Measuring and Monitoring Soil Carbon - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 191

difference. Further, monitoring protocols must
be designed to detect changes in soil proper-
ties over relevant spatial and temporal
scales, with adequate precision and statistical
power. For example, the effect of climate
change on SOC is observed more readily at a
broad scale than at a smaller spatial scale
(Wang et al ., 2010). Alternatively, the preci-
sion required for reporting possible avoided
emissions, expressed as metric tonnes
CO 2 -equivalent, will be different (i.e. higher)
for a strict 'compliance' (e.g. Kyoto type)
than for a 'voluntary' (e.g. Chicago Carbon
Exchange (CCX)) project on the carbon-
offset market (Kollmuss et al ., 2008). For the
latter, C sequestration is generally seen as
an additional benefit next to improving food
security, resilience, biodiversity and human
well-being/livelihood (Milne et al ., 2010).

completed within the framework of the
Env ironmental A ssessment of S oil for M o n-
itoring (ENVASO) project (Morvan et al .,
2007), while the variance of the SOC within
European countries was estimated based on
the SOC map of Jones et al . (2005). Overall,
the variance of SOC concentrations increases
from Mediterranean countries to colder and
more humid north European countries where
extensive areas of organic soils occur. As the
density of the monitoring sites varies widely
between European countries, the MDD is
generally high, ranging from 10 to 30 g C kg − 1
in Nordic countries (Estonia, Luxembourg,
Latvia, Northern Ireland and Finland) to
less than 5 g C kg − 1 in southern countries
or countries with a dense SMN (England
and Wales, Bulgaria, Italy, Greece, Hungary,
Romania, France, Portugal, Spain, Belgium,
Austria and Malta) (Saby et al ., 2008). Given
an expected mean rate of SOC change of
0.6% C year − 1 (Bellamy et al ., 2005), the
relatively dense SMNs in most countries will
detect such a change in close to 10 years.
Similarly, Schrumpf et al . (2011) recom-
mend continuous soil monitoring for SOC
at time intervals of 10 years as a compromise
between detectability of changes and tem-
poral shifts in trends. It should be noted
here, however, that this is longer than the
duration of many land use and management
projects that involve the measurement of
SOC stock changes (i.e. for the baseline and
at the end of the project). Alternatively, some
countries use an interval of 5 years (see
Table 16.2 ) .
Spencer et al . (2011) evaluated the po-
tential of an SMN on US agricultural lands
to detect changes in SOC. They combined
model-based changes in SOC, as produced
by the Century model for the UNFCCC re-
porting, with the variance of the estimates
of the model runs at a subset of the 186,000
National Resources Inventory sites (NRI).
The results of these model runs indicate that
the slope of the standard error against the
sample size declines after model runs at
6000 NRI sites. They argue that an a priori
knowledge of the variance per strata allows
an optimal allocation of sampling units,
resulting in an efficient use of resources for
establishing an SMN.

Sampling design and statistical methods

The main difficulty in assessing changes
in SOC level at the field, landscape and re-
gional scale is not linked to the accuracy
of SOC analysis in the laboratory but to the
design of an efficient sampling system (De
Gruijter et al ., 2006; Conant et al ., 2010).
Garten and Wullschleger (1999) were among
the first to introduce the concept of minimum
detectable difference (MDD). This concept is
based on the Central Limit Theorem and pro-
vides the smallest difference between two
sampling campaigns that can be detected,
taking into account the number of samples
and the variance of the SOC. According to
Spencer et al . (2011), a national SMN should
enable detection of broad-scale changes in
SOC related to multiple drivers such as land
use, management and climate change. Such
an SMN should contribute to the under-
standing of the regional C cycle by detecting
changes in soil fertility and fluxes of CO 2
between the soil and the atmosphere. Saby
et al . (2008) applied the MDD concept to
evaluate the statistical power of SMNs to
detect a certain change in SOC stock. The
parameters of the European SMNs such as
sampling design, the coordinates of the
monitoring sites and the number of sampling
campaigns were obtained from questionnaires