Effects of seasonal rainfall and water table movement on the soil solution composition of tropical peatland

ABSTRACT

Understanding the composition of the soil solution of tropical peatlands is important because it directly affects nutrient availability and environmental degradation. The objectives of this study were to investigate temporal fluctuations in the soil solution composition in tropical peatlands in West Kalimantan and Riau, Indonesia and identify the factors controlling these fluctuations. In each site, we established four study plots consisting of three plots under oil palm (Elaeis guineensis Jacq.) plantation and one plot under uncultivated land as the control. Triplicate soil solution samples were collected at 50 and 200 cm depths, fortnightly. During the drought, the pH at 50 cm was low (3.7–4.0), which was influenced by oxidation reactions such as organic acids and NO₃^? generations. The pH at a depth of 200 cm was high (5.9–6.8), due to reduction reactions such as denitrification. High cation concentrations at both depths would result from organic matter decomposition and the limited downward movement of water. Rewetting the West Kalimantan peatland caused a sharp decrease in pH and ionic concentrations at 50 and 200 cm depths, because of the transportation of ions from the upper acidified layer. However, the lower rainfall levels in Riau than West Kalimantan resulted in a gradual decrease in pH and Ca²⁺ concentration. The higher pH levels and ion concentrations in West Kalimantan than in Riau would be influenced by the enhanced microbial activity due to water supply from the risen water table in this site. This study showed that seasonal rainfall and water table movement were the main factors controlling the fluctuations in the chemical composition of soil solutions.