Emissions of ammonia, nitrous oxide and carbon dioxide from urban gardens in Niamey, Niger

Urban and peri-urban agriculture (UPA) contributes significantly to meet increasing food demands of the rapidly growing urban population in West Africa. The intensive vegetable cultivation in UPA gardens with its high nutrient inputs is often reported to operate at large surpluses of nutrients and presumably high turnover rates of organic matter (OM) and nitrogen (N) losses via emanation and leaching. Many of these claims are lacking solid data which would allow suggesting mitigation strategies. Therefore, this study aimed at quantifying gaseous emissions of ammonia (NH 3), nitrous oxide (N 2O), and carbon dioxide (CO 2) in three representative urban gardens of Niamey, Niger using a closed chamber gas monitoring system. Mean annual N emissions (NH 3-N and N 2O-N) in two gardens using river water for irrigation reached 53 and 48 kg N ha −1 yr −1, respectively, while 25 and 20 Mg C ha −1 yr −1 was lost as CO 2-C. In the garden irrigated with sewage water from the city's main wadi, N 2O was the main contributor to N losses (68%) which together with NH 3 reached 92 kg N ha −1 yr −1, while CO 2-C emissions amounted to 26 Mg ha −1 yr −1. Our data indicate that 28% of the total gaseous C emissions and 30–40% of the N emissions occur during the hot dry season from March to May and another 20–25% and 10–20% during the early rainy season from June to July. Especially during these periods more effective nutrient management strategies in UPA vegetable gardens should be applied to increase the nutrient use efficiency in UPA vegetable gardens.