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The lower E and upper B horizons of sodic, texture-contrast soils are a formidable barrier to most annual and many perennial crops. The research presented here is part of a wider study into the nature of subsoil constraints to root exploration. The aim of this study was to characterize in three dimensions the macro- and meso-porosity across the E horizon–Btn horizon interface of a sodic, texture-contrast soil using X-ray computed tomography (CT). Intact soil cores of 50-cm length and 15-cm diameter were scanned with a medical CT X-ray machine. The pore volume reconstructed from these scans had a resolution of 0.3 × 0.3 × 0.4 cm (in the x, y, z dimensions, respectively). This resolution allowed visualization and quantification of the macroporosity of the intact cores. Undisturbed samples of 1.5-cm diameter and 4-cm length were carefully excised from the interface and scanned with micro-CT X-ray equipment. The reconstructed pore volumes had an isotropic resolution of 19 μm that allowed analysis of the mesoporosity just on the boundary between the E and Btn horizons. Mesoporosity decreased across the interface and increased lower in the Btn horizon. The distribution of the pores at the macro- and meso-scales showed the importance of the smaller pores in the A and E horizons, whereas most of the porosity in the Btn horizon was attributed to the larger pores. Pores in this sodic, texture-contrast soil were not distributed homogeneously at either the macro- or meso-scale. A greater proportion of the pores in the E–B interface were horizontal than in the upper A1, upper E and lower Btn horizons. Some ‘coiling’ of the pores was also apparent in the interface. The shape of some pores (long tubular pores) suggested formation by roots as they drilled through the soil. The orientation of these pores was a function of physical (and possibly chemical) impedance at the interface. [ABSTRACT FROM AUTHOR]

It has been hypothesized that aggregate stability is partly caused by subcritical (i.e. mild) water repellence. We conducted both a field study at a pasture site and an incubation experiment in the laboratory to characterize the relationship between aggregate dynamics and water repellence. In the field study, we investigated 53 samples from a natural texture gradient for water repellence and aggregation. In the 3-week incubation experiment, we added wheat straw to crushed soil material from five of the 53 soils representative of the texture gradient (a sandy loam, two loams, a silt loam and a clay loam), and followed changes in aggregate formation and water repellence. Although there was a dramatic increase ( P < 0.0001) in aggregation in all the soils during incubation (the mean weight diameter increased from about 300 μm to at least 900 μm), we observed a concomitant increase in water repellence only in the clay loam soil ( P = 0.0003). We found no significant correlations between water repellence and aggregation ( n = 53, P > 0.05) in the field. Whereas correlation between aggregate formation and texture was weak in the incubation experiment, we found a significant correlation between aggregation and textural parameters in the field. The amount of large macro-aggregates (> 2000 μm) was positively correlated with clay content ( n = 53, r = 0.53, P < 0.001), and negatively with sand content ( n = 53, r = −0.46, P < 0.001). These results indicate that (i) microbially induced water repellence might only become apparent when enough easily decomposable substrate is available, (ii) soil texture affects aggregate stabilization rather than aggregate formation, and (iii) aggregate formation is not necessarily associated with an increase in water repellence. [ABSTRACT FROM AUTHOR]

Reduced climatic suitability due to climate change in cocoa growing regions of Ghana is expected in the coming decades. This threatens farmers' livelihood and the cocoa sector. Climate change adaptation requires an improved understanding of existing cocoa production systems and farmers' coping strategies. This study characterized current cocoa production, income diversification and shade tree management along a climate gradient within the cocoa belt of Ghana. The objectives were to 1) compare existing production and income diversification between dry, mid and wet climatic regions, and 2) identify shade trees in cocoa agroforestry systems and their distribution along the climatic gradient. Our results showed that current mean cocoa yield level of 288kg ha-1yr-1 in the dry region was significantly lower than in the mid and wet regions with mean yields of 712 and 849 kg ha-1 yr-1, respectively. In the dry region, farmers diversified their income sources with non-cocoa crops and off-farm activities while farmers at the mid and wet regions mainly depended on cocoa (over 80% of annual income). Two shade systems classified as medium and low shade cocoa agroforestry systems were identified across the studied regions. The medium shade system was more abundant in the dry region and associated to adaptation to marginal climatic conditions. The low shade system showed significantly higher yield in the wet region but no difference was observed between the mid and dry regions. This study highlights the need for optimum shade level recommendation to be climatic region specific.

Cocoa agroforestry is perceived as potential adaptation strategy to sub-optimal or adverse environmental conditions such as drought. We tested this strategy over wet, dry and extremely dry periods comparing cocoa in full sun with agroforestry systems: shaded by (i) a leguminous tree species, Albizia ferruginea and (ii) Antiaris toxicaria, the most common shade tree species in the region. We monitored micro-climate, sap flux density, throughfall, and soil water content from November 2014 to March 2016 at the forest-savannah transition zone of Ghana with climate and drought events during the study period serving as proxy for projected future climatic conditions in marginal cocoa cultivation areas of West Africa. Combined transpiration of cocoa and shade trees was significantly higher than cocoa in full sun during wet and dry periods. During wet period, transpiration rate of cocoa plants shaded by A. ferruginea was significantly lower than cocoa under A. toxicaria and full sun. During the extreme drought of 2015/16, all cocoa plants under A. ferruginea died. Cocoa plants under A. toxicaria suffered 77% mortality and massive stress with significantly reduced sap flux density of 115 g cm -2  day -1 , whereas cocoa in full sun maintained higher sap flux density of 170 g cm -2  day -1 . Moreover, cocoa sap flux recovery after the extreme drought was significantly higher in full sun (163 g cm -2  day -1 ) than under A. toxicaria (37 g cm -2  day -1 ). Soil water content in full sun was higher than in shaded systems suggesting that cocoa mortality in the shaded systems was linked to strong competition for soil water. The present results have major implications for cocoa cultivation under climate change. Promoting shade cocoa agroforestry as drought resilient system especially under climate change needs to be carefully reconsidered as shade tree species such as the recommended leguminous A. ferruginea constitute major risk to cocoa functioning under extended severe drought., (© 2017 John Wiley & Sons Ltd.)

Stomatal regulation is a key process in the physiology of Coffea arabica (C. arabica). Intrinsically linked to photosynthesis and water relations, it provides insights into the plant's adaptive capacity, survival and growth. The ability to rapidly quantify this parameter for C. arabica under different agroecological systems would be an indispensable tool. Using a Flir E6 MIR Camera, an index that is equivalent to stomatal conductance (I g ) was compared with stomatal conductance measurements (g s ) in a mature coffee plantation. In order to account for varying meteorological conditions between days, the methods were also compared under stable meteorological conditions in a laboratory and I g was also converted to absolute stomatal conductance values (g 1 ). In contrast to typical plant-thermography methods which measure indices once per day over an extended time period, we used high resolution hourly measurements over daily time series with 9 sun and 9 shade replicates. Eight daily time series showed a strong correlation between methods, while the remaining 10 were not significant. Including several other meteorological parameters in the calculation of g 1 did not contribute to any stronger correlation between methods. Total pooled data (combined daily series) resulted in a correlation of ρ=0.66 (P≤2.2e-16), indicating that our approach is particularly useful for situations where absolute values of stomatal conductance are not required, such as for comparative purposes, screening or trend analysis. We use the findings to advance the protocol for a more accurate methodology which may assist in quantifying advantageous microenvironment designs for coffee, considering the current and future climates of coffee growing regions., (Copyright © 2017 The Authors. Published by Elsevier B.V. All rights reserved.)

The scientific community has recognized the importance of integrating farmer's perceptions and knowledge (FPK) for the development of sustainable pest and disease management strategies. However, the knowledge gap between indigenous and scientific knowledge still contributes to misidentification of plant health constraints and poor adoption of management solutions. This is particularly the case in the context of smallholder farming in developing countries. In this paper, we present a case study on coffee production in Uganda, a sector depending mostly on smallholder farming facing a simultaneous and increasing number of socio-ecological pressures. The objectives of this study were (i) to examine and relate FPK on Arabica Coffee Pests and Diseases (CPaD) to altitude and the vegetation structure of the production systems; (ii) to contrast results with perceptions from experts and (iii) to compare results with field observations, in order to identify constraints for improving the information flow between scientists and farmers. Data were acquired by means of interviews and workshops. One hundred and fifty farmer households managing coffee either at sun exposure, under shade trees or inter-cropped with bananas and spread across an altitudinal gradient were selected. Field sampling of the two most important CPaD was conducted on a subset of 34 plots. The study revealed the following findings: (i) Perceptions on CPaD with respect to their distribution across altitudes and perceived impact are partially concordant among farmers, experts and field observations (ii) There are discrepancies among farmers and experts regarding management practices and the development of CPaD issues of the previous years. (iii) Field observations comparing CPaD in different altitudes and production systems indicate ambiguity of the role of shade trees. According to the locality-specific variability in CPaD pressure as well as in FPK, the importance of developing spatially variable and relevant CPaD control practices is proposed.

The West African cocoa belt, reaching from Sierra Leone to southern Cameroon, is the origin of about 70% of the world's cocoa (Theobroma cacao), which in turn is the basis of the livelihoods of about two million farmers. We analyze cocoa's vulnerability to climate change in the West African cocoa belt, based on climate projections for the 2050s of 19 Global Circulation Models under the Intergovernmental Panel on Climate Change intermediate emissions scenario RCP 6.0. We use a combination of a statistical model of climatic suitability (Maxent) and the analysis of individual, potentially limiting climate variables. We find that: 1) contrary to expectation, maximum dry season temperatures are projected to become as or more limiting for cocoa as dry season water availability; 2) to reduce the vulnerability of cocoa to excessive dry season temperatures, the systematic use of adaptation strategies like shade trees in cocoa farms will be necessary, in reversal of the current trend of shade reduction; 3) there is a strong differentiation of climate vulnerability within the cocoa belt, with the most vulnerable areas near the forest-savanna transition in Nigeria and eastern Côte d'Ivoire, and the least vulnerable areas in the southern parts of Cameroon, Ghana, Côte d'Ivoire and Liberia; 4) this spatial differentiation of climate vulnerability may lead to future shifts in cocoa production within the region, with the opportunity of partially compensating losses and gains, but also the risk of local production expansion leading to new deforestation. We conclude that adaptation strategies for cocoa in West Africa need to focus at several levels, from the consideration of tolerance to high temperatures in cocoa breeding programs, the promotion of shade trees in cocoa farms, to policies incentivizing the intensification of cocoa production on existing farms where future climate conditions permit and the establishment of new farms in already deforested areas., (Copyright © 2016 The Authors. Published by Elsevier B.V. All rights reserved.)