Humans are causing rapid, global environmental changes that modify climate, increase the atmospheric carbon dioxide (CO2 ) concentration, and increase atmospheric nitrogen deposition. To some degree these changes are occurring everywhere. Microorganisms are also everywhere, are abundant, and provide essential ecosystem services that are beneficial to humans. Recently discovered high-affinity methane-oxidizing bacteria are ubiquitous in upland soils and remove methane (CH 4 ) from the atmosphere, the second most important greenhouse gas contributing to global warming. Trophic interactions among soil microorganisms mineralize nutrients required for plant growth and carbon required for accurate estimates of the future contribution of CO 2 to global warming. Effects of global environmental change on these ecosystems services remain unresolved, along with potential feedbacks to global warming. Little is known about: (i) the effects of multiple, simultaneous global environmental changes; (ii) how the climate of an ecosystem influences its response to climatic change, (iii) the role of microbial community structure in explaining responses to climatic change; and (iv) whether global change effects can be generalized across ecosystems.The response of soil CH4 consumption to global environmental change was investigated in two multi-factor field experiments. Eight years of four full-factorial treatments in a California grassland revealed that interactive effects of global change are quantitatively important and can determine whether soil CH 4 consumption responds at all to precipitation, the primary driver of CH4 consumption in this grassland (Chapter 1). Four ecosystems along an Arizona elevation gradient were used to test the influence of climate on the response of CH4 consumption to climatic change. After three years of full-factorial warming (+1.5 °C) and precipitation (+50% and -30%) treatments, warming decreased CH4 consumption in wanner ecosystems and elevated precipitation decreased CH 4 consumption in wetter ecosystems (Chapter 2). These direct effects of climatic change were not caused by a long-term change in the abundance or community composition of methane-oxidizing bacteria (Chapter 3). Results from both experiments warn that humans cannot depend on future global environmental changes to enhance the ecosystem service of soil CH 4 consumption, and a positive feedback to global warming is more likely than a negative feedback.A quantitative review of the literature, or meta-analysis, was used to identify environmental variables that best explain changes abundances of soil organisms caused by global warming, an altered precipitation regime, and rising atmospheric CO2 (Chapter 4). Many ecologists have spent many hours counting soil microorganisms in field global change experiments, and my goal was to synthesize these responses to check for universal patterns. Changes in abundances of soil biota were best explained by climate, ecosystem type, and trophic group for warming, precipitation, and CO 2 effects, respectively. Among the global environmental changes investigated, elevated CO2 is most likely to diminish ecosystem services provided by belowground trophic interactions.Patterns are emerging with respect to belowground responses to global environmental change. Multi-factor interactions in the California grassland always involved the primary global change driver. Clear ecosystem interactions with temperature and precipitation were found along the Arizona elevation gradient. And changes in the overall abundance of soil organisms depended on particular variables for particular global change factors. Field experiments provide a power means of predicting future changes in ecosystem services.