Interpreting, measuring, and modeling soil respiration

This paper reviews the role of soil respiration in determining ecosystem carbon balance, and the conceptual basis for measuring and modeling soil respiration. We developed it to provide background and context for this special issue on soil respiration and to synthesize the presentations and discussions at the workshop. Soil respiration is the largest component of ecosystem respiration. Because autotrophic and heterotrophic activity belowground is controlled by substrate availability, soil respiration is strongly linked to plant metabolism, photosynthesis and litterfall. This link dominates both base rates and short-term fluctuations in soil respiration and suggests many roles for soil respiration as an indicator of ecosystem metabolism. However, the strong links between above and belowground processes complicate using soil respiration to understand changes in ecosystem carbon storage. Root and associated mycorrhizal respiration produce roughly half of soil respiration, with much of the remainder derived from decomposition of recently produced root and leaf litter. Changes in the carbon stored in the soil generally contribute little to soil respiration, but these changes, together with shifts in plant carbon allocation, determine ecosystem carbon storage belowground and its exchange with the atmosphere. Identifying the small signal from changes in large, slow carbon pools in flux dominated by decomposition of recent material and autotrophic and mycorrhizal respiration is a significant challenge. A mechanistic understanding of the belowground carbon cycle and of the response of different components to the environment will aid in identifying this signal. Our workshop identified information needs to help build that understanding: (1) the mechanisms that control the coupling of canopy and belowground processes; (2) the responses of root and heterotrophic respiration to environment; (3) plant carbon allocation patterns, particularly in different forest developmental stages, and in response to treatments (warming, CO2, nitrogen additions); and (4) coupling measurements of soil respiration with aboveground processes and changes in soil carbon. Multi-factor experiments need to be sufficiently long to allow the systems to adjust to the treatments. New technologies will be necessary to reduce uncertainty in estimates of carbon allocation, soil carbon pool sizes, and different responses of roots and microbes to environmental conditions.