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T he topic of global change is found almost daily in newspaper and magazine articles and on television. Global change refers to large-scale changes in the Earth's biological, geological, hydrological, and atmospheric systems, whether of human or natural origin. The primary concern of global change has centered on the rapid increase in atmospheric concentrations of primary greenhouse gases (carbon dioxide [CO2], methane [CH4], and nitrous oxide [N20]) since the Industrial Revolution in the latter part of the 19th century.1 Increased concentrations of all 3 of these gases trap more of the sun's energy close to Earth's surface and lead to global warming, hence the reason that these gases are often referred to as "greenhouse gases." The concentration of CO2 has increased by about 35% compared to preindustrial times and is predicted to reach twice the preindustrial concentration within the 21st century.2 Plants convert CO2 in the air into plant tissue, so CO2 enrichment generally stimulates plant growth and improves the efficiency with which plants use water.3 These changes, in turn, influence plant nutrition and the cycling of carbon and other mineral elements through the soil/plant system4 and may have long-lasting ecological consequences for rangelands and pasturelands. For example, increased plant production with CO2 enrichment5'6 requires additional soil nitrogen, which is limiting in most rangeland and pastureland ecosystems, and this will modify nutrient cycling because less nitrogen is available for soil microorganisms to decompose plant materials. The increase in methane and nitrous oxide concentrations in the atmosphere that have occurred are also a cause for concern as their warming potential as greenhouse gases surpasses that of CO 2.' Global warming is predicted to have numerous impacts on our climate, including altered precipitation patterns and a potential rise in sea levels. As such, global warming may have profound impacts on human activities and enterprises and is thus a concern to many. Rangelands and pasturelands provide much of the world's food and fiber, and because they occupy an extensive land area, changes in how these lands function could increase or reduce the atmospheric load of greenhouse gases. For example, land management strategies that increase the storage of carbon in plant biomass and/or soil organic matter on rangelands and pasturelands, in a process termed "carbon sequestration," offer opportunities to mitigate the rise in atmospheric CO2 concentrations.8 To evaluate potential impacts that increasing concentrations of greenhouse gases may have, the United States Department of Agriculture-Agricultural Research Service (USDA-ARS), in its Natural Resources and Sustainable Agricultural Systems program, established a Global Change |
