Problem Statement

Rationale. Inorganic carbon, as calcium carbonate and dolomite, constitutes one of the largest carbon pools in the Earth's surface environment, comparable in magnitude to the organic carbon pool. In arid and semiarid irrigated regions, the soil inorganic carbon pool is usually several times larger than the organic carbon pool. The importance of inorganic carbon to the global carbon cycle is that it can serve as a long-term source or repository for atmospheric carbon dioxide, thereby affecting the atmospheric carbon dioxide concentration.

What is known. The interaction of agricultural practices and inorganic carbon is of major importance. Liming of soils (application of calcium carbonate) potentially can release significant quantities of carbon dioxide to the atmosphere, but in some instances also may serve as a repository. Irrigation practices, especially in arid and semiarid environments, may result in either carbon dioxide release to the atmosphere or storage of carbon, depending on various site- specific conditions, such as hydrological setting, irrigation and leaching efficiency, source of water, irrigation system, and nutrient management. Similarly, fertilizer and gypsum application impact inorganic carbon storage and release of carbon dioxide. Models exist to predict the carbon dioxide production and transport in the soil, thus the carbon dioxide concentration can be predicted as well. Models also exist to predict the soil solution composition and the amount of precipitation or dissolution of carbonate minerals in the soil. The predicted change in inorganic carbon and carbon dioxide release is related to the irrigation water composition, plant water uptake, and soil carbon dioxide content. These models have not been tested extensively to validate the predicted changes.

Gaps. We have no information on the changes in soil inorganic carbon as a result of agriculture and only rough estimates of the predicted impact of various practices on carbon release to the atmosphere. There is only limited, preliminary information on the impact of irrigation on changes in inorganic carbon and carbon dioxide emissions to the atmosphere. We can predict the long-term impact of liming on carbon release to the atmosphere but have no information or data on the rate at which it is released. We can predict the net effect of irrigation practices on carbon release, but such analyses have not been undertaken for specific locations, and the conclusions cannot be generalized to other irrigation basins or districts. Computer models are available to estimate the amounts of carbon release or storage under different management practices, but this information needs to be integrated into a hydrologic model where the transport of the water to either surface or deep aquifers is determined. Similarly, data on fertilizer applications are available, but there is no information about the interaction of the fertilizer and increased biological activity in the soil on the inorganic soil carbon.

Goals

• Determine the impact of major irrigation projects on inorganic carbon storage and emission of carbon dioxide to the atmosphere;
• Develop economically viable management practices that could either reduce carbon dioxide emissions from inorganic carbon or store carbon dioxide in the soil water system;
• Determine the rate and quantity of carbon dioxide released to the atmosphere as a result of liming and gypsum application and the effect of different management practices on that release; and
• Quantify the impact of different fertilizer products on the emission or storage of carbon relative to agricultural soils.

Approach

Soil cores will be collected at intervals over time in major agricultural regions from both cropped and disturbed sites and analyzed for inorganic carbon. The data will be used to calculate the changes in carbon storage and to determine the net effect on carbon dioxide concentrations in the atmosphere. Models then will be developed to predict carbon changes in present systems and to evaluate the impact of various management changes. Recommendations regarding carbon release will be evaluated in terms of other environmental consequences, such as efficient use of water and salt and nutrient loading to ground and surface waters. We also will measure residual inorganic carbon on limed fields and calculate carbon dioxide emission rates under different conditions.

Outcomes

New management practices on irrigated lands will reduce carbon dioxide emissions or facilitate storage of inorganic carbon in agricultural soils and hydrologic systems.

Linkages to Other ARS National Programs

• Water Quality & Management
• Soil Resource Management