Surface De gradation and. Soil Erosion inRwanda. Rwanda, a tropical highland in the heart of Africa, is one of the most densely populated countries in the world. The average size of smallholdings is less than 0,8 ha, so that peasants are forced to shorten the fallow periods and to cultivate even very steep slopes. This causes severe soil degradation problems that result in a continuous decline in crop yields as well as in accelerated erosion. On many steep sites, this may lead to a total removal of fertile topsoil within the next few decades if no adequate control measures are applied.With R factors between 150 and 400, rainfall erosivity was shown to be very moderate. Nevertheless, soil losses recorded on the reference plots at Butare were extremely high (more than 550t/ha/y on the bare fallow, 246 t/ha/y on the cassava plot). These data give evidence that in former studies in the same region, soil erosion risks were severely underestimated.Based on five years of plot measurements at Butare (Southern Central Plateau) and on a review of recently published preliminary results obtained from other experimental sites, present rates of soil erosion in Rwanda can be estimated. It can be shown that in former studies soil erosion on steep slopes was underestimated by one order of magnitude.Under these conditions, simple mechanical conservation measures such as erosion control ditches, which take a considerable part of agricultural land out of production , have often proved to be inappropriate and may even increase erosion risks on sites susceptible to mass movements. Examining different erosion control measures, the author proposes to replace mechanical erosion control measures wherever possible by productive, biological measures, such as multi-purpose hedges, and to transform traditional land use systems into appropriate agroforestry systems.Plot experiments carried out in an agroforestry system show that biological erosion control measures such as grass infiltration strips and leguminous hedgerows avoid most of the disadvantages, of mechanical measures and cannot only reduce runoff and soil loss to tolerable limits, but are also able to improve soil fertility and thus increase crop production. For example, a system based on Calliandra calothyrsus cropped in alleys five meters wide could reduce runoff to 33 % and soil loss to less than 5 % of the values registered under traditionally cropped cassava (8,7 % total annual runoff, 246 t/hay of soil loss), while the use of leaves as mulch lead to a nutrient intake of 105 kg of N fixed by symbiotic Rhizobium bacteria, and to the recycling of 47 kg of P and 26 kg of K per hectare and year.
Les résultats des recherches récentes sur l'érosion des sols au Rwanda montrent que, dans différentes régions agro-écologiques, l'érosion sur fortes pentes cultivées est de l'ordre de 100 tonnes par hectare et par an. Etant donné que l'érosivité des pluies au Rwanda est assez faible (Rus A entre 1 50 et 400), cette érosion accélérée est due à la mise en production des terres à vocation pastorale ou forestière et à des méthodes culturales mal adaptées, qui ne garantissent pas une bonne couverture du sol.Sous ces conditions, des structures anti-érosives mécaniques comme les fossés d'absorption ne semblent pas être la solution au problème d'érosion, qui -vu les contraintes socio-économiques et écologiques du pays -ne peut pas être examiné d' une manière isolée du problème de la dégradation des sols. Comme stratégie future pour la conservation des sols au Rwanda, l'auteur propose de promouvoir des méthodes biologiques au lieu de remplacer une méthode de protection mécanique par une autre. Les méthodes biologiques, par exemple la culture en couloirs entre des haies de légumineuses, ne sont pas seulement très efficaces en ce qui concerne la conservation des sols, mais peuvent à la fois garantir un recyclage des éléments nutritifs et un approvisionnement continuel du sol en matière organique, et ainsi ralentir le processus de dégradation de la fertilité des sols au Rwanda.
