Soil ecology and agroecosystem studies - Microbial Ecology in Sustainable Agroecosystems - page 4

Structural
C
(3y)

Plant
Residue

L / N

A I-A

Metabolic
C
(.5y)

CO 2 CO 2

.45
SL

.55

BL .55

Active
Soil
C
(1.5y)

F (T)=(.85-.68T)

CO 2

.42

CO 2

CO 2

.55

.3

.03

Slow
Soil
C
(25y)

.004A

I-F(T)-.004

CO 2

SL
BL

Surface Litter
Soil Litter
Lignin to Nitrogen Ratio
Lignin Fraction
Soil Silt + Clay Content
(Fraction)

.55

Passive
Soil
C
(1000y)

L / N

A
T

Figure 1.1 Flow diagram for the C flows in the CENTURY model (from Parton, W.J., D.S. Schimel,
C.V. Cole, and D.S. Ojima. 1987. Analysis of factors controlling soil organic matter levels in Great
Plains grasslands. Soil Science Society of America Journal 51:1173-1179. With permission). Plant residue
is divided into structural and metabolic C, and soil C is considered as three pools with average
turnover times of 1.5 y (active), 25 y (slow), and 1,000 y (passive).

different maximum decomposition rates ( Figure 1.1 ) . The decomposition of aboveground
and belowground structural and metabolic materials is mediated by microbial activity.
The original model functioned on a monthly time step, but subsequent modifica-
tions of maximum decay rates and temperature and moisture factors have allowed the
model to function on a daily time step, which may be more appropriate for agricultural,
pulse-driven systems. The model partitions SOM into active, slow, and passive pools, with
turnover rates of 1-5 y, tens to hundreds of years, and hundreds to thousands of years,
respectively. Interactions between the compartments are influenced by soil texture, mois-
ture, initial SOM content, and plant production dynamics.

1.2.1 The origins of organic material

Organic material enters soils from within and outside the ecosystem. The internal or
autochthonous sources originate from primary production by plants, algae, and microbes.