MARTINA PREDOTOVA1, JENS GEBAUER1, EVA SCHLECHT2, ANDREAS BUERKERT1
1University of Kassel, Organic Plant Production and Agroecosystems Research in the Tropics and Subtropics, Germany
2University of Kassel / University of Göttingen, Animal Husbandry in the Tropics and Subtropics, Germany
Measurements of horizontal nutrient fluxes in African urban gardens show highly positive balances for nitrogen (N) and carbon (C), leading to the assumption that massive nutrients losses occur through gaseous emissions and leaching. To quantify vertical nutrient losses through volatilisation, fluxes of NH3, N2O, CO2 and CH4 were measured in Niamey, Niger using a mobile INNOVA photo-acoustic infrared multi"=gas monitor connected to a custom"=made cuvette. For one year in three gardens, two irrigated with river water and one irrigated with sewage water, six replicate measurements were taken in vegetable"=plots during 6 days in the coldest (6 am) and hottest (2 pm) period of the day. Measurements were repeated every 6 weeks.
For all measured gases, flux rates were lower in the morning than in the afternoon. Throughout the year seasonal effects on emissions (p < 0.001) were more pronounced at midday regardless of ambient temperature.
Within a season, emission rates of NH3 and CH4 during the morning did not differ significantly between the three gardens and afternoon values varied only slightly. Afternoon missions of CO2, in contrast, were significantly different between gardens, peaking in all cases at the end of hot dry season with > 3kg CO2-C ha-1 h-1 for the low input garden receiving river water, and reaching 5.5kg CO2-C ha-1 h-1 in the high input garden irrigated with sewage water. Differences in N2O volatilisation were largest in the afternoon, when the annual averages for the two gardens using river water ranged from 34 to 43kg N2O-N ha-1, while the values for the garden receiving sewage water exceeded 100kg N2O-N ha-1.
The estimated yearly gaseous nitrogen losses were 50kg N ha-1 for the gardens receiving river water and 90kg N ha-1 for the one receiving sewage water. High N volatilisation in the urban gardens are reflecting surplus N application and indicate that nutrient management in these very intensive production systems is inefficient.
Keywords: Carbon, gaseous emissions, Niger, nitrogen, urban agriculture