Conservation tillage is any tillage and planting system in which 30 percent or more of the crop residue remains on the soil surface after planting to reduce soil erosion by water. Where soil erosion by wind is the primary concern, conservation tillage is any system that maintains at least 1,000 kg ha-1 of flat, small-grain residue equivalent on the surface throughout the critical wind erosion period (CTIC, 1998). Conservation tillage can include specific tillage types such as no-till, ridge-till, mulch-till, zone-till, and strip-till systems that meet the residue requirements (see Fact Sheet 4.3). These tillage systems are chosen by farmers to address soil type, crop grown, machinery available, and local practice. Although these systems were originally developed to address water quality, soil erosion, and agricultural sustainability problems, they also lead to higher soil carbon and increased fuel efficiency (Table 4-5) (Lal, 1989, 1997; Blevins and Frye, 1993; Kern and Johnson, 1993; Dalal and Mayer, 1996).
No-till methods were initiated in North America in the 1950s; by 1996, the estimated area of conservation tillage in the United States was 42 Mha-about 36 percent of planted cropland (CTIC, 1998). Conservation tillage also has been widely adopted in China (Luo, 1991), Brazil (Hebblethwaite and Towery, 1998), Argentina (AAPRESID, 1999), the Pacific, and other parts of the humid and subhumid tropics (Lal, 1989). Yearly estimates provided by CTIC (1997, 1998, 1999) suggest that conservation tillage adoption appears to have reached a plateau because the area under conservation tillage is believed to have decreased in the United States during the past 2 years. Nevertheless, conservation tillage is now achieving widespread adoption around the world. In Argentina, conservation tillage started in 1990; the area under this type of cultivation system was 7.3 Mha in 1998-31 percent of the agricultural lands of the country (AAPRESID, 1999). The area under conservation tillage in Brazil has now reached 12 Mha (CTIC, 1998).
The opportunity for carbon emissions reduction and soil carbon sequestration in the United States has been estimated at 0.012-0.023 Gt C ha-1 yr-1 (Lal et al., 1998). Globally, conservation tillage can sequester 0.1-1.3 t C ha-1 yr-1 and could feasibly be adopted on up to 60 percent of arable lands (Table 4-5). These estimates depend on continued use of conservation tillage. Use of intensive tillage or moldboard plowing can negate or offset any gains made in carbon sequestration (Lindstrom et al., 1999).
Soil carbon sequestration can be further increased when cover crops are used in combination with conservation tillage. Cover cropping shares some of the environmental benefits of no-tillage, such as reduced soil erosion and a resulting decrease in surface water runoff. The use of cover crops can also aid in reducing the use of herbicides (e.g., smother crops; Pan, 1999) and fertilizers (legumes; Subak, 1999). The magnitude of carbon sequestration from the use of cover crops can be greater than that of conservation tillage (Donigian et al., 1995; Grant et al., 1997; Paustian et al., 1997b; Buyanovsky and Wagner, 1998).
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