Carbon sequestration in soil and vegetation, as well as temporal and spatial variations in relation to land use and management and their related policy issues have major implications for global food security, climate change, and environmental quality.
30.1.3.1 Environmental and Socioeconomic
Context for Soil Carbon Sequestration
Land degradation is a constant major threat to food security, especially in Africa and Asia. Soil carbon sequestration (SCS) can be a major way to counter increases in atmospheric CO2 and to reduce land degradation. The conservation of tropical forests and reforestation activities can offset fossil fuel use. In fact, drastic reductions in rates of deforestation are needed to protect the positive functions of tropical ecosystems. A serious problem of land degradation exists in the humid tropics, and there is an urgent need to find alternatives to slash-and-burn agriculture in this region. Similarly, SCS in countries in the Sahel region may be an important way to increase carbon sinks, control desertification, and promote sustainable agriculture and improved livelihoods for its small farmers.
Degraded lands have low soil C content. SOC concentration in degraded topsoils of Africa varies from 5 to 20 MT ha-1. An urgent need exists to implement SCS practices in order to improve soil quality and farmer livelihoods while removing CO2 from the atmosphere.
30.1.3.2 Land Use, Soil Management, and Soil Carbon Sequestration
There are several examples of SOC sequestration from experiments that combine SCS practices with alternative food production systems. The rate of aboveground C sequestration ranges from 1 to 5 MT C ha-1 year-1 in the tropics, but C stocks associated with the traditional peanut-based cropping system in Sub-Saharan Africa ranged from 5 to 25 MT C ha-1. The clay content in soils has a strong effect on C stocks.
Overall rates of SCS in the tropics are lower than those found in higher latitudes (Lal, 2002). Most C accrual is accounted for by a limited number of plant species in agro-forestry systems that use fruit and palm trees, timber-pasture combinations, and secondary-growth forest. Soil C accrual rates are difficult to estimate due to the presence of charcoal in fire-prone or fire-dependent ecosystems, such as those associated with traditional land managers in Asia, Africa, and Latin America.
Examples from the humid tropics suggest that ecosystem C stocks may vary in size, being highest in natural systems and then declining in relative terms for agroforestry, fallow, tree crops, and annual cropping systems. Improved fallow practices (with Tephrosia spp.) can lead to substantial SCS, but sequestration rates appear to depend on soil type. High SCS rates can be achieved by adopting recommended practices on clay soils, but rates may be only half as high for coarse-textured soils.
30.1.3.3 Modeling and Extrapolating Soil Carbon Sequestration
Simulation models for crop production and soil processes are imperfect but valuable tools. Useful models can be continuously improved through testing. Models can be particularly useful for testing hypotheses related to management strategies designed to reduce atmospheric CO2 and to soil improvement practices.
The methodology related to SCS modeling in developing countries and to extrapolation from these studies includes: (1) collection of experimental data, (2) use of these data to improve model parameters and thus simulation results, (3) extrapolation of results using remote sensing data, and (4) use of data assimilation techniques that are designed to improve soil carbon estimates and to evaluate prediction uncertainties. Long-term studies are needed to validate the models thus developed, and to relate SCS rates to land use, soil management, and agronomic productivity.
30.1.3.4 Environmental and Socioeconomic Analysis of Soil Carbon Sequestration
SCS is related to the environment in numerous ways. It is a resource conservation practice that needs to be competitively and practically justified. A need exists to include full C accounting procedures in order to determine the suitability of specific SCS practices. Full C accounting should consider
C emissions resulting from the use of farm machinery and other agricultural inputs, as well as relative differences in N2O and CH4 emissions. No-tillage agriculture may have more C-based input than conventional tillage. However, any comparative C accounting procedure must use baseline data for the traditional and improved farming systems that are being compared.