Subject: Sundquist: Re: Soil respiration measurements - response (fwd) Date: Thu, 4 Oct 2001 21:03:49 -0400 From: NACP mail account To: harriss ---------- Forwarded message ---------- Date: Thu, 04 Oct 2001 14:17:32 -0400 From: Eric T. Sundquist To: Elisabeth Holland Cc: Richard Birdsey , Dennis Ojima , Eldor Paul , Mac Post , Dave Schimel , nacp@python.as.harvard.edu, nacp@io.harvard.edu Subject: Re: Soil respiration measurements - response Hi Beth, Thanks for your comments. I agree that the interaction between long-term and short-term dynamics is a critical issue for the whole NACP, not just the soil respiration component. The atmospheric measurements and modeling will pick up mainly the fast turnover, whereas the most important source/sink interactions may be slower. My view is that we must begin by nailing down the fast sources and sinks before we can reasonably attribute the slower fluxes to specific processes. I agree that Century and Biome-BGC provide one route for scaling that should certainly be exploited. Hopefully, NACP will be a rigorous test. We will have to be creative, and I worry about starting with an approach too tightly bound to a particular model structure. No, I did not incorporate all that we know about soil respiration. I was reluctant to take the space that would be required for a literature review. (By the way, soil respiration measurements go back much further than 30 years.) Some of the historical measurements you mention (base absorption) have been shown to be susceptible to extreme inaccuracies, and many others are of undefined accuracy. As for what we have learned from these measurements, maybe it is not enough to say, "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." Could you please suggest what should be added or expanded? Perhaps you could write something about isotope applications? I tried to suggest an approach in broad terms that would link aboveground fluxes (including those inferred from eddy correlation and other atmospheric data methods) with belowground mechanisms, using soil surface measurements as a key linkage. I hope these methods are not, as you say, incompatible. Perhaps you are inferring something more specific than I intended. What would you suggest? Thanks again, Eric ----- Original Message ----- From: "Elisabeth Holland" To: "Eric T. Sundquist" Cc: ; ; "Richard Birdsey" ; "Dave Schimel" ; "Eldor Paul" ; "Dennis Ojima" ; Sent: Friday, September 28, 2001 5:23 AM Subject: Re: Soil respiration measurements > Dear Eric, > > This does not incorporate all we do know about soil respiration. > This is a measurement that has been made for 30 years-first with > base traps, then with GC measurements and now with a LICOR. the > use of isotopic tracers- 14C, 13C, 18O have all shed light on the > problem. The flux does not respond to just short-term drivers, > but includes long -term ones. The interaction between the short > and long-term dynamics is the cruz of the current problem. > Separating plant and heterotrophic respiration contributions is > key to this. We have reasonable mechanistic models (ie Century, > Biome-BGC) that seem to work to a first approximation, but the > rigorous work of quantifying how well they work has not been > done. We have some tools for doing the scaling, lets make use of > them as part of this plan. > > The approach outlined here seems to combines a correlation > approach with mechanistic modeling. The two seem incompatible > with one another. I do not see a convincing argument for why > these are hard to scale articulated, or for what we have learned > in all of these years of doing measurements. > > Beth > > "Eric T. Sundquist" wrote: > > > 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 > -- 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