Date: Fri, 21 Sep 2001 12:04:53 -0400 From: "Eric T. Sundquist" Organization: U.S. Geological Survey To: nacp@python.as.harvard.edu, nacp@io.harvard.edu Cc: Richard Birdsey , Dave Schimel , Eldor Paul , Dennis Ojima , eholland@ucar.edu, wmp@ornl.gov Hello all, Some soil respiration measurement words below. I tried to incorporate feedback from the workshop as well as from the draft I circulated to the writing group shortly before the workshop. Comments/suggestions/criticisms welcome. Eric ======================== Soil respiration measurements (Sundquist, 9/21/01): One of the most challenging aspects of the NACP will be the estimation of soil respiration fluxes. The efflux of CO2 from soils includes both plant respiration (from live roots) and microbial heterotrophic respiration. Together, these CO2 sources may equal or exceed NPP. Determining the net release or uptake of carbon by the ecosystem or biome therefore requires not only careful measurement of soil CO2 efflux, but also evaluation of the specific processes that cycle and transport belowground carbon. The estimation of soil respiration may be the weakest component in our ability to understand temporal and spatial trends in the terrestrial carbon cycle. Whereas spatially integrated estimates of NPP can now be developed in the context of robust mechanistic models beginning at the leaf level, there is no comparable framework for understanding the belowground processes that control soil respiration fluxes over the range of temporal and spatial scales of interest to the NACP. Previous work has delineated soil temperature and moisture as important factors controlling rates of soil respiration. These influences are mediated by heterotrophic substrate supply, soil texture, landscape setting, and physiological constraints on microbial and root metabolism. Quantification of these controls is often based on measurements that are necessarily very local and short-term. Extrapolation in space and time is extremely difficult. Existing local studies employ a diverse range of techniques and protocols that are often difficult to compare. Soils are very heterogeneous – both laterally and vertically – and soil properties are not readily accessible by remote sensing. Maps of some soil properties are available, but they cannot yet be related systematically to soil respiration fluxes. There is no widely accepted mechanistic model linking the flux of CO2 at the soil surface to belowground soil properties and carbon cycling and transport processes. To address these problems, we will need to determine how measurements of CO2 fluxes at the soil surface can be related both to estimates based on atmospheric data assimilation (including eddy correlation) and to carbon dynamics and soil properties below the soil surface. This effort will require particular attention to development of methods and protocols for soil measurements and their interpretation at intensive and intermediate study sites. Significant improvements will be required in our ability to monitor soil-surface and belowground processes and to assure the comparability of measurements from place to place and time to time. The optimum strategy will likely combine selective soil respiration measurements with soil descriptive data (including texture, organic matter content, and landscape setting), dynamic biophysical data (including soil temperature and moisture content, as well as land management activities), and new models linking belowground processes and CO2 fluxes to ecosystem function (controlling substrate supply and metabolic rates). This strategy will be particularly important to estimating overall CO2 exchange in areas of rolling or mountainous terrain, where estimates based on atmospheric measurements are exceptionally difficult. -- Eric T. Sundquist U. S. Geological Survey 384 Woods Hole Road Quissett Campus Woods Hole, Massachusetts 02543 USA Phone 508-457-2397 Fax 508-457-2310