The world's croplands hold ~160 Pg C in soil organic matter to a depth of 1 m (Paustian et al., 1998; Jobbágy and Jackson, 2000). The amount of carbon stored, however, is subject to management influence through effects on carbon inputs and losses via decomposition: any practice that reduces input or hastens decomposition will reduce carbon reserves; and a practice that adds more carbon or slows decomposition will build soil organic carbon (SOC).
Historically, agricultural practices have favoured SOC loss, especially in the decades after initial cultivation. Typically, arable soils have lost ~20-40% of initial reserves in the soil solum (Mann, 1986; Davidson and Ackerman, 1993). Globally, cumulative losses of carbon from cultivated soils amount to ~50Pg C or more (Paustian et al., 1998; Lal, 2003, 2004a,c; Janzen, 2005). Why do these losses occur? First, agriculture deliberately removes large amounts of photosyntheti-
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cally fixed carbon, leaving less litter carbon for replenishment of SOC reserves. Second, arable farming often accelerates decomposition by physically mixing residues into the soil, disrupting protective aggregates, or creating more favourable moisture and temperature regimes. Arable agriculture can also hasten SOC loss via erosion, but much of the eroded SOC may be deposited locally, and the net effect on overall carbon storage is still debated.
Recently, there has been growing optimism that at least some of the lost SOC can be recovered with improved farming practices, notably through the adoption of NT techniques (Paustian et al., 1998, 2000; Machado and de Silva, 2001; Díaz-Zorita et al. 2002; Lal, 2004a,b). Although intensive tillage was once necessary to control weeds and prepare the land for seeding, effective herbicides and new seeding implements now allow many crops to be grown without any tillage beyond the placement of the seed itself. Such NT practices can increase SOC because they minimize soil disturbance, avoiding disruption of protective soil aggregates and the mixing of litter into soil. In some cases, notably where yields are limited by drought, NT can further increase SOC by enhancing carbon inputs through better moisture use efficiency.
Typically, soils under NT will gain carbon at rates of ~0.2-0.5 t C/ha/year for about two decades after starting the practice (Paustian et al., 1997; Smith et al., 1998; Six et al., 2002, 2004; West and Marland, 2002; West and Post, 2002; Smith, 2004a,b; Alvarez 2005; Puget and Lal, 2005). But rates of carbon accrual can vary widely. For example, rates of SOC gain as high as 1 t C/ha/year or more have been reported for cropping systems managed under NT in Brazil (Sá et al., 2001; Diekow et al., 2005). Climate seems to exert a strong influence on SOC response to tillage. In a review of Canadian studies, VandenBygaart et al. (2003) found average rates of 0.32 t C/ha/year in cool, dry regions (western Canada) but no consistent SOC gain under NT in cool, humid regions (eastern Canada). Franzluebbers and Steiner (2002) similarly observed a strong climatic influence on SOC gain under NT across
North America; rates were highest in subhumid regions, and lowest in cold or dry climates. In a comprehensive review, Ogle et al. (2005) estimated mean SOC accrual under NT (relative to conventional tillage (CT)) over 20 years of 1.23 ± 0.05 in tropical moist climates, 1.17 ± 0.05 in tropical dry climates, 1.16 ± 0.02 in temperate moist climates and 1.10 ± 0.03 in temperate dry climates. These and other studies demonstrate that, while NT often elicits significant soil carbon accrual, the SOC gains are not consistent or even assured.
Much of the carbon accumulation under NT occurs near the soil surface. In some cases, this surface accumulation is offset by reduced carbon storage lower in the soil profile so that when the entire solum is considered, net accrual of SOC may be minimal (VandenBygaart and Kay, 2004; Gregorich et al., 2005). Considering only the surface layer could lead to an overestimate of SOC gains under NT (Royal Society, 2001; Puget and Lal, 2005).
How much SOC could be stored globally by adopting NT agriculture? According to Lal (2004c), 'conservation tillage' might store 0.1-1.0 Pg C/year. The wide range indicates that such estimates are tentative because of uncertainty over achievable rates of SOC gains and the extent to which such practices can be adopted. Because SOC gain can only continue for a few decades, soil carbon sequestration under NT (and other cropland practices) can only make a minor contribution to reaching long-term CO2 mitigation targets (Smith, 2004a,b). However, in view of its potential for immediate implementation, and many other benefits to soil conservation and improved farming, NT farming remains a practice widely encouraged and increasingly adopted.
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