Jean Pierre Irenee Bognonkpe, Kouakou Tanoh Hilaire:
Quantifying Native Soil N Losses at Watershed Scale in West Africa using the Crop Simulation Model Stics

JEAN PIERRE IRENEE BOGNONKPE¹, KOUAKOU TANOH HILAIRE^2
1University of Bonn, Plant Nutrition in the Tropics and Subtropics, Germany
²University of Abobo-Adjame, Plant Biology and Crop Production Improvement, Ivory Coast

The current intensification of land use in inland valleys of sub-Saharan Africa for food crop production exacerbates the leaching of nutrients. Recent publications describe a nitrogen fertility which is stolen from upland slopes to the bottomlands where it can be irreversibly lost in drainage water. A detailed study conducted in a representative inland valley in the forest-savanna transition zone close to Bouake (Côte d'Ivoire) showed exponential losses of soil N in relation with increasing valley surface. These N losses were also found to be amplified with rainfall and land use intensity. A quantitative understanding of this loss mechanism and its extrapolation may improve the spatial targeting of technical options aiming at conserving soil fertility and maximizing water/nutrients use efficiency.

The mathematic model STICS was identified as a suitable tool to highlight, quantify and predict the influence of the factors involved in the N loss mechanisms. A recent assessment and literature review were used to gather data of minor factors required for the use of the model STICS. These data are expected to validate the results obtained in the three-year study conducted in a 130 hectare model watershed in the mentioned transition zone where half of the valley bottom was used for permanent lowland rice cultivation (8 ha). The land use of the slopes gradually changed from natural vegetation to maize cultivation: in 2001 around 5% and in 2002 around 10% was converted into maize fields.

STICS is basically a crop simulation model caring about the use efficiency of nutrients input (particularly nitrogen) and its loss in drainage water. Its accuracy in the sol-plant nutrients transfer helps to highlight the faith of N from various sources. Preliminary obtained results through the model application show the relative importance of physical soil factors (grain size, infiltration and surface runoff, as well as soil roughness) which are related to the soil type and have to be considered in the N losses mechanism. The effectiveness of these results will be discussed.

Keywords: Land use, N-dynamics, rice, STICS modelling, West Africa

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Contact Address: Jean Pierre Irenee Bognonkpe, University of Bonn, Plant Nutrition in the Tropics and SubtropicsKarlrobert-Kreitenstrasse 13, 53115 Bonn, Germany, e-mail: j.bognonkpegmx.de