Agroforestry, water and soil fertility management to fight erosion in tropical mountains of Rwanda

African tropical mountains are often overcrowded because the climate is healthy and favorable to intensive agriculture. Consequently the density of population in the mountains of Rwanda and Burundi has reached an exceptional level (150 to 800 inhabitants/km²) that leads to delicate problems of soil protection against runoff and various types of erosion on steep cultivated hillslopes. Previous measurements on runoff plots have shown that sheet and rill erosion risks have reached 300 to 700 t/ha/year on 20 to 60% slopes with regional rainfall erosivity (R_usa = 250 to 700), very resistant ferrallitic soils (K = 0.01 to 0.20) and traditional farming systems (C = 0.8 to 0.3). Curiously, the runoff rate (10 to 30%) is relatively moderate so that it is possible to restrict erosion with a natural or leguminous fallow, a pine plantation (litter effect) or by mulching coffee, banana or cassava plantations. The problem is now to produce enough biomass to mulch the whole surface with the help of agroforestry. A new strategy (GCES = land husbandry) was suggested to meet the major farmer problems: what should be done to increase the soil productivity rapidly and protect the rural environment? A part of the answer is to be found in the efficient management of water, organic matter and soil fertility restoration (Roose et al., 1988). This strategy was first tested in 9 runoff plots (5 × 20 m) on a 23% slope of a very acid ferrallitic soil (pH = 4). Three types of living hedges (leucaena, calliandra, calliandra + setaria) twice replicated, were compared with the international bare standard plot and with the regional farming system (maize + beans during the first season, and sorghum during the second season). After 2 years, living hedges reduced runoff to less than 2% and erosion to 2 t/ha/year: they produced fire wood and high quality leguminous forage (3 to 8 kg/m) and return to the soil as much as 80 to 120 kg/ha/year of nitrogen, 3 kg/ha/year of phosphorus, 30 to 60 kg/ha/year of calcium and potassium, 10 to 20 kg/ha/year of magnesium. Thanks to agroforestry it was possible to reduce erosion hazard but not to restore the soil productivity. Without 2.5 t/ha/3 years of lime to increase the pH up to 5 and reduce the aluminium toxicity, without 10 t/ha/2 years of farm manure and mineral fertilizers to nourish the crops, the yield remains very low (800 kg/ha/season of cereals). Thanks to agroforestry and a mineral fertilizer complementation, erosion hazard was controlled and the productivity of soil and labour intensified more than 3 times.