Monitoring and Verifying Changes of Organic Carbon in Soil

Changes in soil and vegetation management can impact strongly on the rates of carbon (C) accumulation and loss in soil, even over short periods of time. Detecting the effects of such changes in accumulation and loss rates on the amount of C stored in soil presents many challenges. Consideration of the temporal and spatial heterogeneity of soil properties, general environmental conditions, and management history is essential when designing methods for monitoring and estimating future changes in soil C stocks. Several approaches and tools are required to develop reliable estimates of changes in soil C at scales ranging from the individual experimental plot to broad regional and national inventories.

There are two basic methods for determining soil C changes--direct methods and indirect methods. Direct methods include field sampling and laboratory measurements of total C, various physical and chemical C fractions, and C isotopes. Carbon content changes are differences resulting from changes in land management and are expressed as the change in C amount on an area (kg m^-2) or volume basis (kg m^-3). The calculation is not difficult but requires awareness of the vertical and horizontal variability of soil properties in order to avoid systematic errors. Another promising direct method is eddy covariance measurement of CO₂ fluxes. The vertical component of air movements (eddies) over a vegetated surface can be isolated and quantitatively measured as can CO₂ concentration associated with each eddy. By correlating eddy size and CO₂ concentration for each upward and downward moving eddy, the net uptake or release of C by vegetation plus soil can be calculated. The accuracy and precision of this method is improving as more experience is gained.

Direct methods are essential for determining changes in soil C content for different land management treatments at the plot scale. To determine soil C pools and rates of change for large areas, it is necessary to extrapolate the relationships developed at the plot and field scale. Indirect methods are suitable for large areas. Indirect methods include simple and stratified accounting where plot estimates are multiplied by all applicable areas as determined by soil survey, land cover, climate, and other spatial data. Environmental and topographic relationships with C storage and change and modeling approaches can also be applied when available. Remote sensing and other geographical information are valuable in appropriately representing areas in these spatially aggregated estimation methods.

Accurate science-based methods are available for monitoring and verifying changes in soil C. Codifying a set of methods into protocols that can be used to account for C sequestration in public agreements or private trading contracts needs refinement and testing. The level of precision and effort required will vary with the purposes for which the measurements are applied. Examples of purposes include (a) determining compliance with local, regional, and national laws or treaties regulating CO₂ emissions; (b) evaluating joint implementation projects; and (c) assigning C credits and offsets.

Current methods are effective for evaluating soil organic C changes at relatively low precision (20 to 50% error) and at widely spaced time intervals (minimum 3 to 5 years) with levels of effort that are reasonably affordable. Since relatively small amounts of C sequestered in soils could significantly reduce the rate of increase of atmospheric CO₂ and effectively buy time until low-cost methods for reducing CO₂ emissions are available, there will be considerable pressure to increase the reliability and precision of monitoring soil organic C changes on even shorter time scales.

Scientists are very interested in participating in the development of soil C monitoring protocols. However, in order to contribute to the development of these monitoring mechanisms, they need to know the economic and policy rules under which these mechanisms are anticipated to operate. A multi-sector, multi-discipline, and multi-national effort is required if we are to make monitoring and verification of C sequestration in soils a useful and widely used procedure.

Current and future technologies for monitoring soil carbon. (RS = Remote Sensing, LULC = Land Use and Land Cover, PAR=Photosynthetically Active Radiation, exp.= experimental, and SAR = Synthetic Aperature Radar)

¹Oak Ridge National Laboratory, Oak Ridge, TN; ² Pacific Northwest National Laboratory, Richland, WA; ³ EROS Data Center, Sioux Falls, SD

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