Distribution of heavy metals among different bonding forms in tropical peat soils

Abstract

Abstract The objective of this study is to analyze the characteristics of the macro- and micro-nutrient status of tropical peat soils under natural swamp forest and their changes after reclamation. We sampled peat soils from the coastal swamps in southern Thailand and southern Peninsular Malaysia. These soils were at varying stages of reclamation. Soil Fe, Mn, Zn, and Cu were sequentially divided into water soluble, Ca exchangeable, free-oxide-associated, weakly complexed, strongly complexed, carbonate-associated, and sulfide-associated extractable forms. Peat soils sampled in Malaysia were classified into Oligotrophic peat soils due to their low N, P, K, and Ca contents. The Ca, Mg, K, and Na contents did not change appreciably with the soil depth, though the Mn, Zn, and Cu contents decreased with the soil depth down to 30 cm and subsequently the decrease was negligible. Based on the analysis of the distribution of the metal forms, most of Cu and Fe occurred in strongly chelated and non-extractable forms. Average contents of water soluble and Ca exchangeable forms (intensity factor) of Mn, Zn, and Cu were 12, 4, and < 0.1 mg kg-1 of soils, respectively. In the case of fertilized soil (experimental field of MARDI), heavy metal contents of the surface soil were about 3 to 15 times as high as those of the subsoil. Most of the increment occurred in the weakly chelated, strongly chelated, or non-extractable forms, because the pH of the surface soil was higher than that of the subsoil. As the chelating effect of humic substances is likely to be pronounced for a high soil pH, heavy metals applied in fertilizers were changed into non-available forms. Adsorption strength of humic substances in peat for metal ions was as follows: Cu > Fe > Zn > Mn.