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Article Info AGRICULTURE AND NATURAL RESOURCESDrought is the controlling abiotic stress factor affecting crop production in dryland environments and exposes millions of people to food insecurity in Africa and Asia. Although sorghum is drought tolerant, it is not sufficiently known if biochar can reduce drought-related losses in yields in clay soils for this particular crop. The stomatal morphology and gas exchange responses were investigated of a sorghum cultivar, 'Wad Ahmed' (widely grown throughout Sudan and South Sudan), to drought stress and Acacia seyal biochar application in a greenhouse pot experiment. The experiment was set up in a split-plot, randomized block design with two experimental factors: drought stress (60%, 40%, 20% of field capacity) and biochar (no biochar and 10 Mg/ha). The potting soil was clay textured with 5% carbon content. There were eight replicate pots of each treatment which were arranged randomly in six blocks giving a total of 48 pots. The experiment lasted 153 d from sowing, with 127 d of drought treatment. The results showed that while drought stress had a significant (p < 0.05) effect on gas exchange, water use efficiency, biomass and grain yield, biochar had no significant effect, and neither drought stress nor biochar had a significant effect on stomatal size and density. It may be that high doses of biochar are required to benefit crops grown under drought stress, particularly when the soils have initially high soil organic carbon content, with more time needed for any effect to become evident.

The dependency on rainfed agriculture and weak adaptability of the agricultural sector to climate change threaten food security in Sub-Saharan Africa (SSA). Biochar has widely been touted as a relatively easy means of increasing the soil water storage capacity of soils and thereby improving or maintaining crop yields. In this study we simulated the effect of biochar amendment on sorghum aboveground biomass and grain yield at a site in South Sudan. We used the model AquaCrop parameterized using site, soil, and cropping management data from a field experiment carried out at the site in 2011 and 2012, which were both wet years. Changes in soil hydraulic properties due to biochar were based on a published meta-analysis study. In order to investigate whether the response to biochar differed in dry years, simulations were also carried out for 1990, which was the driest year during the period 1979–2014. Measured and modelled biomass and yields with and without biochar for 2011 and 2012 were compared. Simulated and measured yields depended on growing season rainfall and distribution. The simulations showed that biochar amendment had an effect on rooting zone soil water content and sorghum biomass and grain yield in 1990, but not in 2011 and 2012. In view of expected climate change, the results have important implications for sorghum production and the potential use of biochar in SSA. Given the limited response of grain yield to biochar shown in our simulations, careful selection of sorghum variety and cultivar and consideration of planting date may be a more effective means of improving yields than applying biochar.