Carbon cycling in soil

www.frontiersinecology.org © The Ecological Society of America S contain a huge and dynamic pool of carbon (C), that is a critical regulator of the global carbon budget. As the repository for more than three-fourths of the earth’s terrestrial C, soils store 4.5 times the amount of C contained in vegetation (Lal 2004). Small changes in the processes governing soil C cycling have the potential to release large amounts of CO2, a greenhouse gas. Because soil gas release is temperature sensitive, global warming will probably create a positive feedback, causing soils to release more greenhouse gases that will further exacerbate the problem. Global climate change may soon provide an opportunity to study the effects of altering soil–atmosphere interactions on a massive scale – or may be doing so already. Belowground studies pose special challenges because underground structures, organisms, and processes are invisible to humans, who are more used to studying aboveground systems. The types of satellite remote sensors that have advanced aboveground C studies do not penetrate beyond the soil surface, making remote measurements over large areas impossible. Soil is a dynamic, living system, which is altered by removal from its belowground environment. It is a mix of living and dead plant matter, fungi, macroand microfauna, plant exudates, and animal waste products, embedded in a matrix of solids, liquids, and gases. When the “black box” of this soil mélange is dissected to study its component parts, its function is altered. In this paper, we review the issues at the forefront of belowground C cycling research. Critical societal needs are pushing the science, and the North American Carbon Program Plan (Wofsy and Harriss 2002) has been funded to implement research on measuring and understanding sources and sinks of CO2, methane, and carbon monoxide in the US and its adjacent oceans. Also, the ability of soil to sequester and retain C is now being brokered via soil C credits under the US Department of Energy’s Voluntary Greenhouse Gas Reporting Program, which was established by section 1605(b) of the Energy Policy Act of 1992. In the sections below, we review some major advances in the field of belowground processes, describe some of the major unanswered questions, and make recommendations for fostering future progress in this important area of research. This paper is the result of a one-day workshop convened on February 20, 2003, in Arlington, Virginia by Rachael Craig and Knute Nadelhoffer, current and former program officers in the Division of Earth Sciences and Division of Environmental Biology at the National Science Foundation. Additional input occurred at the University of Michigan Biological Station Soil Biology Workshop, held June 16–17, 2004, in Pellston, Michigan.