Irrigation Increases Inorganic Carbon in Agricultural Soils

Inorganic C reactions are among the most important chemical reactions that occur in irrigated soils and may contribute to the total amount of C sequestered in those soils. Because CO2 can escape from soils to the atmosphere or return to precipitate carbonate minerals, soils are open systems with regard to inorganic C. We measured inorganic and organic C stored in southern Idaho soils having long-term land-use histories that supported native sagebrush vegetation (NSB), irrigated moldboard plowed crops (IMP), irrigated conservation (chisel) tilled crops (ICT), and irrigated pasture systems (IP). Inorganic C and total C (inorganic + organic C) in soil decreased in the order IMP>ICT>IP>NSB. We use our findings to estimate that amount of possible inorganic and total C sequestration if irrigated agriculture were expanded by 10%. If irrigated agricultural land were expanded by 10% worldwide and NSB were converted to IMP, a possible 1.60 x 10⁹ Mg inorganic C (2.78% of the total C emitted in the next 30 years) could be sequestered in soil. If irrigated agricultural land were expanded by 10% worldwide and NSB were converted to ICT, a possible 1.10 x 10⁹ Mg inorganic C (1.87% of the total C emitted in the next 30 years) could be sequestered in soil. If irrigated agricultural land were expanded worldwide and NSB were converted to IP, a possible gain of 2.6 x 10⁸ Mg inorganic C (0.04% of the total C emitted in the next 30 years) could be sequestered in soils. Inorganic C sequestered from land-use changes have little potential to make a significant impact on the concentration of atmospheric CO2. However, when coupled with organic C and altering land use to produce crops on high-output irrigated agriculture while selected less productive rain-fed agricultural land was returned to temperate forest or native grassland, there could be reductions in atmospheric CO2.