Problem Statement

Rationale. Under the normal range of environmental conditions, water availability is the most limiting factor for production in crop agriculture, range or grasslands, and forests. Hence, production, and therefore short- and long-term storage of carbon in all managed plant environments, is influenced by any management factor affecting water availability and water use efficiency under both rainfed and irrigated conditions. Water availability and use efficiency can be increased through most cropping system components, including species and cultivar choice, tillage systems, mulching, weed control, rotational strategies and irrigation. Past focus has been primarily on yield or harvestable or forageable biomass production. Research is needed because of the potentially large impact of improved water availability and use efficiency on carbon storage and because management for this outcome is likely to differ from current practices for yield optimization.

What is known. About 85% of the cropped land area in the U.S. and a larger fraction of pasture, range, grasslands, and forests are solely rainfed (nonirrigated). In rainfed agriculture, water management is largely indirect, via choice and timing of various other cultural practices affecting the soil/crop water budget. For cropped land, these choices include selection of species and cultivar, planting dates, stand density, tillage regimes, weed control, mulching, surfactant use, fallowing, multiple cropping, root growth-enhancing practices, and various kinds of evapotranspiration management. Important factors for range and grassland are animal choice, vegetative species mix, stocking rates, grazing intensity or timing, and fire. Rainfed cropping system optimization can significantly increase yield and biomass production. It can either raise or lower soil carbon storage by specific effects on soil respiration and soil organic carbon oxidation associated with various facets of cropping system strategy. A substantial amount of the soil organic matter originally present in most American farmlands has been oxidized as a result of predominately conventional tillage-based farming, especially in areas where alternate-year fallowing was once common for nutrient mining and water accumulation. Original soil carbon equilibrium values can be attained or even exceeded on many of these soils through enhanced water management or combined with other cultural practices that conserve soil carbon.

Irrigated agricultural lands are an important U.S. economic and environmental ecosystem component, representing a potentially large dynamic and highly manageable repository for atmospheric carbon. Most irrigated agriculture in the U.S. is in arid or semiarid areas, where native biomass production is relatively low. Arid and semiarid soils also have relatively low native organic matter contents, typically 1-2%. The predominant environmental factor restricting native biomass production and soil organic matter accumulation on these lands is low amounts of useable annual precipitation. On typical arid or semiarid lands, biomass production increases 3- to 25-fold with irrigated agricultural husbandry, compared to native vegetation without irrigation. Depending on temperature regime, soil organic matter accumulation and hence, carbon storage, can be greatly enhanced by irrigation, especially where night and/or winter temperatures are low.

Gaps. Little is known about the effects of various water-impacting cultural practices on above- and below-ground carbon partitioning and/or long-term retention or loss of carbon stored in soil. Rangeland, grassland, and forest management for these considerations is less well researched than crop management. Irrigation of cool climate arid and semiarid lands has high potential for carbon storage above the native equilibrium values, but cropping strategies and management practices, especially irrigation scheduling criteria that balance yield, profit, and carbon storage, have not yet been undertaken. Many irrigation waters and soils are high in carbonates. The effects of irrigation scheduling and other cultural practices on both organic and inorganic sources of carbon are likely to be highly interactive. They may result in different carbon storage budgets compared to strategies developed solely on the basis of either organic or inorganic carbon storage under irrigation. Also to be considered are salt and specific ion accumulations possible with changes in irrigation strategies.

Goals

• Assess the impacts of direct or indirect management of rainfall and/or irrigation for crop, range, grass, and pasture systems on soil carbon storage to optimize the combination of yield, profit, and carbon storage;
• Quantify evapotranspiration from rainfed and irrigated crop, range, grass, and pasture management systems to achieve optimal water management, including irrigation scheduling for the best combination of yield, profit, and carbon storage; and
• Determine the interactions of organic carbon storage and inorganic carbon management in irrigated systems.

Approach

Field, greenhouse, growth chamber, and modeling studies will be conducted to determine the effects of major cultural practice options on indirect water management and consequent carbon storage effects. Given the large body of data on the effects of cultural practices on water availability and use for yield, market value of crops, and above-ground biomass production, a key focus will be to increase data on below-ground carbon effects from root growth, carbon compounds exuded from roots, and measured soil carbon changes. Once enough data are collected to provide reasonable links between above- and below-ground carbon relationships, modeling can take better advantage of existing data. New evapotranspiration data and irrigation scheduling relationships emphasizing links to soil carbon storage should be developed and data accumulated. Studies should be conducted to determine and extrapolate interactions between management strategies for combined carbon management in irrigated systems where large amounts of carbonates exist in the soil and in the irrigation water but where source repository relationships have not yet been determined.

Outcomes

• Improved water management in crop, range, grassland, and other vegetative systems will increase the amount of carbon storage that can be attained.
• Water management strategies, including irrigation scheduling criteria, for farmers, land managers, extension agents, consultants, government agencies, and policymakers will guide farming and land management practices, with accurate assessment of the potential magnitude of carbon storage.
• New water management tools, practices, and information will help meet carbon storage goals.

Impact

Increased carbon storage with optimized agricultural yield and profitability

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Water Quality and Management
• Soil Resource Management