Studies on sustainable land use and soil ecosystems in tropical peat land

Most tropical peat soil is classified as Oligotrophic peat due to their low N, P, K, and Ca content. The macro- and micro-nutrient status of tropical peat soils in natural forests and under cultivation was studied in Malay peninsular and Sarawak. The average concentrations of P, K, Ca, Mg, and Fe of tropical peat were significantly lower than those of grass peat in Japan. The P, K, Ca, Fe, Mn, and Zn concentrations of surface soils were higher than those of subsoils. Average concentrations of Ca, Mg, and Fe in Sarawak soils were lower than those of Malay peninsular soils for both surface and subsoils. Based on the analysis of the distribution of the metal forms, most Cu and Fe occur red in strongly chelated and non-extractable forms (non-available form). The concentrations of water soluble and exchangeable forms (available form) of Mn, Zn, and Cu were very low. In the case of fertilized soil, heavy metal content of the surface soils were about 3 to 15 times as high as those of the subsoil. Most of this was accounted for by 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 high soil pH, heavy metals applied in fertilizers were changed into non-available forms. The concentrations of N and P were fairly high in the soil solution in tropical peat profile, except for P in the profile near the center of the peat dome in a climax forest. Dissolved P consisted mostly of ortho phosphate, whereas a large part of N was in organic forms. K concentration was not low in the soil solution in the forest as compared with the global average of river water. Low K in soil solution profile was observed at the Sago plantation field in deep peat. Forest clear-cutting disturbs the K cycling in an ecosystem. The concentrations of Ca, Mg, and Fe were fairly low in the soil solution of peat profile as compared with the average of river water in Japan. The potential capacity to supply K, Ca, and P was not necessarily high, in spite of the apparent high level observed for the soil solution composition. Therefore, from the viewpoint of nutrient dynamics, the potential for using reclaimed peat land is rather limited, especially under low input management.