Deforestation, tillage and cropping systems effects on seepage and runoff water quality from a Nigerian Alfisol

Abstract Conversion of tropical forest ecosystems to agricultural land use can have drastic impact on quality of natural waters as related to temporal changes in the sediment load and concentrations of dissolved nutrients. Long-term experiments were conducted in two phases to assess seepage and runoff water quality changes from an Alfisol owing to changes in land use in a sub-humid region of southwestern Nigeria. Phase I, from 1978 to 1981, studied the impact of deforestation and tillage methods, and Phase II, from 1982 to 1987, evaluated the impact of cropping systems and soil restorative practices. The six treatments evaluated in Phase I, involving combination of deforestation and tillage methods, were: (1) manual clearing (MC) with no-till (NT); (2) MC with plow-till (PT), which involved plowing to about 20 cm depth followed by harrowing; (3) shear blade (SB) clearing with NT; (4) treepusher-rootrake (TP) clearing with NT; (5) TP clearing with PT; (6) traditional farming (TF) involving slash-and-burn agriculture without use of fertilizer and amendments. The TF treatment was discontinued during Phase II. Therefore, the five treatments evaluated in Phase II were: (1) alley cropping with Leucaena leucocephala (Lam.) De Wit planted on the contour at 4 m interval; (2) restorative fallowing with mucuna ( Mucuna utilis Lam.) on severely degraded soil; (3) mucuna fallowing on moderately degraded watersheds; (4) ley farming, involving establishment of pastures and grazing, on severely degraded watersheds; (5) ley farming on moderately degraded watersheds. Pastures included a mixture of Guinea grass ( Panicum maximum Jacq.) and centro ( Centrosema pubescens Benth). All treatments were imposed on watersheds of 2–4 ha each, and were replicated twice. Each watershed was equipped with a rate measuring H-Flume, a water stage recorder, a runoff sampler and a storage tank. In addition, a single lysimeter was installed in each treatment to monitor crop water use and nutrient transport in seepage water. Sediment concentration in water runoff was measured for both phases whereas nutrient concentrations in runoff and seepage waters were measured in Phase II only. Land clearing and tillage methods affected sediment concentration in runoff water. Mean sediment concentration during the first season after deforestation was 5.5 gl −1 for MC and 9.7 gl −1 for TP. Within the NT system, mean sediment concentration was 4.0 gl −1 for MC, 3.9 gl −1 for SB, and 8.0 gl −1 for TP. Sediment concentration was also low for NT compared with PT, 5.3 gl −1 vs. 9.1 gl −1 . Alley cropping decreased sediment concentration under maize ( Zea mays L.) from 2.5 gl −1 to 0.44 gl −1 . Mean sediment concentration was 4.3 gl −1 under maize and 0.2 gl −1 under cowpea ( Vigna unguiculata L. Walp) with alley cropping, compared with 1.4 gl −1 under maize and 0.04 gl −1 under cowpea grown after pastures. There were seasonal and cropping system effects on sediment concentration. Both alley cropping and mucuna cover decreased sediment concentration. Nutrient concentration in runoff was influenced by cropping systems treatments, and was higher without than with alley cropping. Although the total runoff was less, nutrient concentration was greater in water runoff from pastures and mucuna fallow than from maize. The maximum nutrient concentrations were much higher in seepage water than in surface runoff. Manual clearing, no-till, alley cropping and use of cover crops decreased transport of sediments and dissolved elements in surface runoff and seepage water.