Deforestation and land-use effects on soil degradation and rehabilitation in western Nigeria. III. Runoff, soil erosion and nutrient loss

Abstract

Runoff and soil erosion were measured in a two-phase experiment for seven consecutive years from 1979 through 1986 on a 44·3 watershed comprising 12 sub-watersheds of 2–4 ha each. These experiments were conducted at Ibadan, Nigeria, which is characterized by two growing seasons per year. The first season lasts from mid-March through mid-July, and the second season from mid-August to early November. Experiments were conducted in two phases comprising different treatments. The six treatments during Phase I (1979–1981), involving different combinations of land clearing and tillage methods, were: (1) manual clearing (MC) with plough-till (PT); (2) MC with no till (NT); (3) shear-blade (SB) clearing with NT; (4) tree-pusher (TP) clearing with PT; (5) TP clearing with NT; (6) traditional clearing and farming (TF). The five farming-system treatments during Phase II (1982–1987) were: (A) alley cropping with Leucaena leucocephala planted on the contour at 4 m intervals, (B) fallowing with mucuna (Mucuna utilis) on severely degraded soils; (C) fallowing with mucuna on moderately degraded soils; (D) ley farming on severely degraded soils; (E) ley farming on moderately degraded soils. The traditional farming treatment of Phase I was discontinued in 1982 at the beginning of Phase II and the land was returned to natural fallowing. Runoff, soil erosion, and nutrient losses in runoff were monitored using a 4·5 feet H-Flume, FW-1 waterstage recorder and Coshocton Wheel sampler. From the mean annual rainfall of 1320 mm, mean annual runoff amounts for the 3-year period from 1979 through 1981 during Phase I were 5·6 mm (0·42 per cent of rainfall) for MC-NT, 5·7 mm (0·43 per cent) for TF, 43·2 mm (3·27 per cent) for SB-NT, 43·5 mm (3·30 per cent) for MC-PT, 68·3 mm (5·17 per cent) for TP-NT, and. 113·1 mm (8·57 per cent) for TP-PT treatments. Runoff losses were consistently more during the first compared with the second growing season. Trends in soil erosion were similar to those of runoff, and annual soil erosion amounts were 0·1 Mg ha−1 for MC-NT, 0·2 Mg ha−1 for TF, 1·5 Mg ha−1 for SB-NT, 3·6 Mg ha−1 for MC-PT, 5·1 Mg ha−1 for TP-NT, and 8·3 Mg ha−1 for TP-PT treatments. Measurements made during Phase II showed that there were no significant differences in runoff or soil erosion among the restorative farming systems adopted. Nutrient concentrations in runoff and nutrient loss were generally high in uncropped treatments, e.g. mucuna fallow during the fallowing phase and ley farming during the grazing phase.