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• APSIM tested against six and eight-year crop rotation trials in southern Africa. • Effect of diversification on average productivity, yield trends, soil carbon and nitrogen properties investigated. • Low input systems benefited from crop diversification that included legumes through increased Nmin content. • Commercial systems faced a productivity-environment trade-off in the short term, but intercropping seemed promising. Diversification of cropping is perceived as a strategy to simultaneously achieve high productivity and maintain environmental sustainability. In southern Africa, however, due to a lack of medium to long-term field trials, there is missing quantitative information. Utilising the capability of agro-ecosystem models to quantify the interactions of crop productivity with management and environmental variables, the APSIM model was evaluated against six and an eight-year field trial datasets comprised of different crop rotations and fertiliser rates under two contrasting agro-ecological conditions in South Africa (clay soil with a mean rainfall of 871 mm versus a sandy soil with a 570 mm rainfall). Model output was compared to observed grain yield, aboveground dry matter, soil organic carbon (SOC), and mineral nitrogen (N min) dynamics. APSIM was able to reproduce the observed grain yield dynamics fairly well as indicated by a Wilmott index of agreement of 0.90. Prediction accuracy, as indicated by the absolute model error across all crops, however, only reached 39%. Simulated N min and SOC dynamics showed similar patterns to the observations. Subsequently, the model was applied in a ten-year simulation experiment with rotation treatments (14 rotations, respectively intercropping systems, including a maize monoculture control), fertiliser levels (zero and 70 kg N ha−1), and residue management (retained and removed) for the two sites. For low input systems, such as smallholder farms, residue management and legume integration are of the utmost importance to maintain SOC and more pronounced Nmin levels, which, for the sandy soil, resulted in an average maize yield increase of up to 1000 kg ha−1. Maize monoculture treatments with residues removed reduced SOC moderately by 0.04–0.08 %, while yields declined strongly (>1000 kg ha−1) over the simulated period of ten years. In commercial, fertilised cropping systems, allocating land to cultivate crops other than maize reduced the simulated total yield performance. This diversification disadvantage has to be considered against the benefits of increased SOC and yields in the medium-term, i.e. a period of ten years. For the commercial systems, maize intercropped with delayed sown oats or cowpea appeared promising. [ABSTRACT FROM AUTHOR]

An open-path eddy-covariance system containing energy balance sensors was installed in a 1.3 ha field to estimate ET of orange-fleshed sweet potato. The study was conducted during the 2014/15 and 2015/16 growing seasons. This is the first study of its kind in South Africa, and first on locally developed OFSP cultivar 'Bophelo'. The average seasonal ET obtained was 356 mm. The time-average Kc values were 0.47 during the initial stage (39 days), 0.47 – 0.97 during the development stage (25 days), 0.97 during middle stage (25 days) and 0.44 during late growing stage (25 days). A heat unit based thermal time equation was successfully developed and validated using three-day running average Kc values. • Daily sweet potato water use was highly affected by changes in canopy cover and weather conditions. • Time-average crop coefficients were derived for each growing period. • A heat unit based thermal time – crop coefficient equation was successfully developed and tested. Accurate quantification of crop water use or evapotranspiration (ET) is crucial in agriculture for improved irrigation scheduling and water resource planning across a wide range of farming conditions. The aim of this study was to quantify ET dynamics for irrigated sweet potato (Ipomoea batatas) in order to determine the FAO-type single crop coefficients (K c) that can be used in models to estimate crop water use under a wider range of semi-arid climatic conditions. An open-path eddy-covariance system containing energy balance sensors was installed in a 1.3 ha field to estimate ET of orange-fleshed sweet potato. The study was conducted during the 2014/15 and 2015/16 growing seasons. Daily ET varied between 0.5 and 5.5 mm (linked closely to canopy cover and weather conditions), with total seasonal measured ET of 361 and 347 mm for the 2014/15 and 2015/16 seasons, respectively. Time-averaged K c values of 0.47 for the initial stage, 0.97 for the mid-season stage and 0.44 for the late growth stage were derived from this study. In addition, a heat unit based Growing Degree Days - K c equation was successfully developed and validated in this study to adjust K c values of sweet potato to specific climates. These K c values can be used in combination with the FAO-56 reference evapotranspiration (ET o) to estimate site-specific sweet potato ET, which is vital for improving water management of irrigated sweet potato cropping systems growing under semi-arid conditions. [ABSTRACT FROM AUTHOR]