Soil Organic Carbon Sequestration Under Different Tropical Cover Types in Colombia

Abstract

Soils are important to the global carbon cycle because they store the largest pool of carbon of terrestrial ecosystems, part of which could be released to the atmosphere as a result of land use changes. This study assesses how soil organic carbon stocks differ as a function of soil depth and the tropical forest successional state. Soil shafts 4 m-deep were excavated in six primary forests (PF), six young secondary forests (SF, 6–12 year), and four pastures (PAS) plots located in a watershed of the Porce River in the Colombian Andes mountains. Soils were sampled from the four walls of each shaft using a progression of 14 depths between 0 and 4 m depth. Clay content, soil bulk density, and C content were determined at each depth. In order to understand the soil organic carbon (SOC) change in these cover types, we modeled the data using non- linear equations and found significant differences in carbon content (Mg C 100 Mg⁻¹ soil; 1 Mg = 10⁶ g) and carbon stocks (Mg ha⁻¹) among cover types. The highest stocks of SOC to 4 m depth were found in soils of PF (227.9 Mg ha⁻¹), followed by SF (192.5 Mg ha⁻¹), and PAS (171.2 Mg ha⁻¹). These results suggest that converting primary forests to degraded pastures over several decades leads to substantial losses of soil organic carbon. Development of secondary forests in the sites of abandoned pastures starts to recover soil carbon stocks at the early stages of forest development. Differences of soil organic carbon inventories between PF and PAS were only 11.5 Mg ha⁻¹ in the first 30 cm but 56.8 Mg ha⁻¹down to 4 m. These results strongly support the need to assess the effects of land use change on SOC by sampling along the whole soil profile. Monitoring the superficial layers alone could yield misleading conclusions.