Soil Carbon Dynamics Following Land Use Changes and Conversion to Oil Palm Plantations in Tropical Lowlands Inferred From Radiocarbon

Tropical forests account for a large portion of the Earth's terrestrial carbon pool. However, rapid deforestation threatens the stability of this carbon. We examine radiocarbon (Delta C-14) and stable carbon (delta C-13) isotopes of soil organic matter to provide insight into rates of carbon turnover, inputs, and losses of pasture-derived (C4) versus forest or oil palm-derived (C3) carbon. Data are presented for natural lowland forests on mineral soil converted to pastures in Peru and to oil palm plantations in Peru, Indonesia, and Cameroon. We additionally examine plots of secondary forests following agricultural use. There were large losses in carbon stocks under both pasture and oil palms. In the plots converted to pasture, our data indicate a preferential loss of relatively young carbon, and a greater loss of forest-derived carbon than replacement with pasture-derived carbon. Natural forests converted directly to oil palm plantations sustained losses in carbon, but Delta C-14 values suggest that the soil may retain a sufficient amount of newly acquired carbon to offset initial losses of young carbon. Furthermore, replacement of pastures with oil palm plantations facilitates the accumulation of young carbon, which may lead to a gradual increase in carbon stocks. The sites examined here are representative of the biophysical characteristics in roughly half of the humid tropics, suggesting that these findings may be applicable to a large area of similarly managed mineral soils in lowland tropical forests.