Erosional effects on nitrogen balance in maize (Zea mays) grown on a volcanic ash soil in Tanzania

Nitrogen and water are both major limiting factors for agronomic production in Africa. The effect of erosion on the availability of N and water for plants is not known for major soils of the tropics. Therefore, a study was conducted to assess erosional effects on N- and water-balance on a Mollic Andosol of 9-17% slope at Lyamungu Agricultural Research and Training Institute (ARTI), Kilimanjaro region, Tanzania. Soil erosion phases were established on the basis of the topsoil depth (TSD) as an indicator of previous erosion: < 15 cm; severely eroded, 16-25 cm; moderately eroded and 26-35 cm; slightly eroded. Fill-in lysimeters (80 cm diameter, 80 cm deep) and field runoff plots (4 x 10 m) were used to monitor evapotranspiration (ET) and seepage, and N losses through erosion, runoff, leaching and plant uptake. Maize (Zea mays var. Kilima) was planted in lysimeters and runoff-plots over two cropping seasons (1995 and 1996). Rainfall and evaporation (E) were measured daily. Sediment samples from runoff plots were analysed for total N, P and C, and water samples from leachate, runoff, and rainfall were analysed for NO3-, and NH4+. There were few effects of severity of past erosion on N and water balance, but the losses of N from the system during the time of observation were very high. More than 100 and 200 kg N ha(-1) was lost in 1996 through erosion and leaching, respectively. Removal of N through plant material was about 120 kg ha(-1) in 1995 and 60 kg ha(-1) in 1996. A net depletion of 125 and 272 kg N ha(-1) was observed in 1995 and 1996, respectively. The data show that the traditional system is far from sustainable and yield decline is expected to occur in the coming years unless measures are taken to improve its N balance.