Effects of Rainfall Seasonal Dynamics on the Chemical Properties of the Soil of a Tropical Southern Humid Rainforest Ecosystem in Nigeria

Rainfall season changes in Nigeria affect soil chemical properties in tropical rain forest, the effect of rainfall variation on soil chemical properties depend on its intensity, duration and time. However, the report on the seasonal variation/changes in rainfall remained sketchy especially for the rainforest zone in Nigeria. Due to this, in the Southern humid rainforest in southwestern, Nigeria, assessment of seasonal changes on soil chemical properties was done. The study was done in Ala Forest Reserve in Ondo State of Nigeria in 2010, 2011 and 2012. Twenty randomly selected sampling plots were chosen at different seasons viz: January (considered as dry season), March (onset of rainy season), June-July and September (peak of rainy season) and November (waning of rainy season). At five selected locations, random sampling was done at the depth of: 0 – 5 cm, 5 – 10 cm, 10 – 15 cm and 15 – 20 cm. Soil organic matter, soil pH, available phosphorus, total nitrogen and exchangeable cations (Ca, Mg and K), increased during the peak of rainy season. Soil nitrogen, organic matter and cation exchange capacity (CEC) decreased down the profile depths. Dry season (January) and peak of rainy season (June/July and September) showed greater influence on soil chemical properties. Phosphorus content was maximum in dry season (January) and at the onset of rainy season (March). However, it remained lower at the peak of the rainy season (June-July and September). On the other hand, soil organic matter and total nitrogen were found to be low in dry season (January) as result of burning of the vegetation. At the peak of rainy season (July and September), nitrogen content increased as a result of nitrogen fixation activities. Also, the decrease in the total exchangeable bases (TEB) at the depths (10-20 cm) might be attributed to their involvement in the tissue synthesis and stem elongation at this active growth period.