COMPARATIVE INVESTIGATION OF SOIL PHYSICAL AND CHEMICAL PROPERTIES OF DIFFERENT FOREST TYPES IN RAINFOREST ECOSYSTEM OF SOUTHWESTERN NIGERIA

Abstract

This study was carried out to investigate soil physical and chemical properties of primary forests (Akure forest reserve and Omo biosphere reserve), degraded forests (Eda Forest reserve and degraded part (OA3) of Oluwa forest reserve), and sacred groves (Osun-Oshogbo and Idanre Hills) in rainforest ecosystem of southwestern Nigeria. Two Line transects of 1000 m each in length and separated by a distance of at least 1000 m from each other were laid approximately at the middle of each site. Temporary sample plots of 40 m × 20 m were laid at alternate sides along each transect after every 250 m interval, giving 4 plots per transect, 8 per site, and 48 for this study. Soil samples were collected at any two edges of each sample plot at four fixed depths of 0–15 cm, 15–30 cm, 30–45 cm, and 45–60 cm using a soil auger. Soil samples from similar depths in each sample plot were bulked together, and representative samples were taken to the laboratory for analysis. Soil particle size distribution was the soil physical property investigated while the chemical properties were soil pH, organic carbon, organic matter, available phosphorus, exchangeable cations, and total nitrogen. The analyses were conducted at the wet laboratory of the Department of Forestry and Wood Technology, the Federal University of Technology, Akure. The results revealed that some of the Physico-chemical properties of the soils varied significantly across the different forest types and soil depths. Generally, the soils of all the study sites were sandy and acidic, which is important for tropical rainforests to maintain soil fertility.Sand contents ranged from 60.07 % to 79.68 %. The sand contents of Akure forest reserve, Oluwa forest reserve, and Idanre Hills sacred grove decreased with an increase in soil depth while the sand contents of Eda forest reserve increased with depth. The sand contents of Omo Biosphere Reserve and Osun-Oshogbo sacred grove did not follow any discernable pattern. The clay contents of the different forest sites ranged from 9.46 % to 23.85 % while the silt contents range from 10.87 % to 18.88 %. The soils of the study sites were acidic. The pH ranged from 4.71 to 5.20 for primary forests; 5.19 to 5.39 for degraded forests and 3.84 to 5.37 for sacred groves. The availability of essential nutrients and exchangeable bases under the study sites were tolerable to supporttree growth and thus, beneficial in planning sustainable forest management. However, there were variations in the soil nutrients between the study sites. For example, organic matter content ranged from 1.75 % to 5.17 % in primary forests, 1.71 % to 2.55 % in degraded forests, and 3.23 % to 5.10 % in sacred groves. This indicates a better ability of primary forests and sacred groves to support tree growth and thus are beneficial in planning sustainable forest management. The low soil organic matter content of degraded forests limits their potential to support tree growth. However, the high concentrations of available P in the soils of the degraded forests can enhance the fertility of the soil and by extension support the growth of trees. Based on the result of this research, the addition of a relatively higher plant residue should be adopted to enhance the total N contents of the degraded forests which will, in turn, improve the soil conditions.