Slash and burn effect on soil quality of an Alfisol: Soil physical properties

Slash and burn method of land clearing, an integral part of the traditional farming system, is widely practiced by over 90% of farmers in Southwestern Nigeria. It is often used as means of land clearing to pave way to minimum or zero tillage. Two experimental sites located on Alfisol were selected to evaluate changes in soil physical properties following burning. Soils from 0 to 0.05 and 0.05 to 0.10 m depths and worm casts on the surface were collected before and after burning. Soil strength, saturated hydraulic conductivity and infiltration tests of pre- and post-burn soils were taken in-situ. In response to burning, pore volumes reduced and concomitantly reduced infiltration rates, sorptivity and hydraulic conductivity. Infiltration rates, sorptivity levels and saturated hydraulic conductivity decreased significantly after burning by 64.3, 58.9 and 64.3%, respectively, in Site 1 and 47.9, 58.9 and 47.8%, respectively, in Site 2. Mean weight-diameter (MWD) decreased significantly following burning by 30.8 and 43.5% in Site 1 at 0–0.05 and 0.05–0.10 m depths, respectively, and 46.2 and 44.7% in Site 2 at 0–0.05 and 0.05–0.10 m depths, respectively. Appreciable but not significant reduction was recorded in water stable aggregates (WSA) for soils in both sites. However, WSA of worm casts in Site 1 increased significantly from 0.765 to 0.873 kg kg−1. Despite significant decrease of 19.7% in organic matter (OM), the stability of worm cast improved after burning. Slash and burn did not affect soil class but there were appreciable changes in particle size distribution of post-burn soils. Ash deposits in the pore spaces likely accounted for 0.8 and 3% reduction in available water capacity (AWC) at 0–0.05 and 0.05–0.10 m depths, respectively, in Site 1; and 15.1 and 6.6% reduction at 0–0.05 and 0.05–0.10 m depths, respectively, in Site 2 following burning. Soil strength increased after burning but not significant at any of the soil layers while slight increase of 0.8 and 4% in bulk density was recorded at the respective depths. Site 2 followed similar trends in soil strength and bulk density except that the penetrometer cone could not be forced to 0.10 m depth. This study showed that slash and burn may have immediate and direct impact on soil physical properties of an Alfisol; while the soil and worm casts on the soil surface responded differently to burning.