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National Council for Air and Stream Improvement (NCASI), P.O. Box 13318, Research Triangle Park, NC 27709-3318
* Corresponding author (evance{at}ncasi.org).
CARBON RESIDING in forest soils is closely tied to long-term site productivity and has gained prominence in recent years for its role as a potential sink and source of atmospheric CO2. Due to its size, only a small change in the global soil C pool could have a significant impact on atmospheric CO2 concentrations. Uncertainties in measuring and detecting changes in soil C pools remain high, however, both at individual sites and in extrapolating site-level data to regional, national, or global scales. This uncertainty is demonstrated by recent estimates of U.S. C sinks in forest litter, woody debris, and mineral soil for 1980 to 1990 that range from 0.03 to 0.15 Pg (billion metric tons) C yr-1, or 7 to 34% of total (median) sink for the contiguous USA (Pacala et al., 2001). As is often the case, these estimates were derived indirectly using modeling approaches because formal soil C inventory data do not currently exist.
Forest soil C pools are inherently variable, both spatially and temporally. This variability impedes the ability of scientists and resource managers to accurately measure these pools and to detect their responses to management practices or changes in land use. For example, root systems influence spatial and temporal patterns of belowground C. Root-derived C may reside in soil for long periods and, whether dead or alive, coarse roots are particularly difficult to excavate and quantify and are rarely included in belowground C inventories. An abundance of stones can also impede accurate measures of site-level soil C pools, yet they are often not directly measured. Forest management practices such as harvesting and site preparation exacerbate natural soil variability by altering surface residues and changing the depth distribution of soil C. Individual soil C fractions also respond very differently to changes in management or land use.
Accurate and precise approaches must be available for assessing the effects of management practices and land use change on soil C if this important pool is to be adopted as part of any future C accounting system. The Kyoto Protocol, for example, limits reporting of C sequestration activities to those that are both "measurable and verifiable." Even if allowed as part of an accounting system, credit for C sequestration activities could be discounted if high standards of measurement cannot be met (IPCC, 2000). A successful accounting system will require simple, reproducible approaches that can be used by resource managers and others outside of the research community. Credible accounting will also require standardized approaches for validating measurements and for developing default parameters and model inputs. In some cases, these goals can be met by improving traditional sampling protocols used in field experiments. In others, new technologies must be developed and applied.
This set of papers, titled "Approaches and Technologies for Detecting Changes in Forest Soil Carbon Pools," is derived from a symposium held at the 2001 ASA-CSSA-SSSA annual meetings in Charlotte, NC. Topics addressed include: (i) current challenges associated with accurately measuring and detecting changes in forest soil C pools, (ii) modifying traditional approaches to enhance their accuracy and precision, (iii) developing, testing, and applying new technologies and novel approaches, and (iv) unresolved issues where new approaches and additional research are needed.
Received for publication January 7, 2002.
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