Agriculture Reference
In-Depth Information
C content in the 0- to 10-cm layer
C %
1.9
1.8
1.7
1.6
1.5
1.4
1.3
1.2
a
a
a
a
a
a
a
b
0 1 2 3 4 5 6
Number of Crops
Earthworm +
Earthworm -
N content in the 0- to 10-cm layer
N %
0.14
0.13
a
0.12
a
a
a
0.11
0.10
a
a
0.09
0 1 2 3 4 5 6
Number of Crops
FIGURE 8.4
Changes in carbon and nitrogen contents in an ultisol of Peruvian Amazonia submitted to
traditional slash-and-burn agriculture in the presence and absence of earthworms
(Pontoscolex corethrurus).
Pairs of data with different letters are significantly different. (From Pashanasi et al. 1996. With permission.)
M
ODELING
Currently available models that describe SOM dynamics do not take into account explicitly the
effects of soil invertebrates. Part of the effects of soil-inhabiting invertebrates may be implicit,
either when describing initial soil conditions (e.g., through how the C:N ratio is influenced by their
activities) or when assessing the decomposition rates of C pools that will actually include the overall
effects of earthworms. An attempt was made to simulate the effects of earthworm activities on the
three kinetically defined organic pools of the CENTURY model (Parton et al. 1988). This model,
which simulates plant production and SOM dynamics in various agricultural and natural systems,
considers three different SOM functional pools. A labile fraction (active SOM) has a rapid turnover
and exists in the form of live microbes and microbial products. The remaining fractions comprise
SOM that is stabilized, either because it is physically protected (slow pool), because it is chemically
resistant (passive pool) to decomposition, or both.
As a first step, the CENTURY model was used to simulate SOM dynamics and plant production
in the savannah of Lamto (Martin and Parton unpublished data) and validated against observed
data values. Then, the model was calibrated for this site and simulated C dynamics in earthworm
casts and a control soil of the same savannah sieved at 2-mm aperture. Observations by Martin
(1991) during a 450-day incubation of earthworm casts and a control surrounding soil sieved at 2-
mm aperture were used as a reference. For the sieved soil, the model outputs were close to the
experimental results, provided slow soil C decomposition rates increased. Conversely, it was
necessary to decrease the rates for both slow and active decomposition rates of soil C to simulate
its dynamics in casts. Earthworm removal was simulated by replacing the active and slow soil C
decomposition rates of the model with those obtained by calibration with the control soil. Under
these assumptions, the CENTURY model indicated that SOM would decrease by about 10% in 30
