Horizontal nutrient fluxes and food safety in urban and peri-urban vegetable and millet cultivation of Niamey, Niger

Urban and peri-urban agriculture (UPA) has often been accused of being nutrient inefficient and producing negative externalities. To investigate these problems for the West African capital Niamey (Niger), nutrient inputs through fertilizer and manure to 10 vegetable gardens and 9 millet fields and nutrient offtakes through harvests were quantified during 24 months, and contamination of irrigation water and selected vegetables with faecal pathogens and heavy metals was determined. Annual partial horizontal balances for carbon (C), nitrogen (N), phosphorus (P) and potassium (K) amounted to 9,936 kg C ha(-1), 1,133 kg N ha(-1), 223 kg P ha(-1) and 312 kg K ha(-1) in high input vegetable gardens as opposed to 9,580 kg C ha(-1), 290 kg N ha(-1), 125 kg P ha(-1) and 351 kg K ha(-1) in low input gardens. In high input millet fields, annual surpluses of 259 kg C ha(-1), 126 kg N ha(-1), 20 kg P ha(-1) and 0.4 kg K ha(-1) were recorded, whereas surpluses of 12 kg C ha(-1), 17 kg N ha(-1), and deficits of -3 kg P ha(-1) and -3 kg K ha(-1) were determined for low input fields. Counts of Salmonella spp. and Escherichia coli yielded above threshold contamination levels of 7.2 x 10(4) CFU 25 g(-1) and 3.9 x 10(4) CFU g(-1) in lettuce irrigated with river water and fertilized with animal manure. Salmonella counts averaged 9.8 x 10(4) CFU 25 g(-1) and E. coli 0.6 x 10(4) CFU g(-1) for lettuce irrigated with wastewater, while these pathogens were not detected on vegetables irrigated with pond water. These results underline the need for urban gardeners to better adjust the nutrients applied to crop requirements which might also reduce nutrient accumulations in the soil and further in the edibles parts of the vegetables. Appropriate pre-treatment of irrigation water would help improve the quality of the latter and enhance the food safety of vegetables determined for the urban markets.