Abstract
Urban population growth is driving the expansion of urban and peri-urban agriculture (UPA) in developing countries. UPA is providing nutritious food to residents but the manures produced by UPA livestock farms and other wastes are not properly recycled. This paper explores the effects of four scenarios: (1) a reference scenario (business as usual), (2) increased urbanization, (3) UPA intensification, and (4) improved technology, on food-protein self-sufficiency, manure nitrogen (N) recycling and balances for four different zones in a small city (Jimma) in Ethiopia during the period 2015-2050. An N mass flow model with data from farm surveys, field experiments and literature was used. A field experiment was conducted and N use efficiency and N fertilizer replacement values differed among the five types of composts derived from urban livestock manures and kitchen wastes. The N use efficiency and N fertilizer replacement values were used in the N mass flow model. Livestock manures were the main organic wastes in urban areas, although only 20% to 40% of animal-sourced food consumed was produced in UPA, and only 14% to 19% of protein intake by residents was animal-based. Scenarios indicate that manure production in UPA will increase 3 to 10 times between 2015 and 2050, depending on urbanization and UPA intensification. Only 13% to 38% of manure N will be recycled in croplands. Farm-gate N balances of UPA livestock farms will increase to > 1 t-ha-1 in 2050. Doubling livestock productivity and feed protein conversion to animal-sourced food will roughly halve manure N production.Costs of waste recycling were high and indicate the need for government incentives. Results of these senarios are wake-up calls for all stakeholders and indicate alternative pathways.
Highlights
The urban population of Africa has increased > 16 times between 1950 and 2018, from 33 million to 548 million
In the URBAN scenarios we assumed a steady crop yield increase of 3% per year[42,43] (Table 2), but because of the increased population growth, food self-sufficiency will drop below 100% indicating the need to import food
To be able to remain food self-sufficient, population growth will have to be limited to 2.5% per year, crop productivity will have to increase by 3% per year[42,43], and animal productivity in terms of feed protein-N conversion (Table 2) has to double, as indicated in the technological development (TECH) scenario (Fig. 2(d); Table 2)
Summary
The urban population of Africa has increased > 16 times between 1950 and 2018, from 33 million to 548 million. During the same period the African component of the global urban population has increased from 4% to 13%. In 2050, Africa will have 22% of the global urban population with about 1.5 billion urban dwellers[1]. This growing urban population is increasing urban food demand[2] and is drawing food and nutrients from the proximate rural and peri-urban areas[3]. Thereby, urbanization is aggravating soil nutrient depletion in rural areas, and this is a major constraint to agricultural production in many subSaharan African countries[4,5]. The increase in average yield of major cereal crops has been stagnating in this region; for example, it is ~ 2 t$ha–1 in Ethiopia, while the global average is > 3.5 t$ha–1[5]
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