Modelling Soil Carbon - Soil Carbon: Science, Management and Policy for Multiple Benefits - page 206

modifiers describing the impact of low pH,
to extend simulations to highly organic as
well as mineral soils (Smith et al ., 2010).
The CENTURY model uses a similar approach
(Zhang et al ., 2002). While CENTURY simu-
lates plant growth and senescence, and there-
fore detrital C entering the soil (subject to
decomposition), ECOSSE is initialized us-
ing an equilibrium run to determine the size
of SOM pools and plant inputs needed to
achieve steady state at the measured C con-
tent. This is a more robust approach for soils
in steady state, but organic soils usually ei-
ther accumulate C, due to the slow rate of
decomposition compared to the plant in-
puts, or lose C, due to a land-use change that
has disturbed the previously accumulating
system. The size of soil C pools can be esti-
mated if the rate of accumulation or degrad-
ation is known. However, the rate of accumu-
lation or degradation is often unknown in
larger-scale simulations. Alternative methods
are needed for dealing with soils that are
not in steady state, especially as the scale of
simulations increases.
Simulation of SOM turnover under
tropical conditions has to date been re-
stricted by the lack of available data due to
the complex and often inaccessible nature
of tropical ecosystems, especially peatlands
(Farmer et al ., 2012). The effect of the
higher temperatures found in tropical sys-
tems on the long-term dynamics of SOM
has not been widely tested. Simulations of
the decomposition of the large fragments of
woody materials often found in tropical
peat swamps have also not been well tested
against long-term data.
The impact of salinity on the rate of de-
composition and plant inputs has also not
been widely incorporated into SOM models.
This was simulated by Setia et al . (2012) us-
ing a rate modifier derived from laboratory
and field experiments and a reduction fac-
tor for the plant inputs. This has not, as yet,
been widely tested against long-term field
experiments due to the lack of suitable data
on a range of saline soils.
Volcanic soils pose another set of chal-
lenges, associated with adequately simulat-
ing the protection of SOM by the inorganic
minerals in the soil. Shirato et al . (2004)

modified the decomposition rate constant of
the humus pool in RothC to account for pro-
tection of the humus by aluminium (Al)
complexation. The rate constant was modi-
fied according to the content of pyrophos-
phate extractable aluminium in the soil.
This modified version of RothC has been
shown to improve simulations of four long-
term experiments on andosols in Japan. Fur-
ther evaluation of this approach against
long-term experiments on other volcanic
soils is needed.

Modelling SOC at Different Scales

The majority of SOC models were originally
developed to model dynamics at the plot
scale, as this is the scale at which experi-
mental data are available. There are there-
fore numerous examples of application of
different SOC models to plots under culti-
vated and native land use and following
land-use transitions (Paustian et al ., 1992;
Li et al ., 1994; Powlson et al ., 1996; Leite
et al ., 2004). Many models have been tested
and parameterized at the plot scale for dif-
ferent climate and soil conditions across the
globe. Plot-scale studies typically represent
homogeneous conditions, and are therefore
limited in their applicability to other sites.
At the other end of the scale, global es-
timates of SOC change have been made us-
ing models that work on large grids (0.5° ×
0.5°) of the global surface. These are then
linked to soil, climate and ecosystem type
and use either a single pool or very general-
ized models to describe SOC turnover (King
et al ., 1997). Problems arise when we try to
use such approaches at a smaller scale due
to generalizations.
With growing interest in the role of soil
C in the mitigation of climate change, a need
to estimate SOC stocks and changes at the
national to subnational scale has been recog-
nized. Large-scale estimates require model-
ling in some form, both to scale up estimates
of SOC stocks and to make projections of
stock changes through time. The IPCC com-
putational method was devised for national
scale GHG accounting including SOC stock
change (IPCC, 2004). It computes changes