Influence of deforestation on the soils of the Pantabangan Area, Central Luzon, the Philippines

Abstract

To analyze the influences of prolonged grassland conditions after deforestation on tropical soils, soils under a natural Dipterocarp forest and grassland in Central Luzon, the Philippines were compared with each other in terms of morphology, clay mineralogy, physicochemical properties, nitrogen fertility, and humus composition. The influence of forest land degradation on the soil was limited to the shallow superficial horizon in the case of the Plinthic Acrisols, but extended to the E or the upper argillic B horizon for the Orthic Luvisols. The soils of natural forest and grassland differed from each other in the following characteristics: 1) The forest soils exhibited strong structural development and various signs of high activity of the soil fauna unlike the grassland soils. 2) The physical properties of the grassland soils had deteriorated compared with those of the forest soils. 3) Comparison of the clay distribution pattern in solum suggested the existence of a weaker clay eluviation and/or more pronounced truncation of the topsoil in the grassland than under forest. 4) The grassland soils showed marked reductions in the values of chemical properties such as pH, base saturation percentage, and contents of exchangeable Ca²⁺, carbon, and nitrogen in comparison with the forest soils. 5) Nitrogen fertility of the grassland soils was lower than that of the forest soils based on the lower content of available nitrogen and the lower rates of nitrogen mineralization and nitrification. 6) The grassland soils contained humic acid humified to a greater extent than the forest soils.