Mechanisms of C sequestration in soils of Latin America

Carbon (C) sequestration, defined as the process whereby atmospheric CO2 is transferred into a long-lived C pool, is an important issue not only in the scientific community but also in the society at large because of its potential role in off-setting fossil fuel emissions. Through photosynthesis this C is stored in plants and through decomposition, trunks, branches, leaves and roots are incorporated in the soil via the action of different soil organisms, i.e., bacteria, fungi and invertebrates. This, together with the C exudates from roots that are utilized by microbial populations, constitutes the natural pathways of incorporating biomass-C into the soil. The amount of C stored in terrestrial ecosystems is the third largest among the global C pools. Soil organic carbon (SOC) up to 3 m is 2,344 Pg C, and global totals for the second and third meters are 491 and 351 Pg C. The SOC pool in soils of the tropics is estimated at 496 and 506 Pg C, approximately 30% of the global SOC pool. The amount of SOC in the tropics is estimated as 201 to 213 Pg C in the first 30 cm, 384 to 403 Pg C and 616 to 640 for the top meter and 0-200 cm depth, respectively. Thus, there is a considerable amount of C stored in tropical soils, higher than in temperate soils. The biomes with the highest contents of SOC are the tropical evergreen forest (158 Pg C) and the tropical savanna (146 Pg C). Abiotic factors, which moderate C sequestration in soils are clay content, mineralogy, structural stability, landscape position, and soil moisture and temperature regimes. On the other hand, biotic factors involved in C sequestration are determined by the activities of soil organisms. However, models do not include the formation, stabilization and lifespan of the aggregates that have been biologically produced, including roots. This is not only due to the lack of studies on this subject, but also to overlooking the role of soil organisms in soil aggregation.