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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. [ABSTRACT FROM AUTHOR]

We compiled quantitative estimates on symbiotic N fixation by trees in agroforestry systems (AFS) in order to evaluate the critical environmental and management factors that affect the benefit from N fixation to system N economy. The so-called 'N-fixing tree' is a tripartite symbiotic system composed of the plant, N-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 N 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 free-growing 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 N fixation was the highest in improved fallow and protein bank systems, 300-650 kg [N] ha. General average for 16 very variable AFS was 246 kg [N] ha, 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 N-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 N-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-crop-microbia interactions is needed for fully understanding and managing N fixation in AFS. [ABSTRACT FROM AUTHOR]

Abstract: Acacia senegal, an important leguminous tree in arid and semi-arid environments, has shown promise as a multipurpose species, including gum production and soil fertility improvement, linked with N2-fixation capabilities. Of particular interest are ontogenetic and edaphic effects on A. senegal performance in natural populations. Our research objectives were to investigate the effect of tree age and site phosphorus conditions on (1) tree N2-fixation and (2) soil N and C dynamics in natural stands of A. senegal var. senegal, Baringo District, in the Rift Valley, Kenya. Sites consisted of A. senegal saplings (9 months) and mature A. senegal trees (7 years) along an edaphic gradient of soil P availability. A single-tree neighborhood approach was employed using a two by two factorial design: site conditions [high and low soil P contents] and tree age class [juvenile and mature]. Soil (N and C pools and fluxes) and plant metrics were quantified. A soil transfer experiment was also employed to confirm age and site effects on soil N mineralization. On the high soil P site, A. senegal had significantly lower foliar (15N levels than neighboring non-leguminous species (Balanites aegyptiaca), while foliar δ15N values in A. senegal on the low P site exhibited no significant difference with our reference plant, B. aegyptiaca. Across P sites, B. aegyptiaca had similar foliar δ15N values. These results indicate that the rate of N2-fixation of A. senegal trees, as determined with foliar 15N natural abundance methodology, increased with increasing soil P availability in these natural populations. However, N2-fixation rates declined with age. Although soil texture and soil CO2 efflux did not differ between sites or across ages, soils under mature A. senegal at the high P site exhibited significantly greater total N content and total C content in comparison to soils at the low P site and under juvenile plants. Furthermore, under mature A. senegal trees, soil N mineralization rates were significantly greater as compared to under saplings. Soil transplants confirmed that soil microbial activity may be stimulated under mature trees as N mineralization rates were 2–3 fold greater compared to under A. senegal saplings. Our findings suggest that tree age and soil P availability are important factors in the nitrogen budget of natural populations of A. senegal, determining N2-fixation rates, and potentially influencing soil total N and C pools and soil mineral N. This study provides information regarding the adaptation of A. senegal under differing edaphic conditions thus increasing accuracy of management support for A. senegal populations as productive agroforests. [Copyright &y& Elsevier]

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 trade-offs between increasing coffee and tree productivity, and between maximising productivity and limiting the impact of the system on the environment. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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 (NO3 −) 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 80–200cm layer and decreased below, whereas the proportion of gibbsite and to a lesser extent of iron oxi-hydroxides increased below 200cm. Nitrate adsorption isotherms were measured. Nitrate adsorption maxima [coefficient a (N max ) of the Langmuir equation] ranged from 1.9 to 1.6cmolc kg−1 between 80 and 200cm depth and decreased below to reach a value of 0.9cmolc kg−1 at 380cm 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 NO3 − 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 NO3 −–N accumulation in the sub-soil (up to 1570kg of NO3 −–Nha−1 retained between 80 and 380cm depth). In the layer 80–200cm, where the clay size fraction was dominated by kaolinite, 55 to 75% of the stored NO3 − could be extracted by shaking soil in distilled water. In this soil layer, the NO3 − content was positively related to the net positive charge. In the layer 200–380cm, where the clay size fraction was dominated by gibbsite, the proportion of NO3 − extractable in distilled water was lower (45%) than above. Using adsorption coefficients obtained from batch experiments, we computed retardation factors for NO3 − leaching up to 16 pore volumes in deep layers. Therefore, NO3 − 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 NO3 −, either after abandoning agriculture, or in agroforestry systems. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

Abstract: Soil moisture and gaseous N-flux (N2O, N2) dynamics in Costa Rican coffee plantations were successively simulated using a mechanistic model (PASTIS) and two process-based models (NGAS and NOE). Two fertilized (250 kg N ha−1 y−1) coffee plantations were considered, namely a monoculture and a system shaded by the N2 fixing legume species Inga densiflora. In situ N2O fluxes were previously measured in these plantations. NGAS and NOE used specific microbial activities for the soils. To parameterize NGAS, we estimated N mineralization via in situ incubations and the contribution of heterotrophic soil respiration to total soil respiration. Potential denitrification rates and the proportion of denitrified N emitted as N2O were measured in the laboratory to define the values of NOE parameters, as well as nitrification rates and related N2O production rates for parameterizing both models. Soil moisture and both NGAS and NOE N2O fluxes were best modelled on an hourly time step. Soil moisture dynamics were satisfactorily simulated by PASTIS. Simulated N2O fluxes by both NGAS and NOE (3.2 and 2.1 kg N ha−1 y−1 for NGAS; 7.1 and 3.7 kg N ha−1 y−1 for NOE, for the monoculture and shaded plantations respectively) were within a factor of about 2 of the observed annual fluxes (4.3 and 5.8 kg N ha−1 y−1, for the monoculture and shaded plantations respectively). Statistical indicators of association and coincidence between simulated and measured values were satisfactory for both models. Nevertheless, the two models differed greatly in describing the nitrification and denitrification processes. Some of the algorithms in the model NGAS were apparently not applicable to these tropical acidic Andosols. Therefore, more detailed information about microbial processes in different agroecosystems would be needed, notably if process-oriented models were to be used for testing strategies for mitigating N2O emissions. [Copyright &y& Elsevier]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Nitrogen fertilization is a key factor for coffee production but creates a risk of water contamination through nitrate (NO) leaching in heavily fertilized plantations under high rainfall. The inclusion of fast growing timber trees in these coffee plantations may increase total biomass and reduce nutrient leaching. Potential controls of N loss were measured in an unshaded coffee ( Coffea arabica L.) plot and in an adjacent coffee plot shaded with the timber species Eucalyptus deglupta Blume (110 trees ha−1), established on an Acrisol that received 180 kg N ha−1 as ammonium-nitrate and 2,700 mm yr−1 rainfall. Results of the one year study showed that these trees had little effect on the N budget although some N fluxes were modified. Soil N mineralization and nitrification rates in the 0–20 cm soil layer were similar in both systems (≈280 kg N ha−1 yr−1). N export in coffee harvest (2002) was 34 and 25 kg N ha−1 yr−1 in unshaded and shaded coffee, and N accumulation in permanent biomass and litter was 25 and 45 kg N ha−1 yr−1, respectively. The losses in surface runoff (≈0.8 kg mineral N ha−1 yr−1) and N2O emissions (1.9 kg N ha−1 yr−1) were low in both cases. Lysimeters located at 60, 120, and 200 cm depths in shaded coffee, detected average concentrations of 12.9, 6.1 and 1.2 mg NO-N l−1, respectively. Drainage was slightly reduced in the coffee-timber plantation. NOleaching at 200 cm depth was about 27 ± 10 and 16 ± 7 kg N ha−1 yr−1 in unshaded and shaded coffee, respectively. In both plots, very low NO concentrations in soil solution at 200 cm depth (and in groundwater) were apparently due to NO adsorption in the subsoil but the duration of this process is not presently known. In these conventional coffee plantations, fertilization and agroforestry practices must be refined to match plant needs and limit potential NO contamination of subsoil and shallow soil water. [ABSTRACT FROM AUTHOR]

Ensuring sustainable agriculture in semiarid Africa requires the implementation of methods to balance nutrients and to conserve soil organic matter (SOM). There is an urgent need to improve the management of all types of SOM input. In this paper; the authors review a wide range of agricultural practices and discuss their advantages, limitations and feasibility. They distinguish ‘traditional systems’ such as traditional fallow, parkland and manuring from ‘improved systems’ such as ‘improved fallow’, forest fallow, alley cropping, cover crops and application of composted manure. Biomass production (BMP) for ‘improved systems’ is mainly linked: (i) for agroforestry, to the tree species used in forest fallow, to the synchronization of nutrient supply by the soil with the cereal demand in alley cropping, and generally to the efficiency of the root system and its development with the depth; (ii) for cover crops, BMP is mainly linked to the initial soil fertility and to the ecological zone: establishment and management of cover crops are not yet fully mastered under some conditions such as an annual rainfall below 800 mm and/or on very clayey soils; (iii) for manure, BMP is mainly linked to the improvement of fallow in order to ensure sufficient forage resources. Because semiarid Africa is mainly a livestock zone, the authors emphasise manure: constraints, quality indicators and tools used to encourage its production, are analysed. Thus it was concluded that the intensification of manure production and its rational use in semiarid African regions, threatened by drought and malnutrition, is very important: this cannot be separated from the production of plant biomass, whose possibilities have been examined above. [ABSTRACT FROM AUTHOR]

A correction to this paper has been published: https://doi.org/10.1007/s10457-021-00615-9 [ABSTRACT FROM AUTHOR]

The published on-line ms "Carbon dynamics of cocoa agroforestry systems in Central Cameroon: afforestation of savannah as a sequestration opportunity." [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Past land-use effects can last for decades in cocoa agroforestry systems (cAFS). • Savannah cAFS (S-cAFS) and forest cAFS (F-cAFS) show typical attribute combinations. • Ecosystem attributes and services (ES) evolve on the long run in S- and F-cAFS. • Several attributes and ES showed contrasted temporal trajectories in S- and F-cAFS. • Several attributes and ES in S-cAFS tended to converge with those in F-cAFS. Abstract 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Particle size fraction C in a target set was predicted using a soil NIR library. • Models calibrated with the spectral library could accurately predict fraction C. • Spiking with extra-weighting systematically improved model accuracy. • Reference data uncertainty affected the assessment of NIRS prediction accuracy. Particle size fractionation enables a better understanding of soil organic carbon (C) dynamics since it separates fractions that differ in composition, residence time and function. However, this method is time-consuming and tedious; thus, its use has been greatly limited. Our objective was to evaluate the ability of an existing soil spectral library (SSL) from different regions of West Africa to predict the C amount in the fractions (gC kg−1 soil) of the samples in a new target set from Benin. The SSL included 181 samples from five countries, and the target set included 94 samples (depth ≤ 40 cm), most of which were coarse-textured; near-infrared reflectance (NIR) spectra were collected for 2 mm sieved samples (non-fractionated samples). The predicted variables were the C amounts in the non-fractionated soil and in the < 20, 20–50, and > 50 µm fractions (F<20, F20–50, and F>50, respectively). Different methods were tested to optimize the predictions: (i) SSL enrichment with 10 or 15 samples selected from the target set (spiking) and replicated six times (i.e. extra-weighted); (ii) locally weighted (local) partial least squares regression (PLSR), which is calibration by the spectral neighbours with the highest weights attributed to closest neighbours, and was compared to "global" (i.e., common) PLSR, where all calibration samples equally contribute; and (iii) spectrum pretreatments (e.g., smoothing, centring, derivatization). In addition, the intermediate precision of the conventional data (standard error of laboratory; SEL int) was estimated through triplicate fractionation of three samples carried out by three operators (one per replicate). When the SSL alone was used for calibration, the predictions were inaccurate for the C amounts in the non-fractionated soil and in F<20; however, the predictions were accurate for the C amounts in F20-50 and F>50, with minimal benefit from the local PLSR over the global PLSR in general. For the non-fractionated soil, F<20, F20–50 and F>50, the ratios of performance to the interquartile range in the validation set, RPIQ VAL , were 1.6–1.8, 1.6–1.7, 1.9 and 1.9–2.1, respectively. Calibration with SSL spiked (i.e., completed with spiking samples) yielded an increase in RPIQ VAL from 33 to 56% for the C amount in the non-fractionated soil and F<20 and from 0 to 20 % for F20-50 and F>50 (RPIQ VAL reached 2.4–2.5, 2.2–2.3, 1.9–2.0 and 2.1–2.3, respectively), and the benefit of local PLSR was still limited. The SEL int was based on a few samples and thus only provided a rough estimation; this estimate represented at least 65% of the prediction error for the C amounts in the fractions. Therefore, the SEL int needs to be determined more extensively to both improve the model accuracy and refine the interpretation of the predictions based on NIR spectra. This library should be enriched with samples from other sites to represent other soil types. [ABSTRACT FROM AUTHOR]

Abstract 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. Graphical abstract Unlabelled Image Highlights • Higher soil pH, inorganic N content and nematode densities in organic coffee systems • Higher soil nutrient bioavailability and fauna densities under N 2 -fixing trees • Shade effects on soil fertility and food web were species-specific only under organic management. • Better soil fertility was related to improved soil food web. [ABSTRACT FROM AUTHOR]

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 N 2 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: 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 (250kgNha−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 systems), soil C in the upper 10cm remained constant in the CIn plantation (+0.09±0.58MgCha−1 year−1) and decreased slightly but not significantly in the CM plantation (−0.43±0.53MgCha−1 year−1). Aboveground carbon stocks in the coffee monoculture and the agroforestry system amounted to 9.8±0.4 and 25.2±0.6MgCha−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±0.1MgCha−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±0.3kgN-N2Oha−1 year−1, respectively). The net GHG balance at the soil scale calculated 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 balance 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±2.20MgCO2 eqha−1 year−1) than in the CM plantation (3.83±1.98MgCO2 eqha−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±2.96MgCO2 eqha−1 year−1 during the first cycle of 8–9 years. [Copyright &y& Elsevier]

Summary: Partitioning of gross rainfall into throughfall, stemflow and rainfall interception was assessed in Costa Rica during two rainy seasons (mean annual rainfall of 2900mm) 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. [ABSTRACT FROM AUTHOR]

Many studies have shown that introducing N 2 -fixing trees (e.g. Acacia mangium) in eucalypt plantations can increase soil N availability as a result of biological N 2 fixation and faster N cycling. Some studies have also shown improved eucalypt P nutrition. However, the effects of N 2 -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 N 2 fixation and root development, acacia introduction was able to change the profile of organic P and LMWOAs in <10 years. The variations between available Pi, Po and LMWOA forms showed that P cycling was dominated by different processes at each site, that are rather physicochemical (via Pi desorption after LMWOAs release) at Itatinga and biological (via organic P mineralization) at Kissoko. Specific patterns of Po and LMWOAs forms found in soil sampled under acacia or eucalypt would therefore explain the effect of acacia introduction in both sites. Unlabelled Image • Po and LMWOAs patterns differed among native ecosystems and planted plots. • Acacia introduction changed the profile of Po and LMWOAs in <10 years. • Changes were stronger at site with a better N 2 fixation and development of acacia. • P cycling depends on P solubilization with eucalypt and Po mineralization with acacia. [ABSTRACT FROM AUTHOR]

We hypothesized that the nitrogen-fixing tree Acacia mangium could improve the growth and nitrogen nutrition of non-fixing tree species such as Eucalyptus . We measured the N-mineralization and respiration rates of soils sampled from plots covered with Acacia , Eucalyptus or native vegetation at two tropical sites (Itatinga in Brazil and Kissoko in the Congo) in the laboratory. We used a bioassay to assess N bioavailability to eucalypt seedlings grown with and without chemical fertilization for at least 6 months. At each site, Eucalyptus seedling growth and N bioavailability followed the same trends as the N-mineralization rates in soil samples. However, despite lower soil N-mineralization rates under Acacia in the Congo than in Brazil, Eucalyptus seedling growth and N bioavailability were much greater in the Congo, indicating that bioassays in pots are more accurate than N-mineralization rates when predicting the growth of eucalypt seedlings. Hence, in the Congo, planting Acacia mangium could be an attractive option to maintain the growth and N bioavailability of the non-fixing species Eucalyptus while decreasing chemical fertilization. Plant bioassays could help determine if the introduction of N 2 -fixing trees will improve the growth and mineral nutrition of non-fixing tree species in tropical planted forests.

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., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)