Biochar application and wastewater irrigation in urban vegetable production of Ouagadougou, Burkina Faso

In West Africa population growth and fast urbanization challenge food security for which urban and peri-urban agriculture plays an increasing role. Adding biochar to soils depleted in soil organic carbon may improve soil quality but its effectiveness in high input systems, such as urban horticulture in West Africa, is unknown. We studied the effects of fertilization, amended biochar from agricultural waste, irrigation water quality and quantity on small-scale urban vegetable production in a multi-factorial split-plot experiment on a Haplic Lixisol in Ouagadougou, Burkina Faso. A single application of corn cob biochar at the rate of 20 t ha−1 was used in a 2-year study covering eleven cropping cycles. Biochar significantly improved total fresh matter yields of two amaranth cycles by 39% and 17%, lettuce by 7% and carrot by 11%. Repeated measures analysis showed that biochar increased average total biomass by 9% and marketable yield by 6%. Biochar was effective on fertilized plots while plant growth on unfertilized plots was limited by nutrients, most likely nitrogen (N). A 33% reduction in irrigation water caused yield declines of 4–23% which was more pronounced in fertilized plots with higher water consumption by the larger plants. Biochar increased potassium (K) concentrations while wastewater reduced phosphorus (P) in plant tissue. The study showed a positive effect of biochar on crop yields and nutrition while wastewater effects were limited to nutrient deficient plots.