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2012 Annual Report

1a. Objectives (from AD-416):
The semi-arid grasslands of the western Great Plains, mixed-grass prairie and shortgrass steppe, provide a tremendous array of ecosystem services, including livestock forage, a diversity of native plants and animals, resistance to biological invasion, and carbon storage. Global change is expected to dramatically change grasslands and associated ecosystem services, but the nature of its impacts, and the mechanisms underlying those impacts, remain difficult to predict. In water-limited ecosystems, elevated CO2 and warming can have particularly strong and complex effects because, in addition to their direct effects, they alter water availability. Two main objectives will drive our research program over the next five years to understand how these changes might impact the ecosystem services of western rangelands. The first objective is to assess effects of predicted global changes on ecosystem services in a northern mixed-grass prairie. This will be accomplished by determining the effects of temperature, CO2 and precipitation on plant productivity, plant diversity, forage quality, community composition, weed invasion and the ability of native plant communities to recover from disturbance. The biogeochemistry underlying these responses will be studied to improve our understanding of ecosystem responses and to improve algorithms in biogeochemical models like Daycent. We will also evaluate whether and how responses of invasive species differ from those of native species. Our second objective is to develop knowledge and tools that allow rangeland managers to minimize greenhouse gas emissions. We will determine how temperature, CO2 and precipitation influence land-atmosphere exchanges of trace gases and soil carbon (C) storage, and evaluate the relative importance of water, nitrogen (N) and C limitation in regulating C storage. We will use this information plus additional soil C and CO2 flux data from long-term grazing experiments to determine the potential to mitigate greenhouse gas emissions through grazing management, and assess tradeoffs between mitigation and rangeland productivity.

1b. Approach (from AD-416):
To address our first objective concerning the responses of rangelands to global changes, we will use a well-replicated Free Air CO2 Enrichment (FACE) and warming experiment to determine how global change influences the northern mixed-grass prairie. We will examine responses of plant production and quality, composition of native plant communities, carbon and nitrogen cycling, and plant invasion. To understand the mechanisms underlying these responses, we will make extensive use of gas exchange, stable isotope, soil water and nitrogen monitoring, and computer simulation methods. We will use additional treatments to learn how seasonality of precipitation influences the northern mixed-grass prairie, and how the magnitude of those effects compares to effects of CO2 and warming. To address our second objective concerning greenhouse gas mitigation tools, we will measure soil respiration and fluxes of nitrous oxide (N2O) and methane (CH4) using static chambers, and net ecosystem CO2 exchange (NEE) using dynamic chambers within plots of the FACE, warming and irrigation manipulative experiment. Results from the static and dynamic chambers will allow us to quantify CO2-enrichment and warming effects on soil trace gas fluxes and ecosystem level CO2 fluxes, and how these fluxes are related to soil moisture and other environmental factors. We will also take advantage of three ongoing NP215 long-term grazing studies to assess the effects of grazing management strategies (stocking rate and season of use) on the size and dynamics of soil C and N pools, and the potential of these strategies to mitigate greenhouse gas emissions in NMP and SGS. We will use natural variation in precipitation to determine the relative influence of above- and below-average years of precipitation on C and N pool changes. The insights provided by these experiments will help scientists and land managers adapt management practices to sustain ecosystem services in the face of global change, and provide critical information for policy makers.

3. Progress Report:
Research under project 5409-11000-005-00D is guided by two objectives: (1) Assess effects of predicted global changes on ecosystem services in northern mixed-grass prairie, and (2) Develop knowledge and tools that allow rangeland managers to minimize greenhouse gas emissions. Both objectives are centered on the Prairie Heating and CO2 Enrichment (PHACE) Experiment in which atmospheric carbon dioxide (CO2) concentration, temperature and soil water are all being manipulated to further our understanding of how semi-arid rangelands respond to multiple global change factors. The second objective includes measurements of soil carbon within a separate, long-term grazing experiment. A core group of scientists from ARS, the University of Wyoming, Colorado State University, and the Biometeorology Institute in Florence, Italy, plus several graduate students and post docs continue to collaborate 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 native and invasive plant species. This plot arrangement allows us to evaluate both the basic responses of this rangeland to climate change, and also to investigate how global changes interact with disturbance and plant invasion. Our results to date suggest that the effects of elevated CO2 and warmer temperatures depend to a large extent on the combined effects of these two factors on soil plant water relations, and that perennial warm-season (plants possessing the C4 photosynthetic pathway) grasses appear to prosper under these future conditions. Due to higher-than-expected water savings from elevated CO2, our results suggest that productivity in this semi-arid rangeland may be greater under climate warming than previously suspected. However, we are also learning how a number of other plant and soil attributes, particularly the cycling of soil/plant nitrogen (N) may determine the ultimate responses of this rangeland to climate change through competition for soil resources. Experimental work on Dalmatian toadflax (Linaria dalmatica, a perennial invasive perennial forb) resulted in finding that elevated CO2 and warmer temperature conditions led to a 13-fold increase in its invasion of mixed-grass prairie. We used open space in the experiment to begin a study of cheatgrass (Bromus tectorum, an annual invasive grass), which has a winter-annual life history, and may therefore be strongly influenced by warming. A Department of Energy grant will extend the PHACE experiment through 2013 and add additional modeling efforts using results from this experiment to evaluate longer term effects of climate change on carbon (C) cycling. We added efforts to address leaf traits and this effort is mostly a literature synthesis from prior global change experiments. In our greenhouse gas mitigation research, we postponed soil sampling from long-term grazing experiments due to the extended drought conditions. We plan to sample these experimental plots next year to determine the long-term effects of livestock grazing on soil C sequestration and storage.

4. Accomplishments
1. Effects of rainfall event size on forage quality. In semi-arid rangelands, the size of individual rain events can be as important as total annual rainfall in controlling the production and quality of forage for livestock. ARS researchers in Cheyenne, WY/Fort Collins, CO and collaborators from Colorado State University studied how different sized rainfall events affected the uptake of soil nutrients by grass roots versus soil microbes in the shortgrass steppe of eastern Colorado. They found that rainfall event size (0.4 or 0.8 inches) did not affect the timing of nitrogen uptake, but did substantially influence whether plants or microbes were more effective in acquiring nitrogen. Plants were most effective in acquiring nitrogen following smaller rainfall events, whereas soil microbes were more effective in acquiring nitrogen following larger rainfall events. Large rainfall events result in more efficient use of water by grasses, and therefore increase the efficiency of forage production. In contrast, smaller rainfall events may be important for plant nitrogen uptake which should maintain high protein concentrations in forage grasses. The findings also indicate that predicted shifts toward larger rainfall events with future climate change may increase forage production but decrease forage quality in semiarid rangelands.

2. Soil organic matter in grasslands is susceptible to climate change. Soil organic matter (SOM) is an important attribute that contributes substantially to the health of rangeland ecosystems. Understanding how rising atmospheric carbon dioxide (CO2) and warmer temperatures affect soil organic matter is important in determining the long-term effects of climate change on plant production. To evaluate the sensitivity of SOM to climate change, ARS scientists in Cheyenne, WY/Fort Collins, CO and collaborators from the University of Wyoming and the University of Sydney, Australia subjected a mixed-grass prairie to temperatures and CO2 concentrations expected to develop during the next 50 years. Plant production was often increased by the CO2 and warming treatments. Decomposition of SOM that is typically most resistant to decomposition also increased which represents a potential decline in soil quality and health of the rangeland ecosystem. These results suggest that rising CO2 and climate change have the potential to increase plant production in this semiarid rangeland ecosystem, but this benefit may occur with the associated tradeoff of decreased SOM.

3. Soil carbon dynamics and rangeland management. Rangelands are the largest and among the most diverse land resources in the United States. They harbor considerable potential to mitigate climate impacts due to their extensive land area and the application of low-input, adaptive management practices. ARS scientists in Cheyenne, WY/Fort Collins, CO, and Lincoln, NE compiled the effects of rangeland management practices on soil carbon as part of the coordinated agricultural research effort through GRACEnet (Greenhouse gas Reduction through Agricultural Carbon Enhancement network, a multi-location project of ARS). Primary findings were 1) rangelands are typically characterized by short periods of high carbon uptake during the growing season (2-3 months) and long periods of carbon balance or small losses during the remainder of the year, 2) dominant mechanisms for rangeland greenhouse gas mitigation will be through carbon storage in soils and by minimization of livestock-related emissions of methane (CH4) and nitrous oxide (N2O), and 3) complex interactions among climate, vegetation, and management practices influence the role of rangelands as sinks or sources of greenhouse gases. These findings are helping climate change scientists and modeling efforts by increasing the accounting of greenhouse gas balances across different rangeland ecosystems, and quantifying the magnitude and direction of greenhouse gas changes due to interactions between management and environment.

4. Will climate change encourage non-native species? Non-native species and climate change each impact agricultural and natural ecosystems, but the two may also interact. ARS scientists in Cheyenne, WY/Fort Collins, CO along with many scientists from multiple institutions synthesized existing knowledge on 1) how extreme climatic events influence invasions by non-native species, 2) predictions on where the wave of future invasions by non-native species will occur and 3) predictions on where the non-native species will come from. For example, increasing use of drought-tolerant species in horticulture may provide a new source of non-native species in dry regions of the world. Resulting predictions from these syntheses will help policy makers and land managers prevent or control developing invasions by non-native species before they become problematic.

5. Soil nitrogen may constrain the ability of rangelands to respond positively to higher levels of atmospheric CO2. Although the rising carbon dioxide (CO2) concentration in Earth’s atmosphere is widely known to enhance plant production, some recent research suggests that this CO2-induced boost in plant production can be short-lived if soil nutrients are not in adequate supply. To evaluate if soil nutrients like nitrogen might constrain the ability of rangelands to respond to CO2, ARS scientists in Cheyenne, WY/Fort Collins, CO and collaborators from the University of Wyoming and the University of Sydney, Australia conducted two experiments. The experiments confirmed that higher CO2 enhances plants growth, but it also can alter the cycling of nitrogen between plants and soils. The studies suggest that prior projections of increased plant growth due to rising atmospheric CO2 concentrations were likely too high, and such predictions may need to be adjusted downwards to reflect how soil nitrogen fluctuations may constrain plant production responses on rangelands to CO2.

6. Precipitation timing and amount determine the capacity of a mixed-grass prairie to sequester carbon. Rangelands have the potential to remove some of the atmospheric greenhouse gas carbon dioxide (CO2) through photosynthesis by storing the carbon in soil organic matter. ARS scientists in Cheyenne WY/Fort Collins, CO and collaborators at Colorado State University and the University of Alaska evaluated how precipitation affects the capacity of a Wyoming mixed-grass prairie to acquire CO2. Increasing annual precipitation enhanced plant growth and the acquisition of CO2, with high amounts of winter snowfall promoting CO2 uptake. Decreasing precipitation resulted in a net loss or release of CO2 to the atmosphere. These results suggest that precipitation patterns greatly affect the ability of mixed prairie grasses to store carbon. This information will help scientists predict how weather patterns and climate change will influence the capacity of similar semi-arid grasslands to store carbon.

7. Climate change and rising CO2 affect carbon storage and trace gas fluxes in agro-ecosystems. Increasing concentrations of atmospheric carbon dioxide (CO2) are predicted to influence carbon storage and trace gas fluxes in agro-ecosystems. An ARS scientist in Cheyenne, WY/Fort Collins, in collaboration with a scientist from the University of Sydney, Australia reviewed findings from 32 elevated CO2 and 13 warming studies. Results were mixed for the consequences of rising temperature on carbon sequestration and the emission of trace gases. Management practices, such as fertilizer application and the inclusion of legumes, may be helpful in taking advantage of rising CO2 for sequestering carbon in soils of agro-ecosystems. These findings provide useful insights for managers, scientists and policy makers in the development and implementation of greenhouse gas practices for agro-ecosystems.

Review Publications
Blair, A.C., Blumenthal, D.M., Hufbauer, R. 2011. Hybridization and invasion: an experimental test with diffuse knapweed (Centaurea diffusa Lam.). Evolutionary Applications. 5:17-28.

Bradley, B.A., Blumenthal, D.M., Early, R., Grosholz, T.D., Lawler, J., Miller, L.P., Sorte, C., D'Antonio, C.M., Diez, J.D., Dukes, J.S. 2011. Global change, global trade, and the next wave of plant invasions. Frontiers in Ecology and the Environment. 10:20-28.

Blumenthal, D.M., Norton, A.P., Cox, S.E., Hardy, E.M., Liston, G.E., Kennaway, L., Booth, D.T., Derner, J.D. 2011. Linaria dalmatica invades south-facing slopes and less grazed areas in grazing-tolerant mixed-grass prairie. Biological Invasions. 14:395-404.

Chimner, R., Welker, J.M., Morgan, J.A., Lecain, D.R., Reeder, J. 2010. Experimental manipulations of winter snow and summer rain influence ecosystem carbon cycling in a mixed grass prairie, Wyoming, USA. Ecohydrology. 3:284-293.

Morgan, J.A., Lecain, D.R., Pendall, E., Blumenthal, D.M., Kimball, B.A., Carillo, Y., Williams, D., Heisler White, J.L., Dijkstra, F., West, M.S. 2011. C4 grasses prosper as carbon dioxide eliminates desiccation in warmed semi-arid grasslands. Nature. 476:202-205.

Carrillo, Y., Pendall, E., Dijkstra, F., Morgan, J.A., Newcomb, J.M. 2011. Response of soil carbon matter pools to elevated CO2 and warming in a semi-arid grassland. Plant and Soil. 347:339-350.

Dijkstra, F.A., Hutchinson, G.L., Reeder, J.D., Lecain, D.R., Morgan, J.A. 2011. Elevated CO2, not defoliation, enhances N cycling and increases short-term soil N immobilization regardless of N addition in a semiarid grassland. Soil Biology and Biochemistry. 43:2247-2256.

Carrillo, Y., Dijkstra, F., Pendall, E., Morgan, J.A., Blumenthal, D.M. 2012. Controls over soil N pools in a semiarid grassland under elevated CO2 and warming. Ecosystems. 15:761-774.

Dijkstra, F.A., Morgan, J.A. 2011. Elevated CO2 and warming effects on soil carbon sequestration and greenhouse gas exchange in agroecosystems: A review. In: L.R. Ahuja, L. Ma (eds). Methods of introducing system models into agricultural research. Madison, WI: ASA/CSSA/SSSA. pp. 267-486.

Diez, J.D., D'Antonio, C.M., Dukes, J.S., Grosholz, T.D., Olden, J.D., Sorte, C., Blumenthal, D.M., Bradley, B.A., Early, R., Jones, S. 2012. Will extreme climatic events facilitate biological invasions? Frontiers in Ecology and the Environment. 10:249-257.