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2010 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
This is the first report for project 5409-11000-005-00D, which replaces the previous bridge project 5409-11000-004-00D that expired in May 2010. As this new project has only been active since May 2010, milestones, all of which have been met, are limited to those listed as “To date”. There have not yet been any documented accomplishments. Rather, recent accomplishments can be found in the annual report for the previous bridge project 5409-11000-004-00D. Research under project 5409-110-005-00D is guided by two major 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 ambient CO2 concentration, temperature and soil water are all being manipulated to further our understanding of how semi-arid grasslands respond to multiple global change factors. The second objective also includes measurements (slated to begin in 2011) of soil carbon within a separate, long-term grazing experiment. We are presently in the fifth year of the PHACE 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 weed 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. Measurements that are being taken can be roughly broken down into three categories: site environmental, plants, and soils (including soil biota). Our results to date suggest that the effects of elevated CO2 and warmer temperatures are experienced to a large extent by the combined effects of these two factors on soil plant water relations. However, a number of other plant and soil attributes that are not water-driven and which appear directly responsive to the warming and/or CO2 treatments are also being documented. These climate change-induced changes in soil water and other attributes and their combined effects on rangeland ecology will continue to be monitored in the final years of this experiment, and will be used to: 1) develop predictions of future changes in rangelands and 2) develop strategies for adaption of rangelands to climate change. In other research, we will return to a long-term grazing experiment where soil samples will be collected and assayed in order to evaluate the effects of cattle grazing on soil C sequestration.

4. Accomplishments