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To quantify carbon (C) and nitrogen (N) losses in soils of West African urban and peri‐urban agriculture (UPA) we measured fluxes of CO₂‐C, N₂O‐N, and NH₃‐N from irrigated fields in Ouagadougou, Burkina Faso, and Tamale, Ghana, under different fertilization and (waste‐)water regimes. Compared with the unamended control, application of fertilizers increased average cumulative CO₂‐C emissions during eight cropping cycles in Ouagadougou by 103% and during seven cropping cycles in Tamale by 42%. Calculated total emissions measured across all cropping cycles reached 14 t C ha⁻¹ in Ouagadougou, accounting for 73% of the C applied as organic fertilizer over a period of two years at this site, and 9 t C ha⁻¹ in Tamale. Compared with unamended control plots, fertilizer application increased N₂O‐N emissions in Ouagadougou during different cropping cycles, ranging from 37 to 360%, while average NH₃‐N losses increased by 670%. Fertilizer application had no significant effects on N₂O‐N losses in Tamale. While wastewater irrigation did not significantly enhance CO₂‐C emissions in Ouagadougou, average CO₂‐C emissions in Tamale were 71% (1.6 t C ha⁻¹) higher on wastewater plots compared with those of the control (0.9 t C ha⁻¹). However, no significant effects of wastewater on N₂O‐N and NH₃‐N emissions were observed at either location. Although biochar did not affect N₂O‐N and NH₃‐N losses, the addition of biochar could contribute to reducing CO₂‐C emissions from urban garden soils. When related to crop production, CO₂‐C emissions were higher on control than on fertilized plots, but this was not the case for absolute CO₂‐C emissions.

Urban agriculture in developing countries contributes to food diversity and security of the urban population. Its importance will increase in the future because of fast-growing urbanization. Little is known about nutrient fluxes and balances of these high input agricultural systems, which are characterized by high fertilizer use, often combined with wastewater irrigation. Adding biochar to soil has shown the potential to decrease nutrient leaching, increase yields and nutrient use efficiency. Therefore, we installed lysimeters in a multi-factorial field experimental in Tamale, Northern Ghana. The treatments included a control (no amendments applied), biochar at 20 t ha−1, mineral fertilization according to the farmers’ practice and a combination of biochar amendment and fertilization. All treatments were irrigated with tap water or wastewater. The results show higher water losses under wastewater irrigation (+ 33%). The addition of biochar had no effects on nutrient leaching, balances or water flux. Leaching losses of nitrogen were around 200 kg N ha−1 when irrigation exceeded the crop demands. When irrigation was more appropriate, the leaching rates were 50–100 kg N ha−1. The leaching of Mg and Ca almost doubled in some seasons and negative mass balances under mineral fertilization entailed soil acidification. Nitrogen balances varied strongly depending on the season, irrigation water qualities or fertilization (− 50 to 222 kg NO3-N ha−1). We conclude that the high nutrient load associated with the commonly-practiced wastewater irrigation entails large leaching losses. These cannot be curbed by biochar application and should be accounted for in fertilizer management in urban vegetable production.

Rapid urban growth in sub-Saharan Africa challenges food supply of cities. As food and other organic matter are transported from production areas to consumption points, water, which has been used for their production, is transported virtually. This study aimed at determining the magnitude and sources of virtual water flows in food trade of two West African cities, in order to better assess food provisioning risks and water resource use and planning. To this end, flows of unprocessed food from local, regional, national and international sources were systematically recorded at all roads leading to Tamale, Ghana and Ouagadougou, Burkina Faso. The survey was conducted within two years covering the peak (November - December) and lean season (March - April), respectively, for six days in a row. Virtual water flows were computed by multiplying the flow quantities (t yr−1) by their respective virtual water contents (m3 t−1). Results showed that virtual water of all food commodities imported to Tamale and Ouagadougou were 514 and 2105 million m3 yr-1 respectively, out of which 68% and 40% were re-exported to other regions of the country. The data also showed major seasonal variation in virtual water flows across the year. Reflecting their dominating role in local diets, cereals contributed most to the total virtual water inflows in both cities. Southern Ghana is the major net virtual water importer from Tamale through cereals, legumes, vegetables, and livestock. The Northern Region of Ghana, on the other hand, is a net exporter of virtual water in all food groups apart from fruits. In Ouagadougou, large flows of virtual water were imported in cereals, specifically rice from Asian countries, via Ivory Coast.

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.

Markets are critical in connecting small towns with agricultural activities in their rural hinterland. This paper presents data from north-eastern Ghana on grain marketing and trade flows, combined with data on expansion of built and transport infrastructure in five linked towns of a periodic market system. Rurally sourced goods flow through historically established small-town and regional markets to national and international markets. Simultaneously, infrastructure and service provision develops in those small towns. These trends are linked by recursive, reinforcing processes of increasing consumption, agricultural commercialisation and economic diversification. Historical structures, such as periodic markets, and the nature of the commodities involved are central to this linkage and interact with policy: the commodities concerned have been promoted in northern Ghana in preceding decades, and contribute to small-town market development by integrating smoothly into existing local systems. Historical continuity is therefore central to the role that small towns play in rural–urban transformation.

Recent studies have examined the urban metabolism of cities using urban consumption as a proxy for food inflows but very few studies have aimed at quantifying the role of cities as trade hubs and nutrient sinks of their hinterlands. We therefore examined the linkages between food and animal feed supply, their places of production and nutrient flows through the urban system in the two West African cities of Tamale (Ghana) and Ouagadougou (Burkina Faso). Using primary data on food and feed flows, and secondary data to assess the transformation of these flows into nutrient terms, we show that, besides urban consumption, the function of the two study sites as trade hubs significantly determines nutrient flows. In Tamale, > 50% of the nutrient inflows was neither consumed nor was lost in situ but left that city again for other destinations. At least 30% of the incoming cereals was stored in the city for later consumption or export. Ouagadougou relied more on imported goods with 40% of N imported from foreign countries compared to Tamale where only 10% of the N was imported, thus contributing to heavier nutrient extraction in remote production areas.

Agricultural production needs to increase, particularly in sub-Saharan Africa, many rural people are undernourished, and the urban population is growing rapidly. It is worrisome that on many West African soils with low cation exchange capacity and soil organic carbon content, mineral fertilization is rather inefficient. Under these conditions, wherever available untreated wastewater is used for irrigation despite the potential health risks to producers and consumers. For intensively cultivated soils with high mineralization rates, biochar application has been advocated as a promising management option. However, the agronomic benefits of wastewater reuse in agriculture and its interaction with biochar have received only limited attention. This study therefore investigated the effects of mineral fertilizer application and biochar amendment at two water quality and quantity levels on soil moisture, plant nutrition and biomass production on a Petroplinthic Cambisol over 2 years. Rice husk biochar applied at 20 t ha−1 significantly increased fresh matter yields in the first five cropping cycles by 15%, and by 9% by the end of 2 years. Compared with clean water, wastewater irrigation increased yields 10–20-fold on unfertilized plots during the dry seasons, while a fourfold increment was observed in the wet seasons. This seasonal difference is likely a result of the high sequence of irrigation events during the dry season. In this study, fertigation with wastewater contributed significantly to plant nutrition and nutrient recovery while yield-increasing biochar effects disappeared over time. Soil moisture was enhanced by up to 9% due to biochar amendments under unfertilized conditions.

In large areas of sub-Saharan Africa crop production must cope with low soil fertility. To increase soil fertility, the application of biochar (charred biomass) has been suggested. In urban areas, untreated waste water is widely used for irrigation because it is a nutrient-rich year-round water source. Uncertainty exists regarding the interactions between soil properties, biochar, waste water and fertilization over time. The aims of this study were to determine these interactions in two typical sandy, soil organic carbon (SOC) and nutrient depleted soils under urban vegetable production in Tamale (Ghana) and Ouagadougou (Burkina Faso) over two years. The addition of biochar at 2 kg m

This study reports and analyzes nutrient balances in experimental vegetable production systems of the two West African cities of Tamale (Ghana) and Ouagadougou (Burkina Faso) over a two‐year period comprising thirteen and eleven crops, respectively. Nutrient‐use efficiency was also calculated. In Tamale and Ouagadougou, up to 2% (8 and 80 kg N ha⁻¹) of annually applied fertilizer nitrogen were leached. While biochar application or wastewater irrigation on fertilized plots did not influence N leaching in both cities, P and K leaching, as determined with ion‐absorbing resin cartridges, were reduced on biochar‐amended plots in Tamale. Annual nutrient balances amounted to +362 kg N ha⁻¹, +217 kg P ha⁻¹, and –125 kg K ha⁻¹ in Tamale, while Ouagadougou had balances of up to +692 kg N ha⁻¹, +166 kg P ha⁻¹, and –175 kg K ha⁻¹ y⁻¹. Under farmers' practice of fertilization, agronomic nutrient‐use efficiencies were generally higher in Tamale than in Ouagadougou, but declined in both cities during the last season. This was the result of the higher nutrient inputs in Ouagadougou compared to Tamale and relatively lower outputs. The high N and P surpluses and K deficits call for adjustments in local fertilization practices to enhance nutrient‐use efficiency and prevent risks of eutrophication.

In response to changing urban food systems, short supply chains have been advocated to meet urban food needs while building more sustainable urban food systems. Despite an increasing interest in urban food supply and the flows of food from production to consumption, there is a lack of empirical studies and methodologies which systematically analyse the actual proportion and nutritional significance of local and regional food supplied to urban markets. The aim of this empirical study therefore was to compare the geographical sources supplying food to the urban population (“foodsheds”) in Tamale, Ghana and Ouagadougou, Burkina Faso, to record the supplied quantities and to assess the level of interaction between the sources and the respective city. The study was conducted over two years, covering the seasons of abundant and short supply, via traffic surveys on the access roads to the two cities, and in the Tamale markets, resulting altogether in more than 40,000 records of food flow. Results indicated that food sources were highly crop- and season-specific, ranging from one-dimensional to multi-dimensional foodsheds with diverse sources across seasons. Across the commodity-specific foodsheds, city region boundaries were established. Within the proposed city region a relatively large proportion of smallholders contributed to urban food supply, taking advantage of the proximity to urban markets. While food provided from within the city region offers certain place-based benefits, like the provision of fresh perishable crops, a larger geographical diversity of foodsheds appeared to enhance the resilience of urban food systems, such as against climate related production failures.