Special Issue from the 4th USDA Greenhouse Gas Symposium.

Agronomic crops are produced across a variety of landscapes and climates and changes in climate impact their production effi ciency and potential greenhouse gas emissions and sequestration. Studies that reported the response of crops to climate change demonstrated the variation in response, the challenges that need to be addressed in evaluating these responses, and the interactions among natural resource components, e.g., water, soil, and temperature, that determine the degree of the response. Similar research eff orts in forestry systems are needed to provide a baseline for how these systems respond to climate change. Increasing our understanding of the response of agricultural and forestry systems to climate change is providing valuable information to help address the information needs to help guide decision making. Th e focus of building on our information base to guide decisions has been a hallmark of this conference and the previous conferences. A critical component of this conference is the description of technologies with a potential of reducing greenhouse gas emissions and discussion of the eff ectiveness of these technologies. Without the development and evaluation of technologies these eff orts will not progress in terms of being potentially applied to diff erent systems. However, technologies and science have to be linked with public policy to evaluate the economic and social implications of diff erent emission scenarios and technologies. Without the continued exchange of information, it will be impossible to continue to make progress in reducing greenhouse gas emissions. Th is special issue represents a cross-section of papers presented at the 4th USDA Greenhouse Gas Conference. Th ese papers have been peer-reviewed by outstanding reviewers and are presented as part of the cutting edge eff orts that are typical of all of the papers presented at this conference. Th is special issue would not have been possible without the dedication and hard work by John Baker and Tim Parkin who served as Associate Editors in this process and guided the review process and revisions for these papers. Without their eff orts, this special issue would not be possible. Th e conference requires the eff orts of many individuals and a special thanks goes to the Symposium Steering Committee of William Holenstein, USDA Global Change Program Offi ce; Jan Lewandrowski, USDA Global Change Program Offi ce; Charles Walthall, USDA ARS; Carolyn Olson, USDA NRCS; Cheryl Butler, USDA Farm Services Agency; Luis Tupas, USDA CSREES; Allen Solomon, USDA Forest Service; Chris Farley, USDA Forest Service; Charles Rice,