The advantages of associating shade trees in coffee agroforestry systems (AFS) are generally thought to be restricted mostly to poor soil and suboptimal 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.

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The objective of this study was to evaluate the effect of N fertilization and the presence of N 2 fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N 2 O), carbon dioxide (CO 2 ) and methane (CH 4 ), 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 N 2 fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N 2 O emissions with 84% of the annual N 2 O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH 4 uptakes. The higher annual N 2 O 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 N 2 fixing shade trees can increase N 2 O emissions. Annual CO 2 and CH 4 fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO 2 ha -1 year -1 , -1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH 4 ha -1 year -1 , respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season.

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. (Résumé d'auteur

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 Sand F-cAFS along >70-year chronosequences. Our results showed that the attributes and services studied followed typical temporal trajectories in Sand 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 20-30 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 Sand 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.

Research on coffee agroforestry systems in Central America has identified various environmental factors, management strategies and plant characteristics that affect growth, yield and the impact of the systems on the environment. Much of this literature is not quantitative, and it remains difficult to optimise growing area selection, shade tree use and management. To assist in this optimisation we developed a simple dynamic model of coffee agroforestry systems. The model includes the physiology of vegetative and reproductive growth of coffee plants, and its response to different growing conditions. This is integrated into a plot-scale model of coffee and shade tree growth which includes competition for light, water and nutrients and allows for management treatments such as spacing, thinning, pruning and fertilising. Because of the limited availability of quantitative information, model parameterisation remains fraught with uncertainty, but model behaviour seems consistent with observations. We show examples of how the model can be used to examine tradeoffs between increasing coffee and tree productivity, and between maximising productivity and limiting the impact of the system on the environment. (Résumé d'auteur