Response of Organic Carbon Fractions in Tropical Soils to Land Use Changes: Evidence from 13-C Natural Abundance

Tropical soils store about one third of the global soil organic carbon. Quantitative knowledge of stabilization and decomposition processes is necessary to understand, assess and predict effects of land use changes on storage and stability of soil organic carbon (SOC). We analyzed the effects of land use (natural forest, pasture, secondary forest) on carbon storage in different organic matter fractions of topsoils developed on different parent material (marine Tertiary sediments and volcanic ashes) in the humid tropics of northwest Ecuador. Soil density fractionation was combined with 13-C analysis to determine the origin and stability of SOC under pasture and secondary forest. Stocks of mineral-associated carbon and particulate, light organic carbon were greater in volcanic ash soils than in sedimentary soils. Conversion of forest to pasture reduced total SOC stocks and it decreased the relative contribution of the light fraction to total SOC storage in both parent materials. Relative changes in SOC stocks were more pronounced in the light fraction than in the total soil carbon. The 13-C abundance in soil carbon fractions revealed that recently incorporated pasture-derived carbon was less stabilized in the volcanic ash soils than in the sedimentary soils. Most of the pasture-derived SOC which accumulated during about 20 years of pasture use was rapidly mineralized under secondary forest in sedimentary soils. The mineral-associated, pasture-derived C was the fraction with the highest stability. In the volcanic ash soils decomposition of pasture-derived SOC was even faster. It was completely lost after 10 to 20 years of secondary forest growth. The results show that the stabilization of recently incorporated SOC depends on the soil type and on the associated mineralogy. They also indicate that recently accumulated SOC stocks in the analyzed topsoils represent rather labile SOC pools.