Some global estimates have been made about the potential for CS in drylands. The total amount of C loss as a consequence of desertification may be 18 to 28 Pg (1015 g, or 1 gigaton) C (Lal, 2001). Assuming that two-thirds of the C lost (18 to 28 Pg) can be resequestered (IPCC, 1995) through soil and vegetation restoration, the potential of C sequestration through desertification control is 12 to 18 Pg C (Lal, 2002). The case studies presented here assess the effect of different management practices on soil carbon stocks in various dryland ecosystems.
The effect of climate and/or land use change can be predicted only through the use of accurate dynamic models. Given the difficulty of measuring changes in soil carbon stocks, modeling is a useful tool and has been used as an effective methodology for analysing and predicting the effect of land management practices on soil carbon stocks. A number of process-based models have been developed over the last two decades and are available (as reviewed by Smith et al., 1997). FAO has developed a model in collaboration with the University of Trent (Canada) as a methodological framework for the assessment of carbon stocks and prediction of CS scenarios that links SOC turnover simulation models (particularly CENTURY and RothC-26.3) to geographical information systems and field measurement procedures (FAO, 2004a).
For the case studies, the CENTURY 4.0 (Parton et al., 1987, 1988) model was used. It has been tested against a variety of long-term agricultural field trials and has also been used in a variety of climatic zones, including dryland regions. The ability of any model to predict accurately into the future depends on the accuracy and quality of the data used to parametize the model (climatic, soil, and land management data). Few studies contain sufficiently detailed information and the complete data set required for modeling purposes, particularly in dryland regions where such studies are scarce.
Data from four distinctly different dryland systems in Nigeria (Kano region), Kenya (Makueni district), India (Andhra Pradesh and Karnataka States), and Argentina (Monte Redondo and Santa Maria provinces) were used in investigations carried out by Essex University. Table 21.1 summarizes the main characteristics of these systems.
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