Modelling Soil Organic Matter Dynamics – Global Challenges - Sustainable Management of Soil Organic Matter - page 68

residue (APSIM-Residue) dynamics (Probert et al ., 1998b), and sugarcane
growth (APSIM-Sugarcane; Keating et al ., 1999). These modules have
been fully described in the aforementioned papers, but a short description
of them follows. The modules are one-dimensional and driven by daily
climatic data. The dynamics of water, N, C and roots are simulated in soil
layers, with water (and associated nitrate) moving between layers where
gradients exist. The soil organic matter is divided into three 'pools', with
fom representing the fresh organic matter (i.e. roots and incorporated plant
residues), biom representing the active biomass in the soil, and hum repre-
senting the humified material. Part of the hum pool is considered inert. The
soil water module is a 'cascading bucket' water balance model, with water
between the drained upper limit ( dul ) and saturation draining to the layer
below. The drainage rate is controlled by the parameter swcon . The lower
limit of plant available water is defined by the parameter ll15 . Evaporation
from the soil follows Ritchie's (1972) two-stage evaporation model. The
presence of plant residues on the soil surface affects runoff (and hence infil-
tration) and evaporation. The sugarcane module uses intercepted radiation
to produce assimilates, which are partitioned into leaf, structural stalk,
roots and sugar. The processes represented in the module are responsive to
radiation and temperature, as well as water and N supply. Farming opera-
tions (such as fertilization, planting, incorporation of crop residues through
cultivation or burning of crop residues) can be specified through the
APSIM-Manager module to represent actual or hypothetical conditions.

APSIM-Residue and its Parameterization

In APSIM, plant residue on the soil surface is considered separately from
soil organic matter. C in the residues is transferred into the soil organic
matter pools upon tillage, which results in incorporation of residue into the
fom pool, or decomposition, which transfers C into the biom and hum pools.
The rate of residue decomposition is controlled by first order kinetics;

d R /d t = − kR (1)

where R is the mass of residue per unit area, t is time and k is the rate
coefficient given by

k = r max . F C:N . F temp . F moist . F contact (2)

where r max is the maximum (or potential) decomposition rate and
F C:N , F temp , F moist and F contact are factors, scaled from 0 to 1, accounting
for the limitations to decomposition of residue C : N, temperature,
moisture and residue-soil contact, respectively. The functions defining the
factors are,

F C:N = exp [ j (1 − C:N/C:N opt )], C:N>C:N opt (3a)