Many reviews have been published recently discussing options available for soil carbon sequestration and mitigation potentials (e.g. Lal et al., 1998; Metting et al., 1999; Nabuurs et al., 1999; Follett et al., 2000; IPCC, 2000a; Smith et al., 2000a; Cannell, 2003; Lal, 2003b; Freibauer et al., 2004). These options include the management practices that are described below.
For forests, carbon sequestration options include the increase of soil carbon stocks through afforestation, reforestation, improved forest management or revegetation. For croplands, options include zero or reduced tillage, set-aside or Conservation Reserve Program, conversion to permanent or deep-rooting crops, improved efficiency of animal manure use, improved efficiency of crop residue use, agricultural use of sewage sludge, application of compost to land, rotational changes, fertilizer use, irrigation, bioenergy crops, extensification or de-intensification of farming, organic farming (a combination of many different individual practices), conversion of cropland to grassland and management to reduce wind and water erosion. For grazing lands, soil carbon sequestration measures include improved efficiency of animal manure use, improved efficiency of crop residue use, improved livestock management to reduce soil disturbance, improved livestock management to maximize manure carbon returns, agricultural use of sewage sludge, conversion to deeper-rooting species, application of compost to land, fertilizer use, irrigation, extensification or de-intensification of farming and improved management to reduce wind and water erosion. On land undergoing revegetation, options include increasing soil carbon stocks by planting vegetation other than trees with higher carbon returns to soil or with litter more resistant to decomposition. Other potential soil sinks are found in wetlands (protection and creation), urban forest or grassland (protection and creation), deserts and degraded lands (improved management), sediments and aquatic systems (protection) and tundra and taiga (protection) (Smith, 2004).
Most of the estimates for the sequestration potential of the activities listed above range from ~0.1 to 0.8 t C/ha/year, but some estimates are outside this range (Nabuurs et al., 1999; Follett et al., 2000; IPCC, 2000a; Smith et al., 2000a; Lal, 2003b). When considering soil carbon sequestration options, it is also important to consider other side effects, including the emission of other GHGs. Smith et al. (2000c, 2001) have shown that as much as one-half of the climate mitigation potential of some carbon sequestration options could be lost when increased emissions of other GHGs (e.g. N2O and methane) were included. Other authors have found a more extreme effect for zero-tillage agriculture (Six et al., 2004). Robertson et al. (2000) also considered this issue, which is further discussed by Lemke and Janzen (Chapter 5, this volume).
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