Soil Organic Matter Management: The Roles of Residue Quality in C Sequestration and N Supply - Sustainable Management of Soil Organic Matter - page 120

suspension; and (iv) cellulose powder. A control treatment consisted of
fresh soil with no C addition. The jars containing the treated soil were incu-
bated in the dark at 20 C. Alkali traps were changed at 1, 3, 9 and 23 days,

and analysed for C content by titration. Nematode counts were made on
initial soils, and on soils from a parallel incubation after extraction by flota-
tion (Griffiths et al ., 1990).

°

Model description

MOTOR is an extension of the model concepts of Jenkinson and Rayner
(1977) and of the SUNDIAL model (Bradbury et al ., 1993). Crop residue
C is split into decomposable plant material (DPM) and resistant plant
material (RPM). The decomposition rate of the RPM decreases according
to the formulation of Parton et al . (1987), to allow for the lignin content
(L s ):
K RPM = K DPM e(−3L s )

(1)

We assumed that in the absence of lignin, the RPM pool (cellulose +
chemical addititives) decomposes with the same kinetics as DPM since,
following the waste treatment process, enzymes should be available in the
PMS for immediate decomposition of cellulose in soil. Crop residue
organic C decomposes to microbial biomass (BIO), non-protected organic
matter (NOM) and CO 2 . The assimilation efficiency was taken as 60%.
Inert organic matter (IOM) is determined in part by the charcoal content
of the soil. NOM decomposes to CO 2 and microbial biomass. Microbial
biomass turns over to form NOM which in turn may become protected
in soil as POM (protected organic matter) with a rate K a . POM must first
be desorbed (rate constant K d ) into the NOM pool before it can decompose
(Table 3.3.1). The maximum size of the POM pool (X) is defined by soil
texture (Hassink and Whitmore, 1997).
Microbial biomass decays by second-order, not first-order kinetics:

)BIO 2

d(BIO)/d t = k 2 α (1 − α (2)

where BIO = biomass C, = proportion of biomass that consists of

primary and secondary consumers (e.g. protozoa and nematodes) and
1

α

= proportion of biomass that is made up of detritivores (bacteria,
fungi, etc.). Where

−α

α is reasonably constant, the expression k 2 α (1 −α )is

also nearly constant, and equation 3 can be simplified to:

BIO 2 d(BIO)/d t = k 2 ′ (3)

based on fitting Equation 3 to
measured soil biomass N and C. PMS-derived biomass is highly labile,

Whitmore (1996) derived a value for k 2 ′