Conservation tillage is a generic term that includes a wide range of tillage practices, including chisel plow, ridge till, strip till, mulch till, and no till (CTIC, 1998). For more detail, see Section 18.104.22.168.
Use and Potential
The practice came into use during the 1950s for row crop production on erodible land in the midwestern United States. In 1998, about 37 percent of the row crops grown in the United States were sown with a conservation tillage system (CTIC, 1998). The upland area managed with conservation tillage is 12 Mha in Brazil and 4 Mha in Argentina. The majority of the area in conservation tillage in Brazil and Argentina is continuous no-till; this is not the case in the United States. There is potential for expansion of cropland under conservation tillage in Asia, Australia, Africa, and Europe.
Current Knowledge and Scientific Uncertainties
The rate of SOC sequestration by conversion from conventional to conservation tillage in North America has been found to differ among soils, cropping systems, and ecoregions and may range from 0.05 to 1.3 t C ha-1 yr-1, with a mean of 0.3 t C ha-1 yr-1. The rate of sequestration for principal soils and ecoregions must be established through monitoring of carbon dynamics on long-term experiments in different ecoregions. The net rate of sequestration must be assessed by taking into consideration the carbon used in herbicide production and application, which differs among tillage systems. The amount of carbon residue returned to the soil is an important factor because such residue is often removed for use as fodder and fuel.
Rates of SOC sequestration for specific types of conservation tillage can be established for predominant cropping systems on the basis of long-term benchmark experiments, on-site sampling, and modeling. The rates differ depending on the amount of soil disturbance and the quality and quantity of crop residue returned (Lal, 1997; Paustian et al., 1997a). The rate of carbon sequestration can be quantified on the basis of ground cover, residue returned, and cropping systems determined through remotely sensed data. Scaling from local to regional and national levels can be done by using a combination of soil maps, cropping reports, yield data, modeling, and GIS.
This practice can increase the SOC stock in the soil profile for 25-50 years or until saturation is reached. The rate of carbon sequestration may be highest in the initial 5-20 years.
Monitoring, Verifiability, and Transparency
The amount of new carbon sequestration and its residence time (turnover rate) can be verified through ground truthing (on-site sampling). SOC content and bulk density can be measured at the same location over a period of time (e.g., 3- to 10-year interval) to a depth of 1 m. Because of the stratification of SOC, soil samples must be taken in small depth increments in the surface layers. For a few sites, the rate and magnitude of newly sequestered carbon can be determined by soil sampling and measurements of residue returned. Monitoring/verification of tillage practices can be carried out through ground surveys and potentially through the use of remote-sensing techniques to assess surface residue coverage. The modus operandi of conservation tillage is well known. The rate and type of herbicide use may differ among soils and ecoregions. Effective weed control and use of proper seeding equipment to ensure a good crop stand are important to ensure successful adaptation.
Reversion to more intensive tillage can cause loss of sequestered carbon.
SOC sequestration through conservation tillage depends on continued use. Reversion to conventional methods (high degree of disturbance) can cause loss of sequestered SOC. Policy measures must be in effect to ensure that conservation tillage is adopted on a continued basis. Adoption of conservation tillage has numerous ancillary benefits. Important among these benefits are control of water and wind erosion, water conservation, increased water-holding capacity, reduced compaction, increased soil resilience to chemical inputs, increased soil and air quality, enhanced soil biodiversity, reduced energy use, improved water quality, reduced siltation of reservoirs and waterways, and possible double-cropping. In some areas (e.g., Australia), increased leaching from greater water retention with conservation tillage can cause downslope salinization. In wet years, planting may be delayed in no-till systems, potentially resulting in a yield reduction.
Relationship to IPCC Guidelines
Tillage effects on soil carbon stock changes are included in the IPCC Guidelines (Reference Manual), and default values for three levels of tillage intensity are provided.
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