Abstract

This s In order to better understand the dynamics of soil organic matter (SOM) in tropical soils and the impact of converting native forest into plantation, we studied the dynamics of different physically separated SOM pools at different depths under natural conditions and 7 years of pasture (Pancium Maximum) cultivated via the replacement of the native C (C3-derived) and pasture C (C4-derived).Organic C stocks of the secondary forest, was (15±3 kg m-2)and pasture (19±3 kg m-2 ) were significantly different, which was attributed to the high biomass production of the tropical pasture and the protective effect of the high clay content (˂138.15 g kg-2 ). We observed that 72.5-50% of the total organic C accumulated in the clay and silt fraction. The replacement of forest -derived C by pasture-derived C was in average 44%, 34%, and 33% for 0-30, 30-60, and 60-90 cm depth, respectively, suggesting a fast turnover rate of organic C regardless of the high clay content. The replacement decreased in the order: coarse, sand and increase silt, clay fraction. In conclusions, after 7 years of pasture, about 56% of the total organic C in the clay in the topsoil was still from C3, indicating that a significant part of this fraction was relatively recalcitrant