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Titre : Estimation of water use efficiency of soybean (glycine max) for biodiesel production in KwaZulu-Natal.

Auteur : Reddy, Kyle Trent Cameron.

Université de soutenance : University of KwaZulu-Natal

Grade : Master of Science in Hydrology 2020

Résumé
The production of biofuels from crops is an alternative approach to that of fossil fuels, which is expected to increase in order to ensure both cleaner energy and energy security. Knowledge of water use and yield of biofuel crops under different crop management practices and rainfed conditions at a smallholder scale is scarce in South Africa. Therefore, the main aim of this study was to estimate the crop water use and yield of soybean (Glycine max L.) as well as the crop’s response to inoculation. A field study was conducted at Swayimane in KwaZulu-Natal (South Africa) to estimate the seasonal water use, seed yield, water use efficiency (WUEC) and biodiesel yield of two genetically modified soybean varieties (CAPG3 and LS6161R). The trial was grown under rainfed conditions with optimum fertilization (100%) and two inoculation levels (0 and 100%). Seasonal crop water use (m3 ha-1 ) was derived from actual crop evapotranspiration (ETC in mm) that was estimated using the soil water balance method. Final biomass production and seed yield were measured at harvest, while biodiesel yield was determined post-harvest using measured seed oil content. The inoculated LS6161R variety consumed 4810 m3 ha-1 of water and produced 4.59 t ha-1 of seed, from which a WUEC of 0.95 kg m-3 was calculated. For the CAPG3 variety, comparable figures of 5083 m3 ha-1 , 4.35 t ha-1 and 0.86 kg m-3 were obtained for water use, yield and WUEC, respectively. Both varieties produced similar theoretical biodiesel yields of 845-850 L ha-1 , based on a seed oil content of 17.9-18.9%. The non-inoculated treatment produced lower seed yields and WUEC. However, there were no statistically significant differences between varieties and inoculation treatments for measured crop water use and yield. Observations of phenological growth stages were used to partially calibrate the AquaCrop model. The model was then used to simulate crop water use, yield and WUEC, which was then compared to observations. Simulated values of WUEC correlated poorly with observed data for both varieties and inoculation treatments. In conclusion, LS6161R is more water use efficient than CAPG3 and thus, may be better suited for biodiesel production under rained conditions for both smallholder and commercial farming systems. CAPG3 produced a higher proportion of biomass instead of seed yield and thus, is less suited for biodiesel production. With the implementation of good crop management practices, the yield gap between smallholder and commercial farmers can be reduced as is evident in this study. Finally, a full calibration of AquaCrop under optimum (i.e. irrigated) growing conditions is recommended for both soybean varieties.

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