188.8.131.52.4. Sampling strategy
The number of soil pits or cores that must be sampled varies according to the
topographic variation and spatial heterogeneity in the distribution of soils.
At the large (regional/national) scale, a stratified random sample scheme is
statistically efficient for soil data, provided that the basis for stratification
accounts for a significant fraction of the variation in soil carbon contents.
It is essential that the sample is accurately placed within the stratum, which
requires the use of Geographic Information Systems (GIS) and Geographical Positioning
Systems (GPS) or accurate field survey. For validation purposes, all sample
locations should be geo-referenced as accurately as possible. The actual plot
location should be marked to allow relocation of the plot reference point, for
resampling in close proximity to prior sampling. High precision is requiring
to avoid confounding by the high level of fine-scale variation that is common
in soil carbon. Where an a priori randomly determined location is found to be
inaccessible, it must be replaced with a sample having similar characteristics.
The number of samples and the choice of location depend on the land-use and
soil management systems.
At the project level, sampling strategies could be based on representative
toposequences or catena for major land uses within the ecoregion of interest.
The sampling strategy should be determined when the spatial distribution of
the overlying plant community is known. Material should be collected at the
end of each growing season (at the maximum stage of decomposition and with the
maximum litter input), but at a minimum at the beginning and end of each commitment
period. Uncertainty associated with differences in stock between two times can
be substantially reduced by using paired samples; in other words, samples taken
at time 2 should be from very close to the samples taken at time 1 and the difference
calculated per pair, rather than on the accumulated averages (Lal et al.,
184.108.40.206.5. Sampling depth
Except in unusual circumstances and in peats, the amount of soil organic matter
declines exponentially with depth (Nakane, 1976). Globally, only about one-third
of the organic carbon in the surface 2 m is found at a depth of 1-2 m (Batjes,
1996). Losses or accumulations of soil carbon are greatest in the upper soil
profile (0-15 cm), which should be sampled most intensively (Richter et al.,
1999). In the case of management changes on agricultural land (e.g., Smith et
al., 1997b, 1998), samples must be taken from lower in the profile because
accumulations of carbon in the surface horizons may be balanced by losses of
carbon at depth (Powlson and Jenkinson, 1981; Ismail et al., 1994; McCarty
et al., 1998). Defining, a priori, a global soil depth to which carbon
should be analyzed is not practical, however. The sampling depth should be below
the depth that significant change in carbon is expected to occur.
Soil carbon changes in some situations may occur at substantial depths (e.g.,
2-5 m in deep, tropical soils), whereas in cropped soils undergoing management
change, almost all of the change may occur in the top 30 cm. The important issue
is that the depth used in one inventory should match the depth used for that
location or land-use type in the next inventory. The depth definition should
be left open, subject to consistency between inventory times. Determining the
effective sampling depth is not easy when the surface level is subsiding or
aggrading (e.g., in highly organic bog soils, soils under arable agriculture,
or soils that are subject to rapid erosion). Simultaneous measurements of bulk
density are essential in all cases (Lal et al., 2000); such measurements
can be used to calculate an effective soil depth in mineral soils. Measuring
soil depth as a function of mass, rather than distance from the surface, may
be desirable because management activities may significantly change soil bulk
density between measurements, substantially altering the amount of carbon found
in different increments of distance from the soil surface. Failure to account
for bulk density changes could lead to large artifacts in apparent carbon storage
(Ellert and Bettany, 1995). In organic soils, an absolute reference level is
required, which must be surveyed from a stable datum.