Leaching and Plant Uptake of Nitrogen from a Volcanic Ash Soil in Kilimanjaro Region, Tanzania

Abstract

ABSTRACT The amount of nitrogen lost through leaching is not known for a wide range of agrosystems of the Tropics. This study was conducted to determine the effect of past erosion on leaching losses of N from a Mollic Andosol at Lyamungu Research Institute, Kilimanjaro region, Tanzania. Lysimeters, 80 cm in diameter and 89 cm deep, were established in August 1994 on a soil with different severity of past erosion. The climate is humid with the most rain falling in April-May. Soil erosion phases were established on the basis of the topsoil depth (TSD) as follows: < 15 cm for severely eroded, 16–25 cm for moderately eroded and 26–35 cm for slightly eroded. Soil was filled in the lysimeters in four layers as per thickness and density of the ambient conditions for each erosion phase. Maize (Zea mays var. Kilima) was sown in the lysimeter in 1995 and 1996. Several soil properties declined significantly from surface layer to 70 cm depth. The severely eroded phase had significantly lower hydraulic conductivity (7.9 mm min□ 1) and higher penetration resistance (1.0 MPa) than the slightly eroded phase (10.5 mm min□ 1 and 0.7 MPa). Bray-P concentration increased with increasing severity of erosion from 2.8 to 4.2 mg P kg□ 1. Total seepage was 528 mm in 1995 and 907 mm in 1996 as an average over all the treatments. Surface runoff from the lysimeter was minor. Concentrations of NO3 □ in leachate ranged from 0.7 to 66 mg N L□ 1, while that of NH4 + from 0.0 to 1.6 mg N L□ 1. Total leaching loss of N was 8.6 g lysimeter□ 1 (167 kg ha□ 1) in 1995 and 12.4 g lysimeter□ 1 (242 kg ha□ 1) in 1996. There were no significant erosional effects on amount of seepage or leaching of N from this soil. These data indicated that previous erosion had no effect on the amount of leaching, but leaching losses of N may be a major contributor to soil fertility depletion of well-drained soils in Tanzania.