Background

Carbon is the element that defines organic compounds, i.e., those derived from plants and animals, and is therefore at the core of life processes. It has been the object of a great deal of research, but much is still unknown about its cycling from the soil into the atmosphere and back into the soil. This cycle includes both organic and inorganic, i.e., derived from minerals, compounds. The increase in atmospheric carbon dioxide, a greenhouse gas, has heightened interest in carbon storage science because of the role of carbon dioxide and other carbon-containing compounds (methane and other hydrocarbons) in climate change. The concentration of carbon dioxide has increased in the Industrial Age from 270 parts per million (ppm) in the atmosphere in 1800 to the current 365 ppm. It is continuing to increase at a rate of approximately half a percent per year. The increase has been due partly to the burning of fossil fuels and partly to changes in land use and management, including deforestation, conversion of grasslands to croplands, and farming practices that accelerated oxidation, or breakdown, of soil organic matter.

Agricultural research has long focused attention on carbon cycling, primarily because of the fundamental role of carbon in plant development but also because of the importance of organic carbon compounds in maintaining soil tilth and productivity. Recently, in response to concerns about global climate change, a strong interest has developed in determining how agricultural activities and practices can be used to store carbon, particularly in soil. Scientific methodology to enhance soil carbon storage requires a holistic investigation of carbon cycle science. Soil carbon storage potential is the subject of both policy and scientific study. A federal multiagency research initiative on the carbon cycle, the U.S. Global Change Research Program Carbon Cycle Initiative, is under development to address carbon cycle science. ARS is participating in this initiative because agricultural activities have a major impact on the carbon cycle

Soil is the largest terrestrial global carbon pool, estimated to be about one-and-a-half trillion tons. This dynamic pool participates in an annual carbon dioxide exchange between the soil and the atmosphere 10 times as large as that emitted by fossil fuel use. In general, a balance is maintained between the carbon dioxide removed from the atmosphere by plants and the carbon dioxide returned to the atmosphere from the decomposition of plant and animal material. However, changes in land use and land management can disrupt that balance. When native forests and grasslands were cleared and plowed to grow crops during the westward expansion of the U.S., plant and soil organic carbon was rapidly decomposed to carbon dioxide; thus, cultivated agricultural ecosystems contributed to the increase in atmospheric carbon dioxide. On average, the carbon content of tilled soils in the U.S. has been reduced about 40% from pre-tillage levels. This historical loss now provides a pool that can be refilled by using management practices such as improved cropping systems, conservation tillage, grass buffers, and measures such as the Conservation Reserve Program (CRP) and wetlands. Soil carbon storage, then, could partially offset fossil fuel and other carbon emissions, at least until soil storage capacity is reached. Moreover, best management practices for both cropped and grazed lands improve soil resources while increasing carbon storage.

We need a better understanding of the dynamic path of carbon storage. The amount of carbon stored in the soil is determined by the balance of two processes--production of organic matter by terrestrial vegetation (photosynthesis) and decomposition (respiration) of organic matter by soil organisms. Each of these processes is controlled by physical and biological factors. For a given plant type, photosynthetic production depends largely on climate (solar radiation, temperature, rainfall), soil water status, nutrient availability, and carbon dioxide concentration, the latter providing a potential positive aspect of rising atmospheric carbon dioxide levels. Land use options for enhanced carbon storage include identification, protection, and selective management of the most productive native and agricultural ecosystems. Genetic improvements in vegetative photosynthetic capacities offer greater potential ecosystem capture of carbon dioxide and ultimate soil carbon storage.

Increased soil carbon storage is a co-benefit of conservation policies and efforts to reduce soil erosion on agricultural lands, which include the retirement of marginal or degraded croplands under the CRP and Highly Erodible Land conservation subtitles of the Food Security Act of 1985. This act allowed the voluntary retirement of more than 36 million acres of erodible lands for reseeding to perennial grass and tree covers. In addition, a projected two million miles of buffers along stream banks will be established to protect water from potential nonpoint source pollution. Key issues facing resource managers and policymakers are how to manage these vast resources for optimal economic returns while maximizing carbon stored under site-specific soil conditions and how to preserve much of the carbon pool upon termination of these programs.

Estimates indicate that each year in the U.S. about one-and-a third billion tons of carbon are removed from the atmosphere as carbon dioxide by the photosynthetic activity of agricultural crops. Furthermore, indications are that the North American continent is potentially a large repository for carbon. A portion of the fixed carbon within plants ultimately enters the soil, but the capacity of soils to store carbon, the length of time the carbon can be stored in the soil, and the rate at which carbon storage could be accomplished are matters of great interest to both scientists and policymakers. Because of the historical loss, there is no doubt that soil can serve as a carbon repository. Scientific studies have shown that proper management practices such as conservation tillage can increase soil carbon levels. The debate centers on the capacity of soils to store carbon while remaining in productive use and on the policies that will maintain agricultural productivity and maximize carbon storage and climate change benefits. If 5 -15% of the U.S. production of nonfood plant components in croplands were stored in soil organic carbon, an annual carbon storage rate of 70 to 200 million tons could be achieved. These are highly significant quantities of carbon, enough to store all the carbon dioxide emitted from agricultural activities and some of the fossil fuel emissions from other sectors of the U.S. economy. U.S. agriculture would be a net repository of greenhouse gases, helping to mitigate potentially harmful global changes.

Vision

Reduced risk of global climate change and enhanced soil resources through soil carbon storage research

Mission

Conduct and transfer the results of research to identify the best practices for storing carbon from atmospheric carbon dioxide in natural soil and plant systems to reduce greenhouse gases and enhance soil resources

Table 2. ARS locations contributing to the Carbon Cycle and Carbon Storage Component of the Global Change National Program

Global Change National Program Components
State	Locations	Carbon Cycle and Carbon Storage	Trace Gases	Agricultural Ecosystem Impacts	Changes in Weather & the Water Cycle at Farm, Ranch, and Regional Scales
AK	Booneville	X
AL	Auburn	X	X
AZ	Phoenix	X	X
AZ	Tucson	X			X
CA	Fresno	X
CA	Riverside	X
CO	Akron	X
CO	Ft. Collins	X	X		X
FL	Gainesville	X
GA	Tifton	X			X
GA	Watkinsville	X
IA	Ames	X	X
ID	Boise				X
ID	Kimberly	X
IL	Champaign	X
IN	West Lafayette	X
MD	Beltsville	X	X		X
ME	Orono	X
MN	Morris	X	X
MN	St. Paul	X
MO	Columbia	X
MS	Oxford	X			X
MS	Stoneville	X
MT	Miles City	X
MT	Sidney	X
ND	Mandan	X
NE	Lincoln	X	X
NM	Jornada	X		X
NM	Las Cruces				X
OH	Columbus	X
OH	Coshocton	X			X
OK	El Reno	X			X
OK	Woodward	X
OR	Corvallis	X
OR	Pendleton	X
PA	University Park	X		X
SC	Florence	X	X
SD	Brookings	X
TX	Bushland	X	X
TX	Temple	X		X	X
TX	Weslaco	X
WA	Prosser	X
WA	Pullman	X	X	X
WV	Beaver	X
WY	Cheyenne	X

Problem Statement

Rationale. Agriculture can both impact and be impacted by global change. One of the major impacts is associated with greenhouse gas emissions. Cultivated agricultural ecosystems are a carbon dioxide emission source, with estimates suggesting that perhaps 40% of the pre-settlement soil organic carbon has been lost to the atmosphere. Agricultural soils are important in the global context not only because of the annual carbon dioxide exchange with the atmosphere but also because carbon storage in these soils is sensitive to management practices such as cropping systems and tillage. With improved conservation management practices, crop production systems also can become soil carbon repositories. However, the roles of tillage and soil erosion in carbon loss are not clearly understood because the erosion mechanics for transport of carbon and tillage-induced carbon loss have not been clearly identified. Agricultural conservation systems accomplish carbon storage relatively quickly and inexpensively and also buy time to develop new technologies to solve larger, long-term greenhouse gas emission problems.

What is known. Carbon can be stored in agricultural soils in organic or inorganic form, with the organic the most dynamic and complex. Higher amounts of carbon in soil enhance soil productivity, fertility, water-holding capacity, and other soil conditions that reduce erosion and control nutrient and pesticide availability and the release of these chemicals into the environment. Among practices that aid organic carbon storage are increased cropping intensity, conservation tillage, cover crops, crop rotations, and manure or other organic amendments. Estimates indicate that U.S. agriculture removes about 200 million tons of carbon as carbon dioxide from the atmosphere each year. A portion of the carbon within plants ultimately enters the soil, where a fraction of it may reside for hundreds of years, but the ability of the soil to store carbon over long periods of time and the rate at which it can be accomplished is a highly debated scientific question. Furthermore, recent measurements indicate that soil carbon can be rapidly lost following certain types of tillage.

Gaps. Key issues for the fate of global carbon dioxide are how the management of agronomic inputs impacts soil carbon storage; how the maximum potential carbon storage of a soil can be estimated; how long it takes to attain the storage potential; how long it resides; what the regional differences are; and how changes in the global environment, such as increased atmospheric carbon dioxide levels and weather patterns, impact soil carbon cycling. The role of plants with genetically modified physical and chemical attributes in soil carbon storage is unknown. The role of the carbon-to-nitrogen ratio of crop residue in greenhouse gas emissions and the dynamics associated with the equilibrium carbon-to-nitrogen value in a specific soil are not clearly understood.

Goals

• Define animal and cropping system effects, including tillage and residue management, on soil carbon storage, rates of soil carbon change, and carbon quality in different soils and climatic zones, including analysis of long-term experiments;
• Quantify inorganic fertilizer and organic byproduct effects on plant growth and soil carbon storage, rates of soil carbon change, and carbon quality in different soils and climatic zones;
• Quantify impacts of global change on soil carbon storage, rates of soil carbon change, and carbon quality in different soils and climatic zones;
• Define environmental and economic co-benefits of agricultural practices that reduce production risks, promote soil carbon storage, achieve agricultural profitability and sustainability, and improve soil productivity; and
• Develop monitoring protocols, sampling frameworks, and verification schemes to evaluate impacts of land use changes and management practices on greenhouse gas emissions and carbon storage and to document potential carbon credits in an emissions trading system.

Approach

Emphasis will be placed on measuring soil carbon storage and understanding carbon dynamics using an interdisciplinary, multidimensional approach that brings together physical, chemical, and biological processes and properties. As a baseline, existing experimental carbon data on crop production systems can be analyzed to determine the state of current knowledge on soil carbon storage in major U.S. agricultural lands. Methods must be developed to estimate upper limits of soil carbon storage by soil type, farming systems, eco-regions, and nationally to quantify potential reduction in greenhouse gas emissions attributed to agriculture. Existing carbon flux methods and networks (systems to measure exchanges of carbon dioxide between soil and the atmosphere) should be expanded to regions not currently covered and to other ecosystems and management strategies. Resulting data will be used to develop and test models that predict effects of shifts in cropping and tillage systems and land use changes on soil carbon input and storage.

Outcomes

• New and improved systems of management practices will promote and preserve stored soil carbon.
• Improved management will increase the amount and rate of soil carbon storage in grazed and cultivated lands.
• Conservation crop production will remain economically viable, meeting the food and fiber needs of a growing population, and will help reduce fossil fuel use and atmospheric carbon dioxide..
• Scientifically based information will be communicated to provide a solid foundation for national natural resource stewardship and energy conservation policies.
• Soil resources and air and surface and ground water quality will be enhanced.
• New tools for a wide range of spatial and temporal scales will lead to regional assessments of carbon and nitrogen fluxes which, in turn, will be used to quantify multiple environmental benefits of soil carbon storage and conservation agriculture.

Impact

Improved cropping and tillage systems that supply high quality food and fiber while reducing agriculture's impact on the environment through reduced greenhouse gas emissions and enhanced soil carbon storage

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Manure and Byproduct Utilization
• Soil Resource Management

Problem Statement

Rationale. Grazinglands (i.e., rangelands and pastures) and conservation seedings comprise grasslands that make up about 40% of the land area of the U.S. They contain large soil organic and inorganic carbon stores and are important because of their contribution to animal forage production and ecosystem health.

What is known. One of the concerns about the increasing atmospheric concentration of carbon dioxide is the effect on plant community-level responses in natural ecosystems. For example, in some instances, livestock grazing has enhanced carbon storage, but in some cases this increase has been at the expense of undesirable plant community shifts and reduced forage production, a trend that would continue under elevated carbon dioxide. Also, the recent invasion of traditional grasslands by shrubs, generally considered a negative change in the plant community, has been attributed by some to increased atmospheric carbon dioxide levels. This major change in the plant community may alter not only the amount of carbon allocated to below-ground processes, but also the distribution of carbon in the soil profile.

Gaps. We lack information on the potential capacity of grazinglands to store carbon and the rate of carbon accretion under various geographic and climatic conditions across the U.S. Key challenges include devising management schemes to maintain or enhance both grazingland production and carbon storage among diverse soil conditions and types. Management schemes should consider the effects of forage accumulation, grazing management, improved species, and fertility management. Under rangeland conditions, we need to consider the effects of stocking density and changing vegetation structure on processes and interactions that limit carbon storage potential, particularly during periods of stressed growing conditions.

 

Goals

• Quantify the magnitude and rate of change of soil carbon storage with different land use management practices, in different ecoregions, and under different plant communities;
• Determine the rate and extent of soil carbon storage on a regional or soil basis, including the potential of restorative management such as CRP and buffer-strip initiatives;
• Identify and quantify secondary benefits of soil carbon storage; and
• Quantify carbon dioxide fluxes on a seasonal basis under different ecosystems.

Approach

Methodology and parameters in future data collection will be coordinated among locations to fill gaps in existing data on carbon storage in grazinglands, CRP, and buffers and to develop experiments. Existing carbon dioxide flux networks (systems to measure exchanges of carbon dioxide between soil and the atmosphere) should be expanded to regions not currently covered and to other ecosystems and management strategies. Tools must be developed and tested to estimate soil carbon storage in grazinglands, CRP, and buffers. These data then would be provided to develop and verify predictive methods describing the effects of ecosystem changes on soil carbon. Methods must be developed to predict changes in species composition, the range of altered ecosystems, and the resulting differences in potential carbon storage.

Outcomes

• Current and potential soil carbon storage will be estimated for various management and climate conditions.
• The rate of increase in atmospheric carbon dioxide will be reduced.
• Soil resources and air and water quality will be enhanced.

Impact

Enhanced quality of grazinglands, CRP, and buffer strips while attaining maximum potential carbon storage

Linkages to Other ARS National Programs

• Bioenergy & Energy Alternatives
• Food Animal Production
• Rangeland, Pasture, & Forages
• Soil Resource Management

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

Problem Statement

Rationale. Agronomically managed tree farms are important U.S. economic and environmental ecosystem components, representing a potentially large dynamic and manageable repository for atmospheric carbon.

What is known. Land management for plantation-style production of pulp and lumber has intensified in the past two decades, particularly in the U.S. Southeast and Pacific Northwest, on industrial and nonindustrial private woodlands. Increasingly, plantation-style woodlot management blurs the conceptual boundaries between agricultural and forestry practices and objectives. Tree farming also is being adapted as an integrated farming technique with intensive animal feeding operations to manage waste nutrients. Woodlot plantation practices routinely involve intensive fertilization, tillage, chemical weed and disease control, and irrigation, with practices tailored to specific soil, site, and system needs. Intensive management of forest and other tree species has increased yields of lumber, pulp, and other wood products. In irrigated Pacific Northwest poplar plantations, the planting-harvest cycle is as short as five years. In Southeastern Loblolly pine, rotation age wood fiber yields are typically doubled. Maintaining tree farm productivity through cycles of planting genetically superior seedlings and managing for enhanced growth with tillage, fertilization, irrigation, herbicide use, clear-cutting, and replanting, gives sustainable high yields over many cycles. Importantly, tree farming also increases short- and long-term carbon storage. Irrigated Pacific Northwest poplar plantations produce 49 tons of trunk and branch wood and 2 tons of leaves per acre by the 4^th year of production. Maintaining tree farm productivity increases the total amount of carbon that can be captured--in the short-term as paper and wood products and long-term as soil carbon from roots, exudates, and decomposed litter. Intensively managed tree farms have a high potential for storing carbon.

Gaps. Sound estimates of the total amount of carbon stored in intensively managed tree plantations are lacking, particularly the quantity and long-term fate or transformation of carbon in litter, below-ground tissues or exudates, and soil. Time scales associated with the fate of carbon in wood products need to be developed. This documentation will be critical for policymakers considering establishment of carbon credits for tree plantations.

Research to increase the productivity of highly managed tree farms is crucial, both to the economics of tree-producing enterprises and to the issue of long-term carbon storage. Land forms and soils that require tillage need to be identified, to include determining proper tillage practices and conditions (especially soil water) and developing specifically suited tillage equipment. Agronomic research is needed to identify management practice interactions to achieve optimal growth, including refining of site-specific management; potential use of animal, municipal, and industrial wastes; soil property requirements and impacts; and overall system productivity and ecosystem response. Virtually no reliable information exists on tree plantation evapotranspiration responses or irrigation requirements for scheduling to optimize selected system outcomes. One of the most critical gaps in tree farm management is in the areas of root response and below-ground carbon storage.

Goals

• Assess the impacts of intensive tree farm management on the amount and longevity of soil carbon storage by measuring the total amount of carbon stored in intensively managed tree plantations, including soil carbon changes and storage in below-ground tissues or removed as products.;
• Quantify evapotranspiration from rainfed and irrigated intensive tree farm plantations for key species and management systems to optimize water management for desired system outcomes;
• Quantify the carbon cycle time scales for all tree-derived products;
• Provide policymakers with accurate current data to establish carbon credits for tree farm plantations; and
• Address land owner/producer needs for broad reliable agronomic research to increase productivity, waste utilization, and below-ground responses, including carbon storage.

Approach

Existing data will be compiled and studied to synthesize an understanding of the impacts of intensive tree farm management on carbon allocation and to identify knowledge gaps. Simulations and field experiments, conducted in cooperation with industrial and other land owners/producers who are already practicing intensive tree farming, can be used to prioritize research and fill knowledge gaps. These field studies should quantify water, fertilizer, and other resource needs and should consider water quality issues as well as carbon storage. The data and new knowledge from these efforts can be provided to policymakers to quantify and explain the benefits of intensive tree farming as a basis for policy decisions to exploit the potential positive impacts of carbon storage.

Outcomes

• Reliable estimates will be made of the amount of carbon sequestered by intensive plantation forestry.
• Technology will be developed to quantify carbon cycle time scales for all tree-derived products and to determine the fate of carbon in tree-based ecosystem.
• A national carbon credit policy will reward landowners for maintaining sustainable, productive tree plantations that contribute to a healthy environment and public welfare.
• The Nation s tree farms will be more productive, meeting increasing demands for wood and fiber; increasing carbon storage to mitigate rising atmospheric carbon dioxide concentration; and allowing sensitive lands to be removed from traditional forest production.

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Water Quality and Management
• Manure and Byproduct Utilization
• Soil Resource Management

Problem Statement

Rationale. Soil is the largest reservoir of carbon in terrestrial ecosystems. Understanding the mechanisms and processes involved in the accumulation and loss of stored soil carbon provides an opportunity to develop management strategies that increase carbon storage and decrease carbon loss.

What is known. Carbon is stored in the soil in both organic and inorganic forms. Neither the organic nor inorganic pools is homogeneous; both are composed of multiple compounds with a wide range of activity within the carbon cycle. Storage of organic carbon in the soil begins with the entrance of plant- and animal-derived material into the soil. Soil microorganisms control organic carbon cycling and storage in the soil by decomposing dead plant and animal matter and releasing carbon dioxide back to the atmosphere. Important factors regulating this microbial activity include the physical and chemical properties of the plant and animal materials entering the soil, the physical and chemical properties of the soil, and climatic conditions (e.g., temperature and precipitation).

Gaps. The above known factors need better definition as they relate to soil carbon storage. To enhance soil storage of organic carbon, we need to better understand the soil ecology (i.e., how the physical, chemical, and biological factors of the soil interact and affect each other). For example, soil aggregation processes contribute to the physical protection of soil organic carbon, but the relationship between the biological and chemical processes involved in soil aggregation is not well understood. Also needed is a better understanding of physical processes (e.g., erosion, fire, or leaching) by which organic carbon is lost from the soil.

Goals

• Determine the factors controlling the rate, mass, and timing of carbon dioxide sequestered by plants and the amounts and biochemical composition of plant compounds partitioned to above- and below-ground plant organs;
• Determine the fate of plant carbon within the soil, including the spatial distribution of plant carbon originating from above- and below-ground plant organs;
• Determine the processes involved in the physical, chemical, and biological decomposition and transformations of plant-derived carbon;
• Determine the rate of production and turnover of short-, intermediate-, and long-term soil carbon pools;
• Determine the processes and mechanisms of soil carbon loss and transport, including understanding of on-site and off-site impacts; and
• Determine the impact of elevated atmospheric carbon dioxide and climate change on biochemical composition and changes in plant structure and on soil carbon storage processes.

Approach

We must use and assess new and emerging technologies to conduct basic and applied laboratory, greenhouse, and field research to fill identified knowledge gaps. Collaboration among research scientists is necessary to integrate research on the mechanisms and processes of carbon storage, model development, and the development of management practices to enhance soil carbon storage. Interdisciplinary approaches will be important in this research. Archived soil samples from previous studies can be reassessed to glean information on soil carbon changes related to specific long-term management practices. This effort also can benefit from literature searches of past studies on soil organic matter and soil carbon changes related to land use and management practices within land uses. Results of these searches can be used to assess the subsequent effects of these management practices on carbon storage potential. Adjacent sites with similar soil but known long-term differences in land use or management practices should be identified to allow comparison of differences in stored soil carbon.

Outcomes

• The uptake of carbon by plants will be increased, and improved strategies will be developed to store plant carbon as soil organic carbon;
• The amount of plant carbon stored as soil organic carbon will be increased through alteration of the quantity and quality of plant carbon in roots, root exudates, plant residue, and litter;
• The mechanisms and processes controlling uptake, decomposition, storage, and losses of soil carbon will be more predictable;
• Factors that control the production and turnover of various carbon pools under agricultural systems will be more predictable, which will contribute to useful approaches for storing carbon;
• Improved information will be made available on soil erosion, overland movement of carbon, losses of dissolved carbon, losses during burning, and losses from degraded and degrading soils; and
• Management strategies will be developed to protect soil inorganic carbon.

Impact

Enhanced soil and plant productivity with maximum soil carbon storage

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Rangeland, Pasture, and Forages
• Soil Resource Management

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

Problem Statement

Rationale. Most agricultural soils in temperate climates have lost significant amounts of original organic carbon because of excessive tillage. Conservation tillage practices that include reduced and no-tillage farming and increased cropping intensity, along with reseeding of marginal croplands to permanent cover, can increase soil organic matter and store a significant portion of the carbon released during the burning of fossil fuels. However, carbon and nitrogen cycles are linked such that storing carbon in soil requires inputs of nitrogen.

What is known. Several sources of nitrogen contribute to the soil nitrogen pool and can be available for incorporation into soil organic matter. Commercial fertilizer is a major source of nitrogen for conventional farming. Production of commercial fertilizer requires large amounts of energy and consumes fossil fuel. The effect of microbial activity on atmospheric nitrogen, associated with legumes for example, is a low input source of significant amounts of nitrogen. Animal wastes are an important source of nitrogen to the soil, but concentrating animals some distance from the production sites has created distribution problems. Rain and snow annually contribute a small amount of nitrogen to all terrestrial systems. Nitrogen in unharvested plant material is the largest single source of nitrogen returned to the soil in most cropping systems. The availability of this nitrogen for plant use depends on the carbon-to-nitrogen ratio and the quality of carbon in the plant residue. The wider the ratio and the more lignin tissue in the plant material, the slower the release of nitrogen during decomposition. The soil is a major repository for atmospheric methane, but ammonia-based fertilizer has been shown to interfere with methane oxidation.

Gaps. Increasing soil organic matter as stored carbon makes nitrogen less available for plant growth. There are economic and/or environmental problems associated with all available sources of nitrogen, and nitrogen transformations in the soil affect both the storage and release of soil carbon. Both the production and application of commercial nitrogen fertilizer require the use of fossil fuels, thus adding to atmospheric carbon dioxide. Legumes will not economically fit into all crop rotations; methods of increasing nitrogen fixation by free-living (neither parasitic nor symbiotic) microbes are poorly understood; animal wastes are concentrated in locations away from production areas; and deposition of nitrogen in precipitation is a small portion of crop needs. It is known that microbial oxidation of ammonia-containing compounds increases soil acidity, but the amount of acidification and the resulting carbon dioxide emissions have not been quantified.

Goals

• Define cropping systems, by location, that can economically incorporate legumes into the rotation;
• Determine how to promote free-living nitrogen-fixing organism in areas or cropping systems not adapted to use of legumes;
• Quantify the acidification that occurs during the oxidation of organic sources of nitrogen in the presence of growing crops;
• Quantify the impacts of plants grown with elevated carbon dioxide on plant protein (nitrogen) content and on nitrogen requirements for decomposition;
• Determine the effects of elevated carbon dioxide on the processes and mechanisms of soil carbon and nitrogen interactions; and
• Determine the duration and magnitude of interference by ammonia-based fertilizer on methane oxidation.

Approach

Existing experimental data on use of legumes in crop rotations in different geographic and climatic areas will be analyzed to determine where it is feasible to incorporate legumes into crop rotations. Economic models will be used to determine where and when legumes can be used economically. Laboratory and field experiments will be conducted to determine how to encourage nitrogen fixation by free-living microorganisms, and laboratory and field experiments will be used to quantify the acidification that occurs from oxidation of organic nitrogen sources. Long-term research plots and natural systems in various climates will be examined to determine the extent and duration of interference by ammonia-based fertilizers with methane oxidation.

Outcomes

• New and improved management practices will promote the use of legumes or free-living nitrogen fixing microorganisms, reducing the need for fossil fuel based commercial fertilizer.
• The amount of soil carbon in cultivated lands will increase in response to increased cropping intensity and the availability of nitrogen.
• Soil acidification by nitrogen fertilizers will be reduced, decreasing the loss of inorganic carbon from applications of lime or from calcium-containing soils.
• Conservation practices will remain economically viable, meet the needs of a growing population, and will contribute to the reduction of fossil fuel use for food production.

Impact

Increased nitrogen availability to store carbon in the soil, improve soil productivity, and reduce fossil fuel use in food production

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Rangeland, Pasture and Forages
• Soil Resource Management

Problem Statement

Rationale. To determine the soil's capacity to store carbon, it is critical to know the amount and rate of carbon exchanges between soil and the atmosphere. Carbon dioxide uptake by photosynthesis has been extensively researched. In contrast, the major limitation to understanding carbon exchanges, within the context of soil carbon storage, is the rate of carbon return to the atmosphere. Long-term conventional tillage of soils is known to deplete soil organic carbon below pre-cultivation levels. However, it is difficult to quantify and chart the time courses involved and to measure total carbon exchanges that result in soil carbon storage generated by land use and management practices.

What is known. A variety of meteorological, gas chamber, and carbon isotope techniques has been developed for measuring atmospheric and soil carbon interchanges and fluxes. Daily carbon dioxide exchanges can vary widely, with large uptakes of carbon by growing plants during the summer, and large emissions of carbon to the atmosphere in the fall as vegetation dies. Several models of carbon cycling in soil are available, although validation of key components has been hampered by lack of data and shortcomings in measurement technology.

Gaps. Most important to determining the amount of carbon stored in soils is the ability to measure soil carbon content and validate changes to that content over time. Adequate measurement of changes in soil carbon must include evaluation of the physical, biological, and chemical characteristics of soil organic matter and soil inorganic carbon. In addition, stabilities of various physical and chemical components of soil organic matter need to be evaluated. Measurement of changes in soil carbon storage must include sampling schemes that address the spatial and temporal variability of soil carbon; soil bulk density (weight per volume); and chemical, physical, and biological soil properties. We also need rapid analytical and field surveillance methods to extend our predictive capability of soil carbon storage and changes.

Thus far, concerns about changes in soil carbon have focused mainly on those resulting from biological processes; however, changes may result from soil inorganic carbon gains and losses caused by soil erosion, downward movement through the soil profile as dissolved organic or inorganic carbon, and burning. We need methods to estimate these abiotic losses.

Systematic methodologies to determine the impact of land use and management practices on soil carbon storage are needed to predict actual and potential soil carbon storage at local, regional, national, and global scales. Techniques and models are needed to estimate and predict soil carbon storage and storage potentials over similar land management areas from field to regional and national scales. Finally, models must be able to predict the concurrent impact of agricultural practices on both carbon dioxide exchange and the exchange of other greenhouse gases to assess the integrated effect on climate change.

Goals

• Develop tools and techniques to measure carbon exchange processes and to quantify soil organic matter, soil carbon, and soil nutrient (e.g., nitrogen) storage or loss for major agricultural ecosystems and
• Develop predictive tools (models) to understand, integrate, and predict the impacts of land use and management decisions and global change on soil carbon storage in agricultural ecosystems from the local to the national scale.

Approach

Field techniques, including micrometeorological methods and destructive sampling, will be used to measure carbon dioxide balances (increase vs. decrease) over representative landscapes for the long term. Soil sampling and chamber techniques will be used to determine land use and management-induced soil carbon losses. Emphasis will be placed on the development of new techniques to measure the physical, chemical, and biological changes in soil organic matter over time. Subsurface soil water sampling will be used to estimate convective losses of soluble carbon. Models will be developed and used to estimate soil carbon storage and storage potentials over similar land use and management areas and for scaling up from field to regional and national level estimates. This effort will include the use of remote sensing and geographical information systems.

Outcomes

• Tools will be developed to determine carbon budgets on short- and long-term time scales and on field-to-regional spatial scales.
• Standard techniques will be available to sample soils, determine bulk density, and analyze soil carbon.
• Methodologies will be developed to quantify the contribution of agricultural land use and management practices to soil carbon storage
• Precision and accuracy of soil carbon storage estimates will be improved.
• Techniques will be improved to assess changes in soil carbon pools resulting from abiotic processes such as erosion.
• Inventories of soil carbon for U.S. agricultural lands will be improved.
• Models and decision support systems will help to determine the specific amount, quality, and value of carbon storage for various agricultural land use and management practices.

Linkages to Other ARS National Programs

• Integrated Agricultural Systems
• Soil Resource Management