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This special issue focuses on terrestrial biogeochemical cycles and their roles in determining current continental-scale budgets and future trends in biogenic greenhouse gases (GHGs) for North America. Understanding the current magnitude and forecasting future trajectories of atmospheric GHG concentrations requires investigation of their biogeochemical origins as well as their responses to (i) environmental changes, including potential feedback effects of climate change, and (ii) changes in human activities, including potential reductions in GHG emissions resulting from improved land management practices. This special issue provides a group of articles describing the current state of continental-scale sources and sinks of biogenic GHGs and potential strategies for managing them in the future. Human activity over the past two centuries, including fossil fuel combustion, clearing of forests, and intensification of agriculture, has led to a dramatic increase in the concentration of atmospheric carbon dioxide (CO2) and other important long-lived GHGs, namely methane (CH4) and nitrous oxide (N2O). In North America today, fossil fuel combustion is the single most dominant source of GHGs. However, both managed and unmanaged terrestrial ecosystems are significant as both sources and sinks of GHGs resulting from complex biogeochemical processes occurring within these systems. The collection of articles in this special issue indicates that North America is currently a net source of CO2, CH4, and N2O to the atmosphere. For CO2 this source is dominated by fossil fuel burning. Approximately 35% of this fossil fuel CO2 is taken up by North America’s terrestrial ecosystems, largely by forest regrowth from logging earlier in the 20th century. Elevated atmospheric deposition of reactive nitrogen onto terrestrial ecosystems also contributes to an increase in CO2 uptake. Biogenic CH4 and N2O emissions are significant and offset at least half, in units of CO2 equivalent, of the current terrestrial CO2 uptake. As a result of climate change, these emissions are expected to substantially increase by the end of the 21st century. In addition, increases in fertilizer nitrogen use in agriculture will contribute to rising atmospheric N2O concentrations. There are substantial opportunities, however, to reverse these trends. Leaving aside the issue of reducing fossil fuel combustion, changes in agricultural and forestry management practices for carbon sequestration can result in important reductions in atmospheric CO2. Additionally, there may be effective strategies to mitigate agricultural N2O emissions that reduce agricultural losses of other reactive N forms, thereby improving the outlook for soil, water and air quality.
