Humus composition and physico-chemical properties of humic acids in tropical peat soils under sago palm plantation

Tropical peatlands in Sarawak are exploited on a large scale for crop plantations. After reclamation of primary forests, large amounts of gaseous and soluble organic compounds could be released from peatlands. These decomposing products are probably related to the composition and properties of peat humus. Furthermore, these properties might be influenced by land use and topography. Peat soils on the surface and beneath the water table were collected from the edge and the center of a peat dome under sago palm plantations located between two tributaries. The content of alkali-soluble carbon in peat soils and the degree of humification of peat were higher on the edge than in the center of the peat dome and in the deeper horizons compared with the surface horizons. A similar trend was also observed for the contents of carboxylic groups and aromatic constituents in humic acids. The opposite trend was observed for the viscosity of humic acids related to molecular weight. These results indicate that the degree of humification of peat progressed at sites located close to tributaries and in deeper horizons. Peat decomposition might presumably be promoted at locations along the tributaries because of the higher microbial activities supported by a higher mineral nutrient supply. Concurrently, the contents of carbohydrates were lower in the surface horizons than in the deep horizons, suggesting that peat decomposition is accelerated on the ground surface after reclamation. Reclamation of primary forests for the establishment of sago plantations has gradually changed the redox conditions and has promoted biodegradation of peat soils.