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United States Department of Agriculture

Agricultural Research Service


Location: Rangeland Resources Research

2008 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
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 third year of this experiment in which ambient CO2 concentration, temperature and soil water are being manipulated to further our understanding of how semi-arid grasslands respond to multiple global change factors. A core group of scientists from ARS (7), the University of Wyoming (2), Colorado State University (2), and the Biometeorology Institute in Florence, Italy (1), plus graduate students are collaborating on this unique project. Two new research grants were funded this past year to expand research of how nutrient cycling is altered by changes in the soil biota resulting from the CO2, warming and irrigation treatments. 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. Measurements being taken can be roughly broken down into three categories: site environmental, plants, and soils (including soil biota). The site environmental measurements includes a suite of measurements that are available to all project collaborators and describe the overall site environmental conditions, including site air temperature, solar radiation, humidity, and wind, which is 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 80cm) are measured continuously. Plant measurements include monthly plot photography to document changes in plant cover; bi-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 soil 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. Two greenhouse experiments were undertaken this past winter to evaluate plant and soil mechanisms involved in the responses of grassland ecosystems to rising atmospheric CO2. Such controlled-environment experiments are necessary to explore aspects of some response mechanisms, like nutrient cycling, that are difficult to evalute in a field environment. One experiment examined the effects of CO2 and water availability on a primarily soil-based trace gas fluxes, a second one investigated how plants and soils respond to the combined effects of CO2 enrichment and defoliation. This research primarily addresses NP Code 204, Agricultural Ecosystem Impacts (Comp. 3), Problem Statements 3 (Grazinglands) and 4 (Pests); and the Carbon Cycle and Carbon Storage (Comp..
1)of the new NP204 Action Plan.

1. Changes in snowfall exacerbate plant invasion: Although global climate changes have been found to facilitate plant invasion, raising the possibility that agricultural ecosystems will be increasingly dominated by invasive plants, very little is known about effects of altered precipitation. We tested the effects of predicted increases in snow, summer precipitation, and nitrogen deposition, on invasion of the North American mixed grass prairie, the largest remaining grassland on the continent. Although all three changes influenced invasion, increased snowfall had striking effects, greatly increasing the success of several invasive species. These results add another aspect of climate change, altered snowfall, to the list of changes known to contribute to invasion. They also allow for more accurate predictions of both the types of invaders that will be most successful and the types of ecosystems that will be most susceptible to invasion under future climatic regimes. This accomplishment addresses National Program 204, Agricultural Ecosystem Impacts (Component 3), Problem Statement 4 (Pests), Goal 2 (Provide multi-variate analysis of elevated carbon dioxide and other abiotic stresses on crop susceptibility to pests).

2. Weed invasions increase greenhouse gas emissions of rangeland soils: Weed invasions are considered one of the highest priority problems facing ranchers and rangeland mangers, and new research suggests that weeds may enhance climate change as well. Soil emissions of two important greenhouse gases, carbon dioxide (CO2) and nitrous oxide (N2O), plus soil carbon and N were monitored in adjacent pastures dominated by Wyoming big sage (Artemisia tridentata ssp. Wyomingensis) with and without cheatgrass (Bromus tectorum) infestations. The results indicated that soil release of CO2 and N2O, plus C mineralization and nitrification all responded more strongly to simulated precipitation events in areas of cheatgrass infestation compared to non-invaded areas where native grasses like western wheatgrass (Pascopyrum smithii) were more abundant. In addition to their negative impacts on forage quality and biodiversity, weed invasions may contribute to the release of greenhouse gases by terrestrial ecosystems. This research addresses Agricultural Ecosystem Impacts (Component.
3)of the NP 204 Action Plan, Problem Statements 3 (Grazinglands) and 4 (Pests).

3. Effects of elevated CO2 on nitrogen cycling in shortgrass steppe: Elevated CO2 has the potential to enhance plant growth and biomass accumulation, but these processes may be constrained by soil nitrogen availability. It is still uncertain how elevated CO2 affects long-term soil nitrogen dynamics. We examined the effect of five years of elevated CO2 on N dynamics in a native shortgrass steppe in northern Colorado using large open-top CO2-fumigation chambers. Plant growth and plant nitrogen uptake remained significantly higher under elevated than under ambient CO2 after five years of study. The indirect effect of elevated CO2 on soil moisture most likely increased nitrogen mineralization in the soil. These results suggest that plant responses to elevated CO2 are less constrained by nitrogen availability in semi-arid grasslands and could be more pronounced and last longer than in ecosystems that receive more moisture. This research addresses the Agricultural Ecosystem Impacts (Component.
3)of the NP 204 Action Plan, Problem Statement 3 (Grazinglands).

4. Grazing and drought impacts on soil carbon and microbial communities in northern mixed-grass prairie. The northern mixed-grass prairie represents the largest remaining intact rangeland in the US. Previous work in this rangeland ecosystem showed that heavy stocking rates increased soil carbon storage compared to light stocking or no grazing over a relatively wet period of years (1982-1992). Subsequent sampling following a dry period of years (1993-2002), including 5 drought years, determined the soil organic carbon in the light stocking and no grazing treatments were largely unaffected, whereas 30% of the soil organic carbon was lost in the heavy grazing treatment. In addition, soil microbial community structure differed with grazing treatments. The heavy stocking rate altered the plant community composition, which has reduced the aboveground productivity potential and subsequently modified the soil microbial community structure and associated biogeochemical (carbon and nitrogen) cycles. This research addresses Carbon Cycle and Carbon Storage (Component.
1)of the NP 204 Action Plan, Problem Statement 3 (Grazinglands, CRP and Buffers)

5. Ecological and Management Implications of Climate Change for American Rangelands: While most now accept that climate change is a reality, and many consider it the defining present-day environmental challenge, others wonder what it can possibly mean to them and what can done to prepare for it. In a special feature edition of Rangelands and also in the U.S. Climate Change Program’s Special Report, “The Effects of Climate Change on Agriculture, Land Resources, Water Resources, and Biodiversity”, we synthesized the current climate change literature and reported on the potential effects of climate change on American rangelands, and presented management options for the ranchers and land managers who will have to deal with this problem. Among the more important rangeland responses to climate change identified were changes in plant species composition, including weed invasions, reductions in forage quality, and in some regions, increased drought and animal stress. Adaptive management strategies for coping with climate change included increased rangeland monitoring, development and use of models for predicting rangeland responses to climate change, better weather forecasting, and changing management objectives. This research addresses the Agricultural Ecosystem Impacts (Component.
3)of the NP 204 Action Plan, Problem Statements 3 (Grazinglands) and 4 (Pests).

6. Realistic Systems for Simulating Global Warming in Terrestrial Ecosystems: Performing experiments in which climate can be manipulated in a realistic field environment is a difficult yet necessary task if we are to understand how terrestrial ecosystems will respond to climate change. This project, led by an ARS collaborator from the US Arid Land Agricultural Research Center, developed infra-red heating array systems to be used in field environments to impose controlled heating for experiments to evaluate the consequences of warming on ecosystem functioning. The heating arrays were deployed over native grasslands at Haibei, Qinghai, China and at Cheyenne, Wyoming, and their efficiency and performance evaluated. The heating systems performed well, achieving target heating temperatures within 0.5°C 75% of their operation time, and with a heating efficiency which surpasses previous field warming systems. The infra-red hearing arrays are a useful method by which scientists can explore the consequences of global warming on crops and native agro-ecosystems, and are beginning to be adopted by other research groups around the world. This research addresses the Agricultural Ecosystem Impacts (Component.
3)of the NP 204 Action Plan, Problem Statements 2 (Cropping Systems) and 3 (Grazinglands).

7. Belowground nematodes resistant to rising atmospheric CO2. Soil organisms can have a profound affect on how changes in the environment, including climate change, affect the ecology of terrestrial ecosystems; they can be important in stimulating nutrient cycling and promoting plant growth, but may also be destructive to plants. In a report of three separate experiments, all conducted in grasslands and in which ambient levels of carbon dioxide(CO2) were increased to simulate CO2 concentrations projected for Earth’s atmosphere in the second half of this century, little evidence was found to suggest nematodes (various threadworms of the Phylum Nematoda) were affected by atmospheric CO2 concentration. These neutral responses to CO2 enrichment occurred despite increased root production in all three experiments, suggesting a simultaneous antagonistic mechanism may have operated, possibly decreased root quality and/or changes in the soil environment. The findings suggest that herbivorous nematodes in grassland ecosystems may sometimes be unresponsive to rising atmospheric CO2. This research addresses the Agricultural Ecosystem Impacts (Component.
3)of the NP 204 Action Plan, Problem Statement 3 (Grazinglands).

6.Technology Transfer

Number of Non-Peer Reviewed Presentations and Proceedings11
Number of Newspaper Articles and Other Presentations for Non-Science Audiences36

Review Publications
Morgan, J.A., Milchunas, D.G., Lecain, D.R., West, M.S., Mosier, A. 2007. Carbon dioxide enrichment alters plant community structure and accelerates promotes shrub growth in the shortgrass steppe. Proceedings of the National Academy of Sciences 104(37):14724-14729.

Morgan, J.A., Derner, J.D., Milchunas, D., Pendall, E. 2008. Management implications of global change for Great Plains rangelands. Rangelands 30(3):18-22.

Norton, U., Mosier, A., Morgan, J.A., Derner, J.D., Ingram, L.J., Stahl, P. 2008. Moisture pulses, trace gas emissions and soil C and N in cheatgrass and native grass-dominated sagebrush-steppe in Wyoming, USA. Soil Biology and Biochemistry 40:1421-1431.

Kandeler, E., Mosier, A., Morgan, J.A., Milchunas, D., King, J., Rudolph, S., Tscherko, D. 2007. Transient elevation of carbon dioxide modifies the microbial community composition in a semi-arid grassland. Soil Biology and Biochemistry 40:162-171.

Blumenthal, D.M., Chimner, R.A., Welker, J.M., Morgan, J.A. 2008. Increased snow facilitates plant invasion in mixed grass prairie. New Phytologist 179:440-448.

Ayers, E., Wall, D.H., Simmons, B.L., Field, C.B., Milchunas, D., Morgan, J.A., Roy, J. 2008. Belowground grassland herbivores are resistant to elevated atmospheric CO2 concentrations in grassland ecosystems. Soil Biology and Biochemistry 40:978-985.

Dijkstra, F.A., Pendall, E., Mosier, A., King, J., Milchunas, D., Morgan, J.A. 2008. Long-term enhancement of N availability and plant growth under elevated CO2 in a semiarid grassland. Functional Ecology 22:975-982.

Ingram, L.J., Stahl, P.D., Schuman, G.E., Buyer, J.S., Vance, G.F., Ganjegunte, G.K., Welker, J.W., Derner, J.D. 2008. Grazing and drought impacts on soil carbon and microbial communities in a mixed-grass ecosystem. Soil Science Society of America Journal 72(4):939-948.

Kimball, B.A., Conley, M.M., Wang, S., Xingwu, L., Morgan, J.A., Smith, D.P. 2008. Infrared heater arrays for warming ecosystem field plots. Global Change Biology, (14):309-320.

Last Modified: 4/16/2014
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