115. There are distinct physical, chemical and biological processes that enhance and protect C sequestration in the soil (Figure 16). The net effect of these processes is the slow turnover and prolonged mean residence time (MRT). In Madagascar, for example, Coq et al. (2007) reported that increase in earthworm activity enhanced soil aggregation, improved soil quality and increased SOC pool. In tropical soils of Brazil and Sub-Saharan Africa, Barthes et al. (2008) reported that both total soil C and fine SOM increased with increase in silt and clay contents. Barthes and colleagues also observed that aggregate stability depended closely on Al-containing sesquioxides. Residue management experiments in sugarcane plantations in Brazil showed that leaving cane residues on the soil surface rather than burning increased SOC concentration at the rate of 2.0 to 2.6 t C/ha/yr, and the increase in SOC pool was strongly correlated with increase in soil aggregation (Luca et al., 2008). According to the hierarchical model, three different classes of SOM, persistent, transient and temporary, are associated with three different physical soil fractions i.e., > 250 µm macro-aggregates, 53-250 µm micro-aggregates and < 53 µm silt-and-clay content, respectively (Tisdale and Oades, 1982). Plante et al. (2006) concluded that micro-aggregate-level-silt-sized fractions best preserved C upon cultivation. Indeed, increases in total SOC under NT over PT management are attributed to both a greater amount of C-rich macro-aggregates (>250 µm) but also to reduced rate of macro-aggregate turnover under NT due to formation of highly stable micro-aggregates within macro-aggregates in which SOC is stabilized and sequestered over the long-term (Six et al., 2000; Denef et al., 2004, 2007). Gillabel et al. (2007) also concluded that increase in SOC pool in irrigated compared with unirrigated land was due to the increase in micro-aggregate-associated C storage since irrigation increased the micro-aggregation. Therefore, irrigation management combined with NT and mulch farming could greatly enhance SCS in arid regions. In California, USA, Kong et al. (2005) concluded that a strong linear relationship between SCS and cumulative C input indicated that these soils had not yet reached an upper limit of C sequestration. Kong and colleagues also observed that C shifted from < 53 µm fraction in low C-input systems to the large and small macro-aggregates in high C-input systems. A majority of the SCS through additional C-input was sequestered in the micro-aggregates-within-small-macro aggregates (Kong et al., 2005). In general, the rate of SCS, at global scale, is estimated at 220 kg/ha/yr (Paustian et al., 1997) to 480 kg/ha/yr (West and Post, 2002). These rates are low in comparison with soil-specific observations made for a wide range of cropping and land use management systems. Thus, the rates of SCS may be increased through adoption of specific SLM technologies for specific soils.
