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 10 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 10 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 10 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. In 1992, nearly all countries of the world signed the Framework Convention on Climate Change. The longterm goal of this legislation is to stabilize the concentration of greenhouse gases in the atmosphere at concentrations that prevent interference with the climate system. To stabilize or reduce CO2 concentrations, emissions of the gas must either be reduced or transferred from the atmosphere to marine or terrestrial ecosystems. Irrigating arid and semiarid regions may be one method to contribute to C sequestration and, ultimately, reduce atmospheric CO2 concentrations. The addition of water to arid and semiarid soils increases plant growth and, ultimately, C deposition into soil. Entry and others (2002) found that if irrigated agriculture were expanded by 10% into irrigated pastures, a potential increase of 9.3 10 Mg organic C (16.3% of the total C emitted in the next 30 years) could potentially be sequestered. If irrigated agriculture were expanded 10% and the same amount of rain-fed land were converted back to native forest or grassland, a potential increase of 3.4 10 or 6.8 10 Mg organic C (6.0% and 12.9%, respectively, of the total C emitted in the next 30 years) could potentially be sequestered,