Productivity of cropland and grazing land can be enhanced by supplemental irrigation in drought-prone ecosystems and by water table management in seasonally wet soils.
Use and Potential
Irrigated crops are grown on about 255 Mha (FAO, 1999). The potentially irrigable land area in sub-Saharan Africa is estimated at 39 Mha (Hillel, 1997), and there is additional potential in Asia and South America. Most irrigation is in areas with low levels of SOC in the native state. Therefore, there is a large potential for carbon sequestration by the use of irrigation (Lal et al., 1998; Conant et al., 2000). Global expansion of irrigation requires successful resolution of socioeconomic and political issues (Scheumann, 1993; Beaumont, 1996; McCown, 1996). Although drainage of wet soils (organic and mineral soils) enhances crop and animal productivity, it reduces biodiversity. Drainage decreases methane emissions but leads to loss of SOC stock. Therefore, adopting judicious methods of water table management, including sub-irrigation and water recycling, is necessary to SOC sequestration.
Current Knowledge and Scientific Uncertainties
Experimental data on soil carbon (SOC and SIC) dynamics in irrigated soils is scanty. Application of water to drylands influences biomass productivity, increases the amount of residue returned, changes mineralization rates, and alters the carbonate balance. Experimental rates of soil carbon sequestration range from 0.05 to 0.15 t C ha-1 yr-1 for SOC (Lal et al., 1998; Conant et al., 2000) and 0.05 to 0.10 t C ha-1 yr-1 for SIC (Wilding, 1999; Nordt et al., 2000). Excessive irrigation, lack of proper drainage, and use of poor-quality irrigation water accentuate the risks of soil salinization. Use of proper irrigation methods and improved cropping systems is therefore essential to reap the benefits of irrigation in enhancing productivity and soil carbon sequestration.
At the plot or field level, soils can be sampled and SOC changes determined by laboratory analysis. Rates can be developed from selected fields by the use of models along with soil maps and other digital databases to expand localized measurements to the watershed or ecoregions and to national scales. Remote sensing can be used to determine the areas under improved water management. (Determining the area under irrigation is very easy with remote sensing, but assessing areas to which improved water table management is being applied is more difficult.)
The soils in arid and semi-arid regions are inherently low in SOC and would take 50-100 years to reach a steady state. With proper water management, dissolved organic carbon (DOC) would continue to move downward in the soil profile, with the potential to bind with soil minerals and form stable organo-mineral complexes.
Monitoring, Verifiability, and Transparency
Monitoring entails using direct sampling and modeling to quantify changes in SOC levels and using remote sensing to determine the area being irrigated. In areas with low initial SOC levels, changes may be relatively easy to measure over short time periods (e.g., 5-year intervals). This practice is very transparent and can be used, monitored, and verified in all areas.
If the irrigation were stopped, the sequestration would stop. If the soil is tilled, the stored carbon would be lost.
The bulk of sequestered carbon will be in the soil, with residence times of years to centuries.
Irrigation enhances biomass productivity in water-limited agricultural systems. In addition to increasing the risk of salinization, irrigation also requires energy input for pumping and distributing water.
Relationship to IPCC Guidelines
Irrigation and water management practices are not explicitly included in the calculations for soil carbon. The effects of irrigation on residue carbon inputs can be expressed through the choice of "Input Factors." The influence of irrigation on carbon turnover rates and inorganic (carbonate) carbon changes, however, are not dealt with and would require appropriate revisions to the IPCC Guidelines.
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