2.4.2.3.6. Analytical methods
Accurate laboratory assessments of soil carbon content require the use of carbon
(combustion) analyzers. Analytical methods based on mass loss on ignition or
wet digestion (see Nelson and Sommers, 1982; Tiessen and Moir, 1993) are prone
to biases. Although these biases can be calibrated (Kalembasa and Jenkinson,
1973), they add another level of uncertainty to the estimate. Methods for sample
preparation and analysis for SOC and SIC are well documented elsewhere (Lal
et al., 2000). The carbon stock per unit area of land can be calculated
from the percentage of carbon and the soil bulk density. For transparency and
verifiability purposes, soil samples can be geo-referenced and archived in a
dried state in a dark, cool place for future carbon analysis, and original data
can be retained. Some archived samples should be re-analyzed at the same time
as repeat samples to confirm that there has been no drift in analytical precision.
Methods also exist to measure different components of SOC (see Section
4.2).
Modern dry combustion procedures have an analytical precision of ±2-3 percent.
The error associated with assessments at the regional and global scales also
depends on errors introduced through spatial variability and sampling error
and therefore will be much higher than 3 percent. Establishing quality protocols
for all methods is important. The cost of analysis varies from region to region,
by number of samples, and by efficiency. On a global scale, the per-sample cost
of carbon analysis may range from US$3 (where labor costs are very low) to about
US$20, with significant proportions of the total cost accounted for by sampling
(high labor cost), sample preparation, bulk density measurement, inorganic carbon
analysis (or pre-acidification of samples to remove carbonate), archival of
samples, and quality control efforts. This cost range is broadly consistent
with that reported for a Canadian study by Izaurralde et al. (1997).
Special consideration is necessary for peatlands and other wetland soils-collectively
known as Histosols-in which the layer of undecomposed organic matter may be
several meters in depth. These soils are vulnerable to large losses of organic
matter upon changes in regional drainage or soil temperature. Particularly in
areas that have been drained, accurate measurement of changes in carbon stock
in peat soils requires reliable measurement of carbon loss through decay of
organic material. Accurate measurement of changes in carbon in peat soils probably
requires fixed reference points for measuring subsidence from decay. When subsidence
is being measured, corrections should be made for depth changes resulting from
changes in soil moisture content. Estimates of changes in the storage of carbon
in peatlands must avoid biases that may result from changes in bulk density
because of compaction of surface layers. Often, the age of peat at a given depth
may be determined by 14C or 210Pb dating, allowing workers to assess additions
or losses of organic carbon above a depth of known age.
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