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United States Department of Agriculture

Agricultural Research Service


Location: Rangeland Resources Research

2009 Annual Report

1a.Objectives (from AD-416)
Objective I: Assess/project changes in the structure and functioning of Great Plains grasslands due to the interactive effects of elevated CO2 and legume N on primary production, N and C cycling, and plant community dynamics, including invasive weeds. Objective II: Develop management strategies that optimize responses of semi-arid rangeland to global change and minimize emission of greenhouse gases (GHGs).

1b.Approach (from AD-416)
We will use novel Free Air CO2 Enrichment (FACE) technology to expose the native northern mixed-grass prairie (NMP) to a gradient of CO2 concentrations from present ambient levels of approximately 370 to 670 parts per million. Within the CO2 gradient, plots consisting of native NMP and NMP inter-planted with combinations of legume and invasive weed species will evaluate how legume N interacts with CO2 to affect soil C and N cycling, trace gas fluxes, water relations, plant physiology/demography/phenology, reproductive and vegetative recruitment, weed invasion, and plant community dynamics. In a related modeling exercise, data from previous CO2 enrichment and flux experiments will be used to predict long-term weather and global change responses of Great Plains grasslands. A second objective will evaluate the impacts of management strategies on C and N cycling and land-atmosphere exchange of greenhouse gases. The responses of soil C and N dynamics to variable grazing intensity and seasonality will be determined. Inter-seeding legumes into rangeland will be evaluated for its potential to enhance biomass production, forage quality, and mitigate greenhouse gas emissions. This information will be used to develop new management practices that consider trace gas emissions, in addition to more traditional rangeland goods and services.

3.Progress Report
Most of our efforts in global change research this past year were devoted to running and sampling from our Prairie Heating and CO2 Enrichment (PHACE) field experiment. This is the fourth year of this field experiment in which ambient CO2 concentration, temperature and soil water are all being manipulated to further our understanding of how semi-arid grasslands are responding to multiple global change factors. A core group of scientists from ARS (7), the U. of WY (2), CO State Univ. (2), and the Biometeorology Institute in Florence, IT (1), plus several graduate students and post docs are now collaborating on this unique project. The experimental plots are divided into two halves, with one side comprised of a native northern mixed-grass prairie, and the other side seeded under different disturbance regimes with various weed species. This plot arrangement allows us to evaluate both the basic responses of this grassland to climate change, and also to investigate possible consequences for weed invasion. Measurements that are being taken can be roughly broken down into three categories: site environmental, plants, and soils (including soil biota). The site environmental measurements include a suite of measurements that are available to all project collaborators and which describe the overall site environmental conditions at the experimental site, and in many cases, within particular experimental plots. This includes site air temperature, solar radiation, humidity, and wind run, which are collected at a site weather station. At each of the 30 experimental plots, additional measurements of air and soil temperatures (several levels, depths), canopy surface temperature, and soil water content (several depths to 80 cm) are measured continuously. Some of these plot measurements will be used to assess the performance of the CO2 fumigation, warming and irrigations systems. Additional instrumentation is being installed in summer, 2009 to better document treatment effects on microclimate, including leaf and canopy air temperatures and near canopy relative humidity. Plant measurements include monthly plot photography to document changes in plant cover; weekly measurements of phenological attributes of key plant species; mid-summer aboveground plant biomass (by species); periodic assessments of root growth by an underground rhizotron camera system; periodic measurements of plant and system water status; and measurements of stand gas exchange, comprised of canopy CO2 exchange (photosynthesis and respiration) and water exchange (evapotranspiration). Soil measurements include assessments of soil C and N, organic matter, soil fluxes of greenhouse gases, and soil microbes. Isotopes of C and N are being sampled and analyzed to provide insights into plant/soil/microbe/atmospheric cycling of C and N. Also, as a result of grant funding and new collaborations (U. of Boston, CSU, U. of Saskatchewan), new research has been initiated at PHACE on.
1)the involvement of microbial community changes in nutrient cycling,.
2)soil amino acid dynamics, and.
3)the effects of treatments on seedling physiology and establishment.

1. First test of the resource-enemy release hypothesis - Reducing the tremendous economic and environmental costs of invasive weeds requires understanding why they are so successful. Several years ago, ARS scientists in Fort Collins, CO, developed a novel hypothesis, originally published in Science, suggesting a mechanistic link between two of the most widely accepted causes of weed invasion: high resource availability and release from natural enemies. This idea was tested for the first time, finding strong evidence that the predicted patterns in fact occur in nature. Comparing enemy release among 243 European plant species that have been introduced to the US, revealed that fast-growing plant species adapted to wet, nitrogen-rich environments escaped many more pathogen species than slow-growing plant species adapted to dry, nitrogen-poor environments. These results suggest that global changes that increase plant resource availability, such as increases in atmospheric CO2 and nitrogen deposition, will favor exotic over native species. Conversely, management which reduces resource availability, such as land restoration or vegetation buffers around wetlands may be key to controlling not just weedy species, but exotic invasive species in particular.

2. Management influences on soil organic carbon dynamics in northern mixed-grass prairie. Due to emerging markets for carbon credits on rangelands, there is a need to determine how grazing management practices may increase soil carbon sequestration which can reduce the rise in atmospheric carbon dioxide, while also increasing the health of soils, the capacity of soils to take up water, and cycle nutrients for plant growth. ARS scientists in Cheyenne, Wyoming and Fort Collins, Colorado, in cooperation with University of Wyoming scientists summarized findings from long-term (>20 years) research projects regarding the effects of grazing and climatic conditions on carbon sequestration in the northern mixed-grass prairie rangeland ecosystem, which is the largest remaining grassland in North America, encompassing approximately 75 million acres. Proper grazing with best management practices maintained a healthy, diverse plant community and resulted in soil carbon sequestration across the climatic conditions. However, improper grazing practices can result in detrimental changes to the plant community composition, reduce the productivity of the system, and make the system susceptible to losses of soil carbon during periods of drought.

3. Elevated CO2, soil water availability, and species composition effects on methane uptake and nitrous oxide emission in a semiarid grassland. Semiarid grasslands are vulnerable to rising CO2 concentrations affecting plant species composition and soil water availability. A big unknown is how elevated CO2 and its effects on plant species composition and soil water availability will alter the consumption/emission of the greenhouse gasses methane (CH4) and nitrous oxide (N2O). A greenhouse experiment was conducted to better understand the relative effects of elevated CO2, soil water availability, and composition of semi-arid grassland species on CH4 consumption and N2O emission. Elevated CO2 increased CH4 consumption by 37%, while soil water availability and plant species composition did not significantly affect CH4 consumption. On the other hand, soil water availability and plant species composition had significant effects on N2O emission, while elevated CO2 had no significant effect. These results indicate that elevated CO2-induced changes in soil moisture and species composition should be considered when predicting the effects of elevated CO2 on CH4 and N2O fluxes in these grassland systems.

4. Problems in quantifying grassland exchanges of carbon dioxide. As interest grows on ways in which U.S. agriculture can lessen its emissions of greenhouse gases into Earth’s atmosphere, more attention is being paid on how best to quantify those emission rates. A field experiment was conducted to evaluate and compare two popular methods for measuring gas-phase fluxes of carbon, the greenhouse gas carbon dioxide, between a native Colorado rangeland and the ambient atmosphere. The results of the research indicate that two popular methods of carbon dioxide flux measurement, Bowen ratio and eddy covariance, both involving instrumented towers for measuring trace gas fluxes over large areas, can give quite different results, due in large part to fundamental differences in the theory underlying the two methods. An empirical method is developed to help reconcile differences between the two techniques, but the suggestion is made that in future analyses, differences between these two methodologies should be considered in interpreting measurement results. This accomplishment advances our understanding of the science of greenhouse gas flux measurements, and provides scientists and policy makers with better information on how to quantify agriculture’s involvement in the exchanges of trace gases between land surfaces and the atmosphere.

5. A model to better understand N cycling in rangelands. Nitrogen (N) influences many ecosystem processes such as net primary productivity, plant species composition, and carbon sequestration. Despite this importance of N, it remains difficult to examine key processes of the N cycle. One way to study the N cycle in terrestrial ecosystems is by adding 15N (stable isotope of N that is rare in natural systems) to the soil and then tracing the 15N afterwards in different ecosystem compartments. A computer model was developed that simulates the flow of 15N in plants, microbes, and soil after addition of 15N to the soil. 15N Fractions measured in plants for five years after addition of 15N to a semi-arid grassland grown under ambient and elevated atmospheric CO2 conditions (368 vs. 720 ppm) in Colorado were accurately simulated with the model. This model provides a useful tool to better understand N cycling in rangeland ecosystems.

6. Ecological controls on rangeland carbon dioxide exchanges. Understanding how fluxes of greenhouse gases like carbon dioxide are affected by weather and the local ecology are required to estimate the involvement of different agro-ecosystems in the land to atmosphere fluxes of those gases, and are also necessary in the development of management practices to limit those fluxes, thereby helping reduce the nation’s greenhouse gas emissions to Earth’s atmosphere. Research was undertaken in a semi-arid grassland of northern Colorado to use a combination of micrometeorological and stable isotope methods to understand how a particular mixture of native plant species affect the exchanges of carbon dioxide between a semi-arid grassland and the ambient atmosphere. The evolution of carbon dioxide from the grassland to the atmosphere was significantly enhanced following rainfall events, and the seasonal contributions of those fluxes from warm- and cool-season grasses depended on their individual, and sometimes, very different reactions to soil water supply. This information clarifies the role individual plant species have in the exchanges of greenhouse gases between the landscape and atmosphere, and will be used to improve ecological models of terrestrial carbon release to the atmosphere.

7. Adapting world agriculture to climate change. While most now accept that climate change is a reality, the discussion in agricultural circles is turning to address questions of how to best adapt agriculture to climate change. An international group of climate change experts, including several ARS scientists, developed an approach for carrying out a series of highly integrated research experiments for optimizing agriculture’s response to climate change. This generation of experiments would focus on adapting crops to the future environment, specifically to increased concentrations of atmospheric carbon dioxide, using the tools of molecular genetics. However, the research program would be integrated with other efforts exploring the responses of crop species to warming and drought. This blueprint for feeding the world in the face of climate change will serve as an important guide to policy makers and scientists.

6.Technology Transfer

Number of Web Sites Managed1
Number of Other Technology Transfer4

Review Publications
Wilby, A., Mitchell, C., Blumenthal, D.M., Daszak, P., Friedman, C., Jutro, P., Maxumder, A., Prieur-Richard, A., Desprez-Loustau, M., Sharma, M. 2009. Biodiversity, food provision and human health. Island Press, Washington, D.C. Book Chapter.

Dijkstra, F.A., West, J.B., Hobbie, S.E., Reich, P.B. 2009. Antagonistic effects of species on C respiration and net N mineralization in soils from mixed coniferous plantations. Forest Ecology and Management 257:1112-1118.

Morgan, J.A. 2006. Climate change and managed ecosystems: (Book review). Journal of Environmental Quality 35:1966.

Ainsworth, E.A., Beier, C., Calfapietra, C., Cuelemans, R., Durand-Tardif, M., Farquhar, G.D., Godbold, D.L., Hendrey, G.R., Hickler, T., Kaduk, J., Karnosky, D.F., Kimball, B.A., Koerner, C., Koornneef, M., Lafarge, T., Leakey, A. D. B., Lewin, K.F., Long, S.P., Manderscheid, R., McNeil, D.L., Meis, T.A., Miglietta, F., Morgan, J.A., Nagy, J., Norby, R.J., Norton, R.M., Percy, K.E., Rogers, A., Soussana, J., Stitt, M., Weigel, H., White, J.W. 2008. Next generation of elevated [CO2] experiments with crops: A critical investment for feeding the future world. Plant Cell and Environment. 31:1317-1324.

Svejcar, A.J., Angell, R.F., Bradford, J., Dugas, W., Emmerich, W.E., Frank, A.B., Gilmanov, T., Haferkamp, M.R., Johnson, D.A., Mayeux Jr, H.S., Mielnick, P., Morgan, J.A., Saliendra, N., Schuman, G.E., Sims, P.L., Snyder, K.A. 2008. Carbon fluxes on north american rangelands. Rangeland Ecology and Management. 61:465-474.

Shim, J.H., Pendall, E., Morgan, J.A., Ojima, D.S. 2009. Wetting and drying cycles drive variations in the stable carbon isotope ratio of respired carbon dioxide in semi-arid grassland. Oecologia. 160:321-333.

Alfieri, J., Blanken, P., Morgan, J.A., Smith, D.P. 2009. Concerning the Measurement and Magnitude of Heat, Water, Vapor, and Carbon Dioxide Exchange from a Semiarid Grassland. Journal of Applied Meteorology and Climatology 48(5):982-996.

Dijkstra, F.A. 2009. Modeling the Flow of 15N After a 15N Pulse to Study Long-Term N Dynamics in a Semi-Arid Grassland. Ecology 90:2171-2182.

Last Modified: 4/23/2014
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