, soil-and data on the physiological characteristics of the coffee plants and trees. Our review showed that the current literature is insufficient to allow full parameterisation of a process-based model for any coffee-tree combination. Information on weather, coffee and trees is highly limited, but soil information seems more adequate. A regional network of replicated multi-factorial experiments, focusing on the interactive effects of different environmental factors, may help address the main knowledge gaps.

The published on-line ms “Carbon dynamics of cocoa agroforestry systems in Central Cameroon: afforestation of savannah as a sequestration opportunity.”

International audience; The objective of this study was to evaluate the effect of N fertilization and the presence of N2 fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N2O), carbon dioxide (CO2) and methane (CH4), related soil parameters (temperature, water-filled pore space, mineral nitrogen content, N mineralization potential) and litterfall in two highly fertilized (250 kg N ha−1 year−1) coffee cultivation: a monoculture (CM) and a culture shaded by the N2 fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N2O emissions with 84% of the annual N2O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH4 uptakes. The higher annual N2O emissions from the shaded plantation (5.8 ± 0.3 kg N ha−1 year−1) when compared to that from the monoculture (4.3 ± 0.1 kg N ha−1 year−1) was related to the higher N input through litterfall (246 ± 16 kg N ha−1 year−1) and higher potential soil N mineralization rate (3.7 ± 0.2 mg N kg−1 d.w. d−1) in the shaded cultivation when compared to the monoculture (153 ± 6.8 kg N ha−1 year−1 and 2.2 ± 0.2 mg N kg−1 d.w. d−1). This confirms that the presence of N2 fixing shade trees can increase N2O emissions. Annual CO2 and CH4 fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO2 ha−1 year−1, −1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH4 ha−1 year−1, respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season.

Agroforestry should be a major climate-smart agriculture option as it combines sustainable production with adaptation and mitigation of climate change. In recent decades, cocoa and coffee cultivation have been responsible for the loss of more than 30 million ha of primary and secondary forests, and thus for increased greenhouse gas emissions. However, they also have a substantial mitigation potential via the 20 million ha currently in production, only part of which is managed under agroforestry. These agroforestry plantations are more stable over time and resilient against climate change and price volatility of agricultural products, by combining ecological services with diversified production. This chapter illustrates these features through research results obtained on three continents and proposes recommendations on the management of these systems and on public policies—from the farm to the territory.

International audience; L'agroforesterie compte parmi les pratiques traditionnelles d'exploitation des sols existant depuis des siècles sur tous les continents et de très nombreux agriculteurs des pays du Sud en vivent. Aujourd'hui l'agroforesterie constitue une des solutions à mettre en oeuvre pour utiliser durablement les ressources naturelles limitées, préserver les zones de forêt tropicales et faire face au changement climatique et à la globalisation des marchés. L'objet de cette présentation est d'illustrer, à travers différents exemples de pratiques agroforestières des zones tropicales humides et sèches, l'impact des arbres en rotation ou en association permanente, sur les productions vivrières ou marchandes (service d'approvisionnement) et d'autres services écosystémiques (production primaire, cycle des nutriments et séquestration du carbone). Dans ces exemples, ont été analysés les processus agro-écologiques relatifs aux interactions entre espèces/strates pour l'utilisation des ressources et au fonctionnement biogéochimique du système, ainsi que leurs effets sur les services. Selon les lieux, les contextes et les espèces utilisées, les arbres peuvent avoir un impact positif sur le rendement des cultures associées (facilitation). Ils peuvent au contraire avoir un impact négatif (compétition pour les ressources), qui sera acceptable dans la mesure où il sera compensé par une diversification ou une meilleure qualité des productions et une plus grande résilience du système. En stimulant la production primaire et en faisant varier l'allocation relative du carbone et des ressources vers les différents puits que sont les produits récoltés, la biomasse permanente et la litière, l'agroforesterie influe sur l'ensemble des services écosystémiques. Le rôle bénéfique des légumineuses arborées sur le cycle de l'azote et du carbone et la productivité des cultures en association ou en rotation a été mis en évidence. Cependant, les bénéfices tirés des pratiques agroforestières mises au point en zone tropicale sont encore insuffisamment étudiés et valorisés. Ainsi, l'impact économique des systèmes au niveau local et national, les facteurs écologiques à l'origine des gains de productivité, la fourniture de services écosystémiques, la sélection d'espèces et de variétés adaptées aux contextes agroforestiers, l'adaptation des systèmes face aux changements climatiques sont autant de champs d'investigation à poursuivre.

One of the strategies farmers can adopt to reduce their vulnerability to coffee price volatility while preserving natural resources is to conquer new markets and produce eco-certified products. Does any other economically viable strategy exist to increase the income of coffee farmers? The presence of shade trees contributes significantly to the economic sustainability of coffee farms in Central America, mainly through the diversification of revenues: either through governmental subsidies and/or through the sale of tree products. The production and the sale of timber or fuel wood are an almost inevitable choice for coffee farmers in Central American regions at low to medium altitudes, where coffee plants are usually heavily shaded to buffer unfavourable ecological conditions. In addition to a diversification of revenues, the presence of shade trees on coffee farms has a positive impact on coffee quality as well as beneficial environmental effects. Indeed, wood extraction from coffee plantations reduces the exploitation of forest reserves and tree fallows.

The effect of tree species on the characteristics of the herbaceous stratum, during the first five years of a fallow, was evaluated in the North of Cameroon (average annual temperature 28.2 °C, total annual rainfall 1050 mm). Treatments included a natural grazed herbaceous fallow, a natural ungrazed herbaceous fallow and three planted tree fallows (Acacia polyacantha Willd. ssp. campylacantha (Hochst. ex A. Rich.), Senna siamea Lam. and Eucalyptus camaldulensis Dehnh.), which were protected against grazing. Because tree species influenced light interception in different ways, as well as having different root patterns, they had different effects on the herbaceous stratum in terms of species composition and biomass. The grazed herbaceous fallow maintained the greatest species richness. Protection against grazing or the introduction of tree species associated with the absence of grazing induced both a progressive evolution to a particular species composition. The ungrazed herbaceous fallow consisted mainly of Andropogon gayanus Kunth, which provided the greatest biomass (8 t dry matter ha–1 at the end of the fallow period). E. camaldulensis provided little shade and the lowest fine root mass in the top layer allowing the growth of A. gayanus and thus a greater herbaceous biomass (3.5 t DM ha–1) than that found under the other tree species. Under the heavy shade of A. polyacantha, the herbaceous stratum consisted mainly of annual Pennisetum spp. (2.2 t DM ha–1) and showed the greatest N concentration (1.3%), probably due to N2 fixation by the tree species. After the fourth year, despite the relatively open tree canopy, S. siamea, which showed the highest fine root mass, had a strong depressive effect on the herbaceous stratum.

Scalable indicators are promising to assess ecosystem services. In a large (660 ha) coffee agroforestry farm, we calibrated the relationship between the Normalized Difference Vegetation Index (NDVI), calculated on a High Resolution (HR) satellite image and ground-truth LAI, providing a 2-layer (shade trees and coffee) LAI calibration with LAI 2000 and a new technique based on the cumulative distribution of LAI along transects. The effective and apparent clumping of coffee leaves were computed (0.76 and 0.89, respectively). We also calibrated the relationship between the derived HR-LAI farm map and NDVI from the Moderate Resolution Imaging Spectroradiometer (MODIS) in order to re-construct LAI time-series (2001–2011). Coffee LAI, as derived from MODIS after substracting the contribution of shade tree LAI varied seasonally between 2.4 and 4.4 m2 m−2, with a maximum by the end of wet season (peak of harvest), steep decline during the drier-cooler season, minimum after annual coffee pruning, recovery during the next rainy season and pause during the grain filling period. MODIS also detected significant inter-annual variations in LAI originating from annual pruning, or plot renovation followed by a progressive LAI recovery during up to 4 years. We related the coffee-LAI time-series with farm registries to examine the impacts of management on LAI and on selected ecosystem services, namely yield and hydrological services. Nitrogen fertilization was adjusted annually by the farmer and appeared as the best yield predictor (R2 = 0.53). Combining N-fertilization with LAI from 6 significant months of the year, the prediction was improved (R2 = 0.74), confirming LAI as an important co-predictor of yield. We ended up with a yield prediction model including also the percentage of pruned resprouts (R2 = 0.79), with potential uses for regional yield mapping or reconstruction of historical yield time-series. The impacts of varying LAI (from nil to double actual values) on hydrological services were simulated. LAI affected the partitioning between green water (evapotranspiration) and blue water (infiltration, aquifer recharge, streamflow), thus the water provisioning. We discussed how LAI was influenced by natural factors (phenology, interaction between vegetative and reproductive components, climate) and by management (pruning, renovation).We confirmed LAI as a powerful scalable indicator for several key ecosystem services.

In intensive cultivation of coffee (Coffea arabica L.), large N fertilizer inputs are thought to increase nitrate (NO3 (-)) water contamination and greenhouse gas emissions. This study was carried out (1) to evaluate the nitrogen use efficiency of a highly fertilized C. arabica-Inga densiflora agroforestry system on an Andosol and (2) to determine the control mechanisms of N fluxes and losses. Nitrogen pools and fluxes were monitored for one cropping season in a coffee plantation (density 4,722 plants ha(-1), height 2.1 m), shaded by regularly pruned leguminous trees (density 278 trees ha(-1); height 8 m), in the Central Valley of Costa Rica. The fate of N fertilizer (250 kg N ha(-1) year(-1)) was traced by adding N-15-urea at 1.61 kg N-15 ha(-1). The labeled urea was rapidly nitrified or immobilized in soil organic matter with 20.8 % recovered in organic form at the end of the cropping season in the top 2 m of the soil. There was high net N mineralization and nitrification in the top soil (a parts per thousand 200 kg N ha(-1) year(-1) in 0-10 cm) and up to 257 kg NO3 (-)-N ha(-1) were found in the top 2 m of the soil. Only 25.2 % (63 kg N ha(-1)) of the applied fertilizer (N-15 recovery) was taken up by the two plant species (13.5 % in the coffee plants, 9.6 % in the shade trees and 2.1 % in the litter). Total N export in the coffee fruit harvest accounted for 110 kg N ha(-1) but only 17.6 kg N ha(-1) came from the applied fertilizer (7 % of N-15 recovery). During this year of high coffee production, the coffee plant acquired most of its N from mineralized soil N rather than from N fertilizer. High fertilization resulted in a low rate of N-2 fixation by I. densiflora, estimated at 22.7 kg N ha(-1) year(-1) (Ndfa of 16.1 %). As a result of high water drainage (1,745 mm for a total rainfall of 2,678 mm), the main fate of N fertilizer was NO3 (-) leaching (33-55 % of N-15 recovery). The annual NO3 (-)-N leaching at a depth of 120 cm was 157.2 kg N ha(-1) year(-1)(including 82.8 from applied N) and the N2O-N emission was 5.8 kg N ha(-1) year(-1). These results clearly showed that the system was N saturated, leading to low use efficiency of the N fertilizer and significant losses of N, principally through NO3 (-) leaching. This study provided an insight on how to reduce the negative environmental impact of N fertilization in intensive coffee cultivation and increase N use efficiency.

Gum arabic production from Acacia senegal is lower in sub-humid areas than arid areas. Water stress is thought to be the reason for higher yields in arid areas. The application of ethephon is thought to mimic the effect of water stress in other plants. The objective of this study was to determine if the application of ethephon would increase the gum yields of Acacia senegal under sub-humid conditions in Cameroon. Trees receiving 40 or 120 mg ethephon were compared to controls in field experiments at a semi-arid and a sub-humid location in Northern Cameroon, over two seasons. Two provenances from drier areas (Sudan) were compared to the local one. In the first season, gum yield of the local provenance treated with ethephon was increased by 400–600 % compared to the untreated trees. Gum yield at the semi-arid location was 77, 313 and 214 g/tree with 0, 40 and 120 mg ethephon/tree, respectively, while at the sub-humid location, it was 30, 186 and 114 g/tree with 0, 40 and 120 mg ethephon/tree. However, in the second season, the effect of ethephon was not significant in the semi-arid area, whereas it was evident in the sub-humid area (up to 478 g/tree). Moreover, ethephon did not affect gum yield of provenances from drier areas (Sudan). This showed that the water-stress hypothesis has to be refined. The development of ethephon-based tapping systems is promising, but requires further studies with a wider range of environmental conditions and A. senegal provenances.

Agroforestry represents an opportunity to reduce CO2 concentrations in the atmosphere by increasing carbon (C) stocks in agricultural lands. Agroforestry practices may also promote mineral N fertilization and the use of N2-fixing legumes that favor the emission of non-CO2 greenhouse gases (GHG) (N2O and CH4). The present study evaluates the net GHG balance in two adjacent coffee plantations, both highly fertilized (250 kg N ha?1 year?1): a monoculture (CM) and a culture shaded by the N2-fixing legume tree species Inga densiflora (CIn). C stocks, soil N2O emissions and CH4 uptakes were measured during the first cycle of both plantations. During a 3-year period (6-9 years after the establishment of the sys-tems), soil C in the upper 10 cm remained constant in the CIn plantation (+0.09 ± 0.58 Mg C ha?1 year?1) and decreased slightly but not significantly in the CM plantation (?0.43 ± 0.53 Mg C ha?1 year?1). Above-ground carbon stocks in the coffee monoculture and the agroforestry system amounted to 9.8 ± 0.4 and 25.2 ± 0.6 Mg C ha?1, respectively, at 7 years after establishment. C storage rate in the phytomass was more than twice as large in the CIn compared to the CM system (4.6 ± 0.1 and 2.0 ± 0.1 Mg C ha?1 year?1, respectively). Annual soil N2O emissions were 1.3 times larger in the CIn than in the CM plantation (5.8 ± 0.5 and 4.3 ± 0.3 kg N-N2O ha?1 year?1, respectively). The net GHG balance at the soil scale calcu-lated from the changes in soil C stocks and N2O emissions, expressed in CO2 equivalent, was negative in both coffee plantations indicating that the soil was a net source of GHG. Nevertheless this bal-ance was in favor of the agroforestry system. The net GHG balance at the plantation scale, which includes additionally C storage in the phytomass, was positive and about 4 times larger in the CIn (14.59 ± 2.20 Mg CO2 eq ha?1 year?1) than in the CM plantation (3.83 ± 1.98 Mg CO2 eq ha?1 year?1). Thus converting the coffee monoculture to the coffee agroforestry plantation shaded by the N2-fixing tree species I. densiflora would increase net atmospheric GHG removals by 10.76 ± 2.96 Mg CO2 eq ha?1 year?1 during the first cycle of 8-9 years.

We compiled quantitative estimates on symbiotic N2 fixation by trees in agroforestry systems (AFS) in order to evaluate the critical environmental and management factors that affect the benefit from N2 fixation to system N economy. The so-called ''N2-fixing tree'' is a tripartite symbiotic system composed of the plant, N2-fixing bacteria, and mycorrhizae-forming fungi. Almost 100 recognised rhizobial species associated with legumes do not form an evolutionary homologous clade and are functionally diverse. The global bacterial diversity is still unknown. Actinorrhizal symbioses in AFS remain almost unstudied.Dinitrogen fixation in AFS should be quantified using N isotopic methods or long-term system N balances. The general average ± standard deviation of tree dependency on N2 fixation (%Ndfa) in 38 cases using N isotopic analyses was 59 ± 16.6 %. Under humid and sub-humid conditions, the percentage was higher in young (69 ± 10.7 %) and periodically pruned trees (63 ± 11.8 %) than in freegrowing trees (54 ± 11.7 %). High variability was observed in drylands (range 10-84 %) indicating need for careful species and provenance selection in these areas. Annual N2 fixation was the highest in improved fallow and protein bank systems, 300-650 kg [N] ha-1. General average for 16 very variable AFS was 246 kg [N] ha-1, which is enough for fulfilling crop N needs for sustained or increasing yield in low-input agriculture and reducing N-fertiliser use in large-scale agribusiness. Leaf litter and green mulch applications release N slowly to the soil and mostly benefit the crop through long-term soil improvement. Root and nodule turnover and N rhizodeposition from N2-fixing trees are sources of easily available N for the crop yet they have been largely ignored in agroforestry research. There is also increasing evidence on direct N transfer from N2- fixing trees to crops, e.g. via common mycelial networks of mycorrhizal fungi or absorption of tree root exudates by the crop. Research on the below-ground tree-cropmicrobia interactions is needed for fully understanding and managing N2 fixation in AFS.

The profitability of hydropower in Costa Rica is affected by soil erosion and sedimentation in dam reservoirs, which are in turn influenced by land use, infiltration and aquifer interactions with surface water. In order to foster the provision and payment for Hydrological Environmental Services (HES), a quantitative assessment of the impact of specific land uses on the functioning of drainage-basins is required. The present paper aims to study the water balance partitioning in a volcanic coffee agroforestry micro-basin (1 km2, steep slopes) in Costa Rica, as a first step towards evaluating sediment or contaminant loads. The main hydrological processes were monitored during one year, using flume, eddy-covariance flux tower, soil water profiles and piezometers. A new Hydro-SVAT lumped model is proposed, that balances SVAT (Soil Vegetation Atmosphere Transfer) and basin-reservoir routines. The purpose of such a coupling was to achieve a trade-off between the expected performance of ecophysiological and hydrological models, which are often employed separately and at different spatial scales, either the plot or the basin. The calibration of the model to perform streamflow yielded a Nash-Sutcliffe (NS) coefficient equal to 0.89 for the year 2009, while the validation of the water balance partitioning was consistent with the independent measurements of actual evapotranspiration (R2 = 0.79, energy balance closed independently), soil water content (R2 = 0.35) and water table level (R2 = 0.84). Eight months of data from 2010 were used to validate modelled streamflow, resulting in a NS = 0.75. An uncertainty analysis showed that the streamflow modelling was precise for nearly every time step, while a sensitivity analysis revealed which parameters mostly affected model precision, depending on the season. It was observed that 64% of the incident rainfall R flowed out of the basin as streamflow and 25% as evapotranspiration, while the remaining 11% is probably explained by deep percolation, measurement errors and/or inter-annual changes in soil and aquifer water stocks. The model indicated an interception loss equal to 4% of R, a surface runoff of 4% and an infiltration component of 92%. The modelled streamflow was constituted by 87% of baseflow originating from the aquifer, 7% of subsurface non-saturated runoff and 6% of surface runoff. Given the low surface runoff observed under the current physical conditions (andisol) and management practices (no tillage, planted trees, bare soil kept by weeding), this agroforestry system on a volcanic soil demonstrated potential to provide valuable HES, such as a reduced superficial displacement-capacity for fertilizers, pesticides and sediments, as well as a streamflow regulation function provided by the highly efficient mechanisms of aquifer recharge and discharge. The proposed combination of experimentation and modelling across ecophysiological and hydrological approaches proved to be useful to account for the behaviour of a given basin, so that it can be applied to compare HES provision for different regions or management alternatives.

The shade impact by Inga densiflora on water use and drainage in a coffee agroforestry system (AFS) was compared to coffee monoculture (MC) in Costa Rica. Rainfall interception, transpiration, runoff and soil water content were monitored during 3 years. Runoff was lower in AFS than MC (5.4 and 8.4% of total rainfall, respectively) and a higher water infiltration was observed under AFS. Still, the higher combined rainfall interception + transpiration of coffee and shade trees in AFS resulted in a lower drainage than in MC. No coffee water stress was recorded either in AFS or MC as relative extractable soil water remained above 20% during the dry seasons. Time course of soil water content showed enhanced access to soil water between 100 and 200 cm depth in AFS. This suggests complementarity for soil water between coffee and shade trees. The model HYDRUS 1D predicted that drainage at 200 cm depth accounted for a large fraction of annual rainfall (68% for MC and 62% for AFS). Climatic scenario simulations showed (1) a potential competition for water between coffee and shade trees when the dry season was extended by 4–6 weeks compared to actual, and (2) a severe reduction in annual drainage, but without competition for water when rainfall was reduced down to 40% of the actual.

The advantages of associating shade trees in coffee agroforestry systems (AFS) are generally thought to be restricted mostly to poor soil and sub-optimal ecological conditions for coffee cultivation whereas their role in optimal conditions remains controversial. Thus, the objective of this study was to investigate, under the optimal coffee cultivation conditions of the Central Valley of Costa Rica, the impact of Inga densiflora, a very common shade tree in Central America, on the microclimate, yield and vegetative development of shaded coffee in comparison to coffee monoculture (MC). Maximum temperature of shaded coffee leaves was reduced by up to 5°C relative to coffee leaf temperature in MC. The minimum air temperature at night was 0.5°C higher in AFS than air temperature in MC demonstrating the buffering effects of shade trees. As judged by the lower relative extractable water (REW) in the deep soil layers during the dry season, water use in AFS was higher than in MC. Nevertheless, competition for water between coffee and associated trees was assumed to be limited as REW in the 0-150 cm soil layer was always higher than 0.3 in shaded coffee compared to 0.4 in monoculture. Coffee production was quite similar in both systems during the establishment of shade trees, however a yield decrease of 30% was observed in AFS compared to MC with a decrease in radiation transmittance to less than 40% during the latter years in the absence of an adequate shade tree pruning. As a result of the high contribution (60%) of shade trees to overall biomass, permanent aerial biomass accumulation in AFS amounted to two times the biomass accumulated in MC after 7 years. Thus provided an adequate pruning, Inga-shaded plantations appeared more advantageous than MC in optimal conditions, especially considering the fact that coffee AFS provides high quality coffee, farmers' revenue diversification and environmental benefits.

A nitrogen budget study of a highly fertilized coffee plantation growing on an Acrisol (Ultisol: Ustic Paleohumult) in Southern Costa Rica provided clear evidence of low nitrate (NO 3 − ) leaching to groundwater, in spite of a large annual rainfall and fairly high soil permeability. In this study, conducted at the same site, we measured soil properties in relation to nitrate adsorption. The clay content and proportion of kaolinite were high in the 80–200 cm layer and decreased below, whereas the proportion of gibbsite and to a lesser extent of iron oxi-hydroxides increased below 200 cm. Nitrate adsorption isotherms were measured. Nitrate adsorption maxima [coefficient a ( N max ) of the Langmuir equation] ranged from 1.9 to 1.6 cmol c kg − 1 between 80 and 200 cm depth and decreased below to reach a value of 0.9 cmol c kg − 1 at 380 cm depth. Anion exchange capacity and nitrate adsorption maxima were positively correlated with the clay and kaolinite contents, and negatively with gibbsite content and the net charge, assessed from the difference between soil pH in KCl and in distilled water. Nevertheless, the sorption affinity of NO 3 − to mineral surfaces, (coefficient b of the Langmuir equation), was highly correlated with the net charge and with the occurrence of gibbsite and iron oxi-hydroxides. We measured in the field a large NO 3 − –N accumulation in the sub-soil (up to 1570 kg of NO 3 − –N ha − 1 retained between 80 and 380 cm depth). In the layer 80–200 cm, where the clay size fraction was dominated by kaolinite, 55 to 75% of the stored NO 3 − could be extracted by shaking soil in distilled water. In this soil layer, the NO 3 − content was positively related to the net positive charge. In the layer 200–380 cm, where the clay size fraction was dominated by gibbsite, the proportion of NO 3 − extractable in distilled water was lower (45%) than above. Using adsorption coefficients obtained from batch experiments, we computed retardation factors for NO 3 − leaching up to 16 pore volumes in deep layers. Therefore, NO 3 − adsorption has already delayed and may further delay groundwater contamination over decades in these tropical Acrisols. Deep rooting perennial species may take advantage of this subsoil NO 3 − , either after abandoning agriculture, or in agroforestry systems.

En Amérique Centrale, les systèmes agroforestiers représentent une alternative à la monoculture intensive de café fondée sur l'utilisation importante d'intrants et qui peine à concilier à la fois production, respect de l'environnement et rentabilité économique. Les systèmes agroforestiers ont un potentiel de stockage de carbone mais aussi de réduction du lessivage de nitrates et d'émissions d'oxyde nitreux, un important gaz à effet de serre. Néanmoins, au Costa Rica, beaucoup de plantations caféières en monoculture ou sous ombrage de légumineuses fortement taillées sont encore gérées conventionnellement et reçoivent de grandes quantités d'engrais azoté (200 kg N/ha/an). Des pratiques alternatives utilisant des engrais organiques ainsi qu'une gestion plus raisonnée des légumineuses d'ombrage sont toutefois en expansion. Comprendre les mécanismes et quantifier les changements dans la dynamique de l'azote en fonction de l'espèce ligneuse associée, de sa conduite, des apports d'engrais et des conditions de site est de première importance pour évaluer l'efficience d'utilisation de l'azote et la contamination des nappes par les nitrates et les émissions d'oxyde nitreux. Pour évaluer l'impact des pratiques sur la dynamique de l'azote, l'UR " Ecosystèmes de Plantations " du CIRAD (UMR Eco&Sols) a établi en collaboration avec ses partenaires, le CATIE et l'ICAFE des bilans de flux d'azote dans différents systèmes agroforestiers caféiers de référence comparés à des monocultures. Les études ont montré pour les systèmes conventionnels en général une faible efficience d'utilisation de l'engrais par le caféier et l'arbre d'ombrage ainsi qu'un fort potentiel de pertes de nitrates dans les eaux de drainage et des émissions d'oxyde nitreux non négligeables. Les travaux ont montré également par rapport à la monoculture un effet très positif de la légumineuse d'ombrage (Erythrina poeppigiana) sur la disponibilité en azote du sol, et la production de café en particulier dans les systèmes organiques. Ainsi la légumineuse d'ombrage permet un niveau élevé de fixation d'azote atmosphérique et un recyclage important de cet azote lors de l'élagage des branches feuillées. (Texte intégral)

International audience; Partitioning of gross rainfall into throughfall, stemflow and rainfall interception was assessed in Costa Rica during two rainy seasons (mean annual rainfall of 2900 mm) in two coffee systems: (1) a monoculture (MC) and (2) an agroforestry system (AFS) including Inga densiflora as the associated shade tree species. Coffee architecture, not LAI, appeared to be the main driver of stemflow as stemflow was higher for shaded coffee plants (10.6% of incident rainfall) than for coffee plants in MC (7.2%), despite the fact that these shaded plants had lower LAI. The presence of Inga trees modified coffee architecture with shaded coffee plants presenting larger stems and branches resulting in higher coffee funneling ratio under shade. In AFS, coffee plants and trees accounted respectively for 88% and 12% of total stemflow which represented 11.8% of incident rainfall. AFS displayed larger cumulative stemflow and smaller total throughfall compared to MC. Cumulative throughfall expressed in % of the gross rainfall, differed between systems and monitoring periods and the trend showed a decrease with increasing LAI. Nevertheless, as stemflow measurement and interception loss estimation were done only during the second year of the study, the shade tree showed a low influence in increasing interception loss, as the combined LAI of coffee plants and shade trees was rather similar in AFS as that of coffee in MC. Furthermore, coffee plants accounted for the largest fraction of the interception loss in AFS as the coffee LAI was more than 3-fold that of shade trees.

The impact of Erythrina poeppigiana on soil characteristics, at three different positions relative to the shade tree and from three different soil depths, was evaluated in pairs of comparable Costa Rican coffee farms (organic and conventional) in 2000 and 2004. In the conventional system at 0–5 cm, higher C and N concentrations were found close to the shade tree versus the positions 2 m from the trunk (5.04 vs. 4.18%). This positive effect could influence only 20% of the farm area when high population of E. poeppigiana were used. This finding highlighted the importance of E. poeppigiana in maintaining SOM levels. In contrast, the organic system showed similar C and N concentrations for all positions probably due to an even distribution of pruning residues and to the use of organic amendments. A trend to higher total C and N concentrations for organic farms in comparison to conventional farms was found. No significant temporal changes in soil C or N concentrations were found between 2000 and 2004.

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In Central America, coffee (Coffea arabica) is commonly grown under different leguminous Inga tree species which are regularly pruned for shade control, and fuelwood production. Understanding tree-crop interactions in these systems, such as belowground competition for water and nutrients is limited by the difficulty to quantify the proportion of roots belonging to the different species. For example root morphologies of coffee and Inga densiflora are similar and root sorting by eye almost unfeasible. The aim of this study was to quantify the specific fine root mass distribution of crop and tree in a C. arabica - I. densiflora agroforestry system using near infrared spectrometry. The study was carried out in a 7 year old coffee plantation (4722 plants ha-1) shaded by I. densiflora (278 trees ha-1), established on a Costa Rican Andosol at 1200 m elevation under annual rainfall of about 2300 mm. The fine root (diam < 2 mm) biomass of mixed samples of coffee and Inga was investigated in soil cores (10 cm height, 8 cm diameter) down to 1m depth taken along the coffee rows at 50 cm from coffee plants and at 3 distances from the base of Inga trees. Pure samples of coffee and Inga roots (attached to the shoot) were also taken in the field. All samples were then oven-dried at 60°C and ground to a fine powder. Binary root mixtures containing known proportions (from 2.5 to 97.5%) of each plant species were artificially prepared by weighing and mixing root powders of the two species. Root samples of each species, artificial mixtures and all mixed root samples collected in the field were scanned with a NIRSystems6500. The spectral data were recorded at 10-nm intervals from 400 to 2500 nm. Coffee and tree species were spectrally different, particularly between 400 and 1300 nm. The calibration equations obtained by NIRS to estimate the proportion of species in root mixtures were accurate for the two species. These equations were consequently used to determine the specific mass ratio of field root samples and deduce the fine root mass distribution of both species. Coffee contributed to more than 85% of the total fine root mass. 70% of coffee roots were in the top 50 cm, while Inga fine roots were evenly distributed in the soil profile.

We used the inhibitor acetylene (C2H2) at partial pressures of 10 Pa and 10 kPa to inhibit autotrophic nitrification and the reduction of nitrous oxide (N2O) to N2, respectively. Soils (Andosol) from a Coffea arabica plantation shaded by Inga densiflora in Costa Rica were adjusted to 39, 58, 76 and 87% water-filled pore space (WFPS) and incubated for 6 days in the absence or presence of C2H2. Soil respiration, nitrification rates and N2O emissions by both processes were measured in relation to soil moisture conditions. At all WFPS studied, rates of N2O and N2 productions were small (4.8; 14.7; 23 and 239.6 ng N–N2O g−1 d.w. d−1 at 39, 58, 76 and 87% WFPS, respectively), and despite a low soil pH (4.7), N2O was mainly produced by nitrification, which was responsible for 85, 91, 84 and 87% of the total N2O emissions at 39, 58, 76 and 87% WFPS, respectively. At the three smaller values of WFPS, a linear relationship was established between WFPS, soil respiration, nitrification and N2O released by nitrification; no N2 was produced by denitrification. At more anaerobic conditions achieved by a WFPS of 87%, a large rate of N2O production was measured during nitrification, and N2 production accounted for 84% of the gaseous N fluxes caused by denitrification.