Drylands have particular characteristics that will affect their capacity to sequester carbon. Drylands often experience high temperatures, low and erratic rainfall, minimal cloud cover, and small amounts of plant residues to act as surface cover to minimize radiation impact. As a result, soils in the drylands are, generally, both inherently low in organic matter and nutrients and rapidly lose large proportions of those small quantities as CO2 when exposed by tillage and other conventional practices. Exposed and loosened soils are also highly prone to soil erosion, particularly rainfall patterns that include intense, storm precipitation after long dry periods. The key issue in drylands is therefore to maximize the capture, infiltration, and storage of rainfall water into soils by promoting conditions that accumulate organic matter and increase soil biodiversity. Drylands are particularly prone to soil degradation and desertification, with 70% of the agricultural land degraded. This means that soils have lost considerable amounts of carbon. As a consequence, the C stock of most dryland soils is less than 1%, and in many cases less than 0.5% (Lal, 2002). Increasing soil quality is therefore the main strategy for CS in drylands. Because drylands cover approximately 43% of the Earth's land surface (FAO, 2000), and dryland soils have lost carbon as a result of land degradation, they offer a great potential to sequester carbon (Scur-lock and Hall, 1998; Rosenberg et al., 1999). Furthermore, soil carbon decomposition is also dependent on soil moisture, so dry soils are less likely to lose carbon (Glenn et al., 1993), and consequently the residence time of carbon in drylands is much longer than, for instance, in forest (Gifford et al., 1992). Whereas forest and intensive farming systems may be important carbon sinks, increasing carbon in degraded agricultural soils of dryland regions would also have direct environmental, economic, and social benefits to the local people and smallholders that depend on them.
Although most of the research on soil organic matter dynamics and processes has been conducted in temperate zones, several reviews have highlighted the potential offered by drylands and degraded lands to sequester carbon (Izaur-ralde et al., 2001; Lal, 2001). Agricultural productivity in drylands is not only limited by natural constraints, but also by low input management as a result of limited resources and technologies. The depletion in soil carbon in agricultural soils as a result of land misuse and soil mismanagement, can be reversed. Important strategies to improve productivity include (1) growing adapted species, (2) enhancing water-use efficiency and water retention in soils, (3) managing and enhancing soil fertility, and (4) adopting improved cropping systems (Lal, 2001). Improved cropping systems include crop rotations, planted fallows, residue mulch, conservation of trees, and growing leguminous species. Recommended practices involve soil water conservation and management, irrigation management, soil fertility management either by adding inorganic fertilizers or organic inputs, residues management, and reduced or zero tillage (Lal, 2003). Some of these practices are the main principle of conservation agriculture, which has been proven to be effective in increasing productivity and CS. Furthermore, the fact that conservation agriculture requires much less external inputs makes it more attractive to poor farmers. The case studies presented here analyse the effect of such practices on soil carbon stocks in various dryland systems.
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