National Program 204: Global Change
|FY 2001 Annual Report
Considerable attention was focused on global change science in the United States and around the world in 2001. President George W. Bush's announcement in June that the United States would address climate change through two new national climate change research and technology initiatives, rather than through obligations under the international Kyoto Protocol, set some intense research planning activities in motion at the Federal agency level. As background papers and draft proposals for the Administration's new initiatives were prepared, members of the Agricultural Research Service (ARS) Global Change National Program Team and ARS scientists provided the scientific expertise on many aspects of the role of agricultural research and technology development in understanding, responding to, and mitigating global change. Members of the National Program Team continued to represent ARS and the Department of Agriculture (USDA) in the numerous interdepartmental and interagency committees that coordinate global change research throughout the Federal government, including the USDA's own Global Change Task Force, as well as various interagency working groups of the U.S. Global Change Research Program (USGCRP).
In accordance with the action plan, the following components are addressed within the USGCRP: 1) Carbon Cycle and Carbon Storage, 2) Trace Gases, 3) Agricultural Ecosystem Impacts, and 4) Changes in Weather and the Water Cycle at Farm, Ranch, and Regional Scales
Carbon Cycle and Carbon Storage
Carbon sequestration in croplands and grazinglands continues to be of great interest to the science, policy, and agriculture communities. Much of the increase in concentration of CO2 in the atmosphere during the late 19th and early 20th centuries can be attributed to changes in land use, particularly tillage practices and clearing of forests for agriculture. In the latter half of the 20th century, changes in tillage practices have changed North American croplands from net sources to net sinks of carbon, in effect, "refilling" the reservoir of soil carbon that had been released. Carbon storage in the land removes carbon from the atmosphere, possibly slowing the rate of increase in global warming, and providing broad benefits to soil, water, and air quality nationwide.
ARS is a leader in terrestrial carbon cycle and carbon storage research. The National Program Team actively participates in identifying and prioritizing goals in this area of science through the USGCRP Carbon Cycle Interagency Working Group. This working group completed a draft of an interagency implementation plan for carbon cycle research and initiated planning for a major initiative in understanding the carbon cycle in North America. The agricultural science community will be essential to the success of these plans. In international activities, a member of the National Program Team was the U.S.side coconvenor of the 9th U.S.Japan Workshop on Global Climate Change, entitled "Carbon Cycle Management in Terrestrial Ecosystems." Several ARS scientists led working groups at this invitational meeting, held in Tokyo, to develop research recommendations and foster international cooperation in research on managing carbon in the land.
Understanding the duration and fate of carbon pools in soil is a key element of managing the land to sequester carbon from the atmosphere. Together, croplands and rangelands comprise more than half of the land area of the 48 coterminous United States, providing an important opportunity for managing resources for carbon storage. ARS scientists at the ARS National Soil Tilth Laboratory (NSTL), Ames, Iowa, studied different aspects of carbon turnover in cornsoybean fields. One experiment has been initiated to quantify CO2 and water vapor exchanges in corn and soybean crops over different soils and nitrogen management. Beginning after the Fall 2000 harvest, CO2 exchanges were monitored through the winter and the 2001 growing season. As much as 40 percent of the carbon incorporated into crop biomass originated from carbon respired from the soil. As the plant residues decompose, a small amount of carbon is incorporated into stable compounds that may persist in soil for centuries. Other experiments at NSTL showed that one class of stable compounds, accounting for approximately 20 percent of the total soil organic carbon pool, consists of highdensity complexes of carbon and metal with a high ratio of carbon to nitrogen, making them very resistant to microbial breakdown. Another class of compounds accounting for approximately 45 percent of the soil carbon, is made up of lowdensity coatings with low carbon/nitrogen ratios on the surfaces of clay particles, which are available for microbial use. However, since soil particles are easily moved by subsurface water, stable forms of carbon might be lost through movement of soil in water. To examine those potential losses, ARS scientists at the North Appalachian Experimental Watershed Research Unit in Coshocton, Ohio, measured soil organic carbon concentrations in spring water from pastured watersheds and in percolate from lysimeters placed in cornsoybean rotation. Concentrations of soil organic carbon in these water samples were extremely low, indicating that subsurface flow in soils of the north Appalachian region contributes to minimal losses of soil carbon. Other environmental factors are important in carbon cycling in soil. Scientists at the ARS Soil Dynamics Research Laboratory (SDRL) in Auburn, Alabama, showed that temperature and the concentration of CO2 in the atmosphere can influence the turnover of organic carbon in soil. Soil samples were collected at the end of a 5year study of sorghum crop responses to CO2 enrichment and incubated at 20, 25, or 30C. Higher temperatures increased carbon turnover and nitrogen mineralization, but the temperature effect was not influenced by the CO2 concentration to which the crop had been exposed. Although increasing CO2 itself may tend to increase carbon storage in soil, the increase in temperature accompanying the rise in CO2 may offset increased storage due to more rapid rates of nutrient cycling. Even gases that do not contain carbon may influence carbon cycling. In two experiments conducted by ARS scientists at the ARS Air QualityPlant Growth and Design Laboratory, Raleigh, North Carolina and SDRL, leaf tissues from soybean plants grown in air enriched with ozone (O3, an air pollutant that contributes to global warming and is highly toxic to plants) was 45 to 60 percent slower than residues from plants grown in air cleaned of most of the O3. In one of the experiments, leaf residues from plants grown in a CO2enriched atmosphere decomposed 40 percent slower than residues from plants in an ambientCO2 atmosphere. Results show that chemical changes in the atmosphere expected during the 20th century may affect rates of carbon cycling. These studies provide insights into the complexities of understanding how changing environmental conditions may alter aspects of carbon cycling through influences on the sources, forms, and transformation rates of carbon in important agricultural systems of the United States.
Much of the carbon dioxide released by the burning of fossil fuels each year is absorbed by plants on the North American continent. The extent to how much a society can depend on this massive continental "sink" for sequestering carbon from the atmosphere and mitigating global change is a critical question for land managers and policy makers. Grasslands in the Northern Great Plains are vast ecosystems, yet their role in the overall storage of carbon in North America has not been quantified. Carbon dioxide is removed from the atmosphere by grassland vegetation during only about half the year, but prairie grassland soils are biologically active and release carbon dioxide back into the atmosphere yearround. ARS scientists at the ARS Natural Resource Management Research Laboratory in Mandan, North Dakota, assessed the impact of carbon dioxide from the soil on annual exchange between the land and the atmosphere for a grazed prairie ecosystem. The plant canopy gained an average of 1.7 grams of carbon dioxide per square meter of soil surface each day during the sixmonth growing season. Losses from the soil during the dormant winter season averaged the same amount of carbon dioxide or greater, depending on the measurement method used. These results show that it is essential to measure the exchange of carbon dioxide between the land and the atmosphere even during the dormant season if the role of grassland ecosystems in mitigating greenhouse gases is to be determined.
The volume of literature on the terrestrial carbon cycle and the influence of land management on soil carbon continues to grow rapidly. ARS scientists meet a great need to analyze and draw conclusions from a wide range of information becoming available from disparate experiments. An ARS scientist at the ARS Environmental Plant Dynamics Laboratory (EPDL), Phoenix, Arizona, collaborated with scientists in Italy and Japan to analyze carbon sequestration data from freeair CO2 enrichment (FACE) experiments conducted on cotton and wheat at Maricopa, Arizona, (by ARS scientists and cooperators) and on ryegrass and clover in Switzerland. Variability among the measurements was high, but in 13 out of 13 reports, CO2 enrichment increased the average carbon content of the soil. In an aggregate analysis of these reports, CO2 enrichment under conditions in which nitrogen was not a limitation to plant growth increased soil carbon by 11 percent. This analysis suggests that the rate of increase in atmospheric CO2 concentration is being slowed somewhat by increasing retention of carbon in soil. Another significant contribution to the synthesis and integration of soil carbon research is a new book edited by an ARS scientist with other USDA and university collaborators, "Assessment Methods for Soil Carbon" (Lewis Publishers, Boca Raton, FL; 2000), which includes many chapters written by ARS researchers. The book provides some standardized methods for measuring the different soil C pools to address problems with C and related data, provides means to scale point data for use at many different levels, provides a method for sampling to measure the different pools and flux rates, addresses schemes for C trading, and provides methods for verifying changes in C stocks and rates of sequestration. This book is the latest in a series by these scientists and offers the science and policy communities a single authoritative source for information on measuring carbon for different purposes.
After CO2, methane is the most important gas in terms of its contribution to global warming, accounting for 9 percent of all greenhouse gas emissions in the United States. Certain agricultural processes, especially digestion by ruminant livestock, are significant sources of methane. Because of its residence time in the atmosphere and its heattrapping properties, a given amount of methane contributes 21 times the global warming potential as the same amount of CO2. Nitrous oxide, another greenhouse gas arising from microbial processes in soil, is even more influential in contributing to global warming, with 310 times the global warming potential as CO2. Thus, management of agricultural resources in ways that prevent emissions of methane from livestock, manurehandling systems, or watersaturated soils, or nitrous oxide from soils, can make significant contributions toward mitigation of global change.
Rice is one of the world's most important crops. Certain microbes in flooded rice fields are a significant source of methane around the world, although this contributes only 0.2 percent of the greenhouse gas emissions in the United States. Other microbes in paddy soils can break down methane by oxidizing the gas, which is an important natural feature for mitigating methane emissions to the atmosphere. Fertilization with ammonium‑based compounds can decrease the oxidation capacity of rice field soils, thereby increasing net methane emissions to the atmosphere. ARS scientists at the ARS Soil, Plant and Nutrient Research Laboratory (SPNRL) in Ft. Collins, Colorado, conducted a series of studies on the effect of ammonium fertilizer on methane oxidation in rice field soils from China. The inhibitory effect of ammonium fertilizer was short‑lived, depending upon the methane concentration. Considering the fact that methane concentration in paddy soils is generally high when fields are drained, it is apparent that the initial effect of inhibition of methane oxidation by ammonium fertilizer is a temporary phenomenon and has little impact on net methane emissions from rice fields globally.
Technologies developed by ARS scientists in Watkinsville, Georgia, for measuring trace gas emissions from agricultural systems are being used to study greenhouse gas emissions for nongovernmental organizations and foreign government agencies. These technologies are intended to be noninvasive or noninterfering to evaluate emissions from difficulttomeasure systems, such as confined animal production systems and openfield cropping or grazing conditions. Those examining the technologies include the Utah Pork Producers Council (to investigate producer compliance with government emissions regulations) and research institutions of Canada and Japan (for evaluation of greenhouse gas emissions from animal and cropping systems). These techniques will greatly facilitate the measurement of trace gases from agriculture by using less expensive sensors and reducing laborintensive efforts of other techniques.
Agricultural Ecosystem Impacts
Ecological processes contribute to the causes of global change, and components of ecosystems are indicators that environmental changes occur. Agricultural ecosystems are embedded in the larger landscape and are important due to their land coverage. They are significant in the scientific community's efforts to quantify, understand, respond to, and mitigate global environmental changes. Articles in the scientific literature and the popular press continue to suggest that the welldocumented increases in atmospheric CO2 concentration will only benefit food and fiber production. However, ARS research demonstrates the risks in drawing simple conclusions about the complex interactions among atmospheric chemistry, weather variability, crop and livestock characteristics, pests, soil properties, and other variables. In February 2001, a member of the National Program Team provided ARS' expertise on ecosystems and their goods and services as part of the U.S. State Department delegation to a session of the Intergovernmental Panel on Climate Change (IPCC). This IPCC session convened in Geneva, Switzerland, to review, edit, and ratify the Summary for Policy Makers of IPCC Working Group II Impacts, Adaptation, and Vulnerability. USGCRP research on ecosystems and global change is informed directly by ARS research; a member of the National Program Team cochaired the interdepartmental team that prepared the ecosystem science section of the USGCRP's draft 10year strategic plan and serves as Chair of the USGCRP's adhoc Ecosystems Interagency Working Group.
Rising atmospheric CO2 concentrations can increase photosynthesis of many plants which under otherwise favorable conditions may lead to increased growth and productivity. However, predicted climate changes anticipated with the rise in CO2 may drastically decrease productivity and offset the gains from CO2. Projecting food availability and quality depends on studies that examine several factors at once. In cooperation with University of Florida researchers, ARS scientists at the ARS Crop Genetic and Environment Research Laboratory in Gainesville, Florida, measured leaf photosynthetic capacity, carbohydrate metabolism, and growth of 2‑year old sweet orange trees grown for 29 months at ambient or doubled‑ambient CO2 in controlled, temperature‑gradient greenhouses at 1.5 and 6.0 degrees Celsius above Gainesville temperatures. Increased CO2 caused decreased water loss from leaves and increased photosynthesis and growth, despite a decrease in amount and activity of an important photosynthetic enzyme. Elevated temperature caused increased water loss from leaves but no change in photosynthesis or growth, despite a decrease in the amount, but not the activity, of the same enzyme. Results of this study indicate that citrus, an important crop grown on 1.1 million acres in the United States, should function well under rising CO2 and with moderate increases of temperature, in the absence of other stresses or limitations. In contrast, this same research team found that twiceambient CO2 increased soybean seed yield by 30 percent, but increased temperatures expected to accompany the rise in CO2, suppressed the number of seeds produced, inhibited seed development, lowered seed carbohydrate content, and altered fatty acid ratios in the seeds. These experiments illustrate how expected environmental changes may alter yield and quality of different crops in different and unanticipated ways.
Two other potential difficulties in generalizing about global change and crop yield have been addressed by an ARS scientist at the EPDL, Phoenix, Arizona, with university and international collaborators. Studies of crop responses to increasing CO2 in the atmosphere have been conducted in different locations with different experimental methods, leading to uncertainties about crop yield projections. The research team analyzed reports from experiments conducted around the world over the last decade with freeair carbon dioxide enrichment (FACE), and compared the FACE results with those obtained from experiments in which chambers were used. Generally, the magnitude of the growth and other responses varied among crop types and water and nitrogen status. However, except for lower rates of water vapor loss from leaves and greater stimulation of root systems in FACEgrown plants, the relative responses to elevated carbon dioxide were consistent between FACE and chamberbased experiments. In another analysis, projection of wheat growth and yield by a collection of wheat simulation models, used in attempts to project wheat production under different global change scenarios, were tested for their ability to match results from actual FACE experiments. Simulated and observed results were considered in good agreement, providing evidence in support of model validity. When analyses of results obtained by different methods and models support similar conclusions, the science and policy communities gain confidence in projecting the direct effects of elevated carbon dioxide on crop yields in assessments of future agricultural productivity.
Understanding and projecting the availability of food and fiber as global environment changes requires research at different levels of biological organization. ARS research at the genetic and subcellular levels provides insight and possible tools for studying plant response to environmental changes anticipated in the 21st century. In Beltsville, Maryland, ARS researchers found a modern soybean line and an ancestral line that exhibit greater yield increases in response to CO2 enrichment than many other lines. These lines offer genetic variation for researchers interested in underlying mechanisms of crop response to CO2, as well as for crop breeders who develop new varieties for the 21st century. Although rising CO2 is expected to cause an increase in the global mean temperature, some regions of the world may actually experience cooling conditions during the cropping season as largescale atmospheric and oceanic circulation patterns change. Scientists from the ARS Soybean Genomics Improvement Laboratory, Beltsville, Maryland and the ARS Plant Science Research Laboratory, Raleigh, North Carolina, have identified one of the fundamental mechanisms of freezing adaptation in plants. This process involves the flow of water out of the cell, which increases the concentration of solutes inside the cell and depresses the temperature at which the cell contents freeze. In freezetolerant wheat, there is a water channel in the membrane of cells in the plant crown that is induced by low temperatures; this channel is absent from other tissues in the wheat plant or in wheat varieties that do not adapt to freezing temperatures. This knowledge might be used to identify or develop coldtolerant plants for regions where "climate change" may not involve warming.
At the plant chemistry level, ARS researchers have demonstrated how several important crop properties can be altered in response to global change factors, including characteristics affecting carbon cycling as noted above. Researchers at the ARS Phytonutrients Laboratory, Beltsville, Maryland, demonstrated that relatively mild increases in temperature or severe drought during seed development increased the amount of Vitamin E, an antioxidant in soybean seeds that is an important phytonutrient in soybean products intended for human consumption. In other research on antioxidants in soybean tissues, ARS scientists at SDRL found that activities of antioxidant enzymes in soybean plants changed with CO2 enrichment, but these changes differed between two soybean genotypes with differential resistance to a pathogenic fungus. These effects of CO2 and temperature have implications for how global change may alter plant resistance to pests, diseases, and environmental stresses, as well as affect the nutritional value of food.
How plant roots respond to various aspects of global change may be very important to whether crops and rangeland communities provide expected growth, yield, or other characteristics. Typically, aboveground growth of certain grasses common in warm regions responds most strongly to CO2 concentration when soil water is limiting, but ARS researchers at the ARS Grassland, Soil and Water Research Laboratory (GSWRL), Temple, Texas, found that the increase in root growth caused by CO2 enrichment for two of those grasses dominant in the tallgrass prairie was the same whether soil moisture was abundant or limiting. However, this type of response cannot be generalized to all plant species. ARS scientists at SDRL Auburn, Alabama, integrated data from numerous studies to assess changes in root growth and distribution by plants exposed to CO2 enrichment. The review article published in a prominent international journal discussed consequences of CO2 increases on root distribution by crop plants, which is a critical determinant of plant resource acquisition and subsequent growth. The paper offers an important resource to researchers working on mechanisms of plant responses and projections of crop growth and yield.
ARS scientists working on plant ecology have shown the importance of global change to higher levels of organization in agroecosystems. Researchers at the ARS Alternate Crops and Systems Laboratory, Beltsville, Maryland, found that pollen production by common ragweed is increased by CO2 enrichment, even within the range of CO2 increases that occurred during the 20th century. Production of more pollen by ragweed affects weed ecology through distribution of genetic material into the weed population. An implication of this finding for human health is that increased ragweed pollen production may worsen symptoms of allergy sufferers as CO2 concentration continues to rise. Effects of global change on interactions of plants with microorganisms in the environment can have varied and unexpected consequences. ARS scientists at the ARS Conservation Laboratory in Phoenix, Arizona (with university cooperators) determined the impact of CO2 enrichment and soil moisture on beneficial mycorrhizal fungi associated with sorghum roots in a FACE facility. Compared to ambient CO2, the CO2 enrichment increased the length of fungal hyphae by 109 percent when soil water supply was ample and by 267 percent when moisture was low. Hyphal length is important because the hyphae act as extensions of the root system, benefitting the crop primarily by increasing the soil volume exploited for phosphorus, so greater hyphal length may result in plants with access to more phosphorus. These fungal hyphae also improve soil structure. Waterstable aggregates were increased an average of 30 percent by CO2 enrichment, attributable in part to an increase in the production of glomalin, a substance made by the mycorrhizal fungi that acts as a "glue" for soil particles. Such a change in soil structure could lead eventually to increased water infiltration and decreased soil erosion, conferring indirect benefits of CO2 enrichment to the environment. ARS scientists at the ARS Rangeland Resources Research Laboratory in Cheyenne, Wyoming and SPNRL, Ft. Collins, Colorado (along with university cooperators) have found that doubling of CO2 over ambient levels affects several aspects of the ecology of the shortgrass prairie. Needle and thread, an abundant grass, exhibited increased growth under CO2 enrichment, while other dominant grasses (western wheatgrass and blue grama) did not. Over time, such a differential response could lead to significant changes in the composition and structure of the shortgrass prairie community, including a decline in the dominance of blue grama, which has good forage quality and is adaptable to the harsh, semiarid climate. One consequence of global change in some regions may be an alteration in the success of invasive weed species. The emergence and fate of seedlings of four annual and five woody perennial species of invasive legumes in an undisturbed pasture of coastal bermudagrass were examined by ARS researchers at GSWRL. All annuals died at first frost before they produced seeds, but all perennials survived the season under both wellwatered and dry conditions and resumed growth the following spring. As climatic conditions change in western rangelands, a gradual change in the timing of frosts may alter plant community structure.
Collectively, these studies from ARS scientists working on agricultural ecosystems impacts of global change demonstrate that the ultimate outcome of rising CO2 concentrations and changing climate must be determined from a combination of experimentation and modeling that allows consideration of multiple interacting factors and a complex series of responses at all levels of biological organization.
Changes in Weather and the Water Cycle at Farm, Ranch, and Regional Scales
Water availability and weather are major drivers of agricultural productivity and environmental quality at all scales. As the concentrations of greenhouse gases continue to rise, average weather conditions are projected to change in varied ways in different regions, and weather variability is expected to increase. Projecting water supplies and favorable weather conditions for production of food and fiber in environmentally sound ways is a critical goal of the Global Change National Program. Another is determining the best ways to use scarce water resources to ensure sustainable production and nonagricultural uses in a changing environment. Feedbacks between agricultural systems and the atmosphere that affect the exchange of water and energy between land and air also are of great importance to understanding the global water cycle at various scales. ARS researchers are engaged in research on these topics to provide tools to enable decision makers to manage resources on the farm or in the watershed, and policy makers to formulate policies that meet diverse interests across the nation.
Using seasonal weather forecasts as part of a risk management approach for water resources in agriculture is a goal of ARS scientists at the ARS Grazinglands Research Laboratory in El Reno, Oklahoma. Past seasonal precipitation forecasts for different regions of the country from the National Oceanic and Atmospheric Administration were analyzed for their ability to forecast precipitation amount and variability. Across the continental United States, the forecasts showed a range of success in matching actual precipitation. The analysis identified regions with the best potential for applying forecasts to agricultural planning. Potential benefits of this research include risk management decisions to plan agricultural activities based on forecasted departures from normal precipitation in the regions where the forecasts accurately predicted precipitation.
Longterm field experiments to determine impacts of global change are difficult and expensive. Analyses of the impact of changing weather and climate on crop production are constrained by the lack of longterm field experiments in which production inputs (including technological developments such as improved varieties and increased rates of fertilization) are held constant. An alternative strategy is the use of crop simulation models, which are based on the underlying physiological processes governing plant growth and development. Simulated crop growth was used by ARS researchers at the Pasture Systems Watershed Management Research Laboratory, University Park, Pennsylvania, in cooperation with university partners to study the impact of weather and climate on typical crops grown in the Great Lakes region over the past 100 years without the influence of technological improvements. Simulated corn and soybean yields were found to increase since the late 1930s at most of the study sites due to increased precipitation and more humid conditions. No consistent trends were found for alfalfa. The simulated yields support previous research identifying a period of favorable climate for crop production in the region from 1954‑1973. The study suggests that at least part of the observed yield increases in the region during recent decades has occurred as the result of wetter, less stressful growing season weather conditions. A better understanding of the effect of historical climate change improves our ability to predict how future climate change may affect food production, including crops grown as livestock feed.
Water scarcity in semiarid regions threatens environmentally sustainable growth and agricultural production in many areas of the western United States. The impacts of global change could be especially severe in these regions, where many agricultural and natural ecosystems are already water stressed. ARS led the SemiArid LandSurfaceAtmosphere (SALSA) Research Program, involving scientists from 20 states, 15 nongovernmental organizations, and six foreign research agencies and institutions, in field experiments in the San Pedro Basin of the United States and Mexico. A series of SALSA Program products were released in 2001, including a spatial data archive on CD; a bilingual conference proceedings; a bilingual, multimedia, interactive CD; and an entire issue (including 21 papers) of the Journal of Agricultural and Forest Meteorology. Collectively, these products document advances in: quantifying land use and land cover change in the San Pedro Basin; using advanced technologies to map heat and water losses; developing models to provide daily maps of grassland health and soil condition for improved resource management; and measuring and predicting daily water use by riparian vegetation. The residents, policy and decision makers, and resource managers of the San Pedro Basin were involved directly in the SALSA Program. Data and information products of this program allow resource managers to monitor and project the availability of scarce water resources in the San Pedro Basin. Land use and weather related phenomena such as El Nino and global climate change introduce uncertainties in decision making for water management, and products and tools from the SALSA Project permit scientists, residents, and government officials at several levels to plan appropriately for water uses. The impact of the SALSA Program was recognized by receiving the 2001 USDA Secretary's Group Honor Award for maintaining and enhancing the Nation's natural resources and environment, as well as paving new ground in developing effective means of infusing science into the local policy and decision making process.