Accretion of organic carbon in the acid soils of the eastern plains of Colombia

The present study investigated some of the mechanisms that could explain the differences between the native savanna and the introduced pastures in term of carbon accumulation in the soil in Colombia. The C accumulated in the soil must come from net primary productivity (NPP). Therefore, anything that constraints NPP ultimately reduces C accumulation in the soil.; This study demonstrated that the small amounts of fertilizers applied for establishing pastures and the regular maintenance fertilizers applied at 2-year intervals increased below ground net primary productivity (BNPP). Increases in BNPP ranged from 2 to 2.5 times.; Increases in soil organic carbon (SOC) in pastures compared with native savanna were reflected by the differences in BNPP. The total amount of additional SOC to a depth of 1m in pastures was 31.7 Mg ha-1 to 58.9 Mg ha-1 more than in the native savanna. The large amount of live roots in the pastures contributed to more C accumulation in soil upon root death and decomposition. Lower contribution is expected from native savanna and degraded pastures because they have lower root biomass.; The increase in root biomass production also had a positive effect on aggregate stability. In fertilized pastures, aggregation depends to a great extent on binding by roots. Grass root systems improve aggregate stability. Compared with native savanna and degraded pasture, micro-aggregates were converted to macro-aggregates in the fertilized pastures.; There are two options in considering the replacement of native savannas with more nutritious species: modify the soil by the application of fertilizer and/or other soil amendments (lime, dolomite or gypsum) so that more demanding and nutritious species can be grown, or select higher-quality plants that are adapted to the soil constraints. The economics of cattle production are such that the only real option in the Llanos is to use adapted plants. A possible exception is where pastures are integrated with crop systems. This option creates a win-win scenario, that is, improving the soil chemical and physical properties and increasing both the agricultural production and C accumulation in the soil.