changes in plant species composition. In Minnesota old fields, nitrogen additions simulating moderate to heavy atmospheric deposition led to the replacement of C4 by C3 grasses, with important decreases in carbon storage per unit of nutrients as well as changes in seasonality.48

Manipulative experiments have been important in marine as well as terrestrial systems. 49 In some oceanic ecosystems, supplies of major nutrients such as nitrogen and phosphorus persist throughout the growing season. In the late 1980s it was hypothesized that these regions might be iron limited.50 Additions of iron in the equatorial Pacific, together with a tracer allowing the fertilized region to be tracked, increased phytoplankton and microbial activity, affecting both community composition and carbon cycling.51 Experiments of this type, following both natural (e.g., atmospheric deposition events) and artificial fertilization experiments, with major as well as minor nutrients, have long been contemplated as prospects for ocean perturbation studies that can be followed with satellite ocean color data.

Multiple Stresses on Ecosystems

The dominant concern originally motivating global change research was global climate change. As ecologists and their colleagues from other disciplines began to address ecology at regional and larger scales, the importance of multiple large-scale environmental changes became apparent. Changes to climate, air quality, and land use may have synergistic effects, modifying the vulnerability of ecosystems to change. The concept of the metro-agro-plex, a region characterized by dense human populations, urbanization, and intensive agriculture, evolved in this context.52 Examination of the geographies of regional air quality problems, urbanization, population growth, and intensive agriculture indicates that all are taking place in the same regions—for example, in Southeast Asia, the eastern and western seaboards of the United States, and Western Europe. These areas tend to have high air pollution levels, with elevated ozone, carbon monoxide, sulfur, and nitrogen. Urbanization and industrialization lead to air pollution and its precursor inputs, but the proximity of cities and agriculture makes the agricultural system vulnerable to damage from ozone, acidification, and other stresses. However, intensive agriculture is itself a source of trace gases to the atmosphere (NO, NH3, and particulates) that may interact with regional air chemistry, particularly at the margins of the metro-agro-plex. Although air pollution is often thought of as an acute local or regional problem, metro-agro-plexes cover sufficient area and contain a high enough proportion of the world's population that they are properly thought of as a global problem.

Air pollution also has a direct global component. Recent work using observations and modeling has shown that nitrogen pollution has become a global problem and that likely future changes in fossil fuel and fertilizer use will lead to high nitrogen deposition worldwide. 53 Beginning in the 1980s, 54 anthropogenic

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