VARIABILITY OF SOIL PROPERTIES UNDER FOREST AND TREE PLANTATIONS IN OKOMU FOREST RESERVE, EDO STATE, NIGERIA

Abstract

Tropical Rainforest (RF) and forest reserves are commonly converted to Oil-Palm (OP) and Rubber Plantations (RP). Variations in soil properties have been studied under different land uses, but studies on the impact of plantations on variability in soil physical and chemical properties is limited. This study analysed soil variability under OP, RP and RF in Okomu Forest Reserve, Edo State, Nigeria. Plant-soil model provided the framework. Using standard procedure, a 200 m x 300 m plot was established in each plantation type. Within each plot, five transects (300m) were established at 50 m interval. A total of 315 soil samples were collected (OP=105, RP=105 and RF=105) from topsoil (0–15 cm), subsoil (15–30 cm) and deep subsoil (30–60 cm) at each interval. Tree height, diameter-at-breast-height (DBH) and density were measured in ten sub-plots in the upper, middle and lower slope positions. Using standard procedures, soil physical (sand, silt and clay) and chemical properties (pH, organic carbon (SOC), total nitrogen (TN), available phosphorus (P), exchangeable calcium, magnesium, potassium, sodium, aluminum (Al), exchange acidity, effective cation exchange capacity (ECEC), extractable manganese, iron (Fe), copper and zinc) were analysed. Spatial variability in soil properties and vegetation parameters were analysed using descriptive statistics, Multivariate analysis and Geostatistical analysis at p≤0.05. Tree height in RF was 16.75 m, OP was 17.21 m, RP was 26.97 m, DHB in RF was 29.97 cm, OP was 154.50 cm and in RP was 60.18 cm. Organic carbon in RF was 2.1%, OP was 1.4%, and RP was 0.8%. Available phosphorus in RF was 8.71 mg/kg, OP was 3.13 mg/kg, and RP was 2.77 mg/kg, while potassium in RF was 0.08 mg/kg, OP was 0.15 mg/kg, and RP was 0.10 mg/kg. Tree height varies significantly with topography in RF (F=5.11) and OP (F=52.34), while variation in DBH was significant in RF (F=1.43). Tree height, DBH and ground elevation were significantly related with pH (r=-0.53, -0.55, 0.53), SOC (r=0.54, 0.54, -0.64), TN (r=0.52, 0.53, -0.59), P (r=0.54, 0.54, -0.55), Al (r= 0.45, 0.51, -0.73), ECEC (r= 0.47, 0.52, -0.75) and Fe (r= 0.46, 0.46, -0.64) in RF respectively. The relationship were not significant in OP and RP. Soil properties variability was similar at different depths in RF (F=0.01), OP (F=1.32) and RP (F=2.09). There was a significant spatial variability of soil properties in topsoil of RF, OP and RP (F=6.17). Significant spatial variability of soil properties were also observed at different slope positions in RF (F=9.94). Spatial variability of soil properties was greater in the topsoil of RP than in OP (F=0.46). Variability in soil properties under rainforest, oil-palm plantation and rubber plantation was limited to the topsoil and was influenced by position along the slope. Keywords: Variability of soil properties, Tree plantation, Rainforest in Edo State, Nigeria Word count: 448