Agriculture Reference
In-Depth Information
Gaseous
Loss
Crop
Fertilizer
Litter
Runoff
Root
Microbial
Biomass
Soil
Organic
C and N
Available
C and N
Stable
Aggregates
Plant Uptake
Matrix and Bypass
FIGURE 9.4
Ecosystem budget model to examine pools and Þuxes of C and N in the absence of earthworms.
Note: Bold boxes indicate pools and Þuxes that are predicted to be affected most by the absence of earthworms.
from casts can contribute to overall losses of N from the system, the formation of stable soil
aggregates from casts could lead to longer term protection of C and N.
In the absence of earthworms (Figure 9.4), our model predicts that surface litter and soil organic
C and N pools are larger than when earthworms are present. The microbial biomass C and N pool
is also larger but with a predicted slower turnover rate. The increase in size of these pools, coupled
with longer residence times, could lead to a decrease in available C and N. As a consequence, there
would be less loss of C and N in gaseous and leaching Þuxes. Leaching, in general, would be
expected to be less because of the absence of earthworm burrows, which contribute to bypass Þow.
However, the absence of earthworm burrows, which open to the soil surface, could also lead to
increased overland Þow and associated loss of nutrients in runoff. The role of earthworms in
effecting overland Þow of nutrients has rarely been investigated, but Sharpley et al. (1979) reported
increased losses of N and P in surface runoff in pastures without earthworms (treated with carbaryl)
compared with that in pastures with earthworms present. Before it can be determined whether
earthworms contribute to a net gain or loss of nutrients from a system, their inÞuence on these
various nutrient pools and Þuxes needs further quantiÝcation.
F
E
UTURE
XPERIMENTS
To provide data to validate the simulation and nutrient budget models, we suggest experimental
approaches that are Ýeld based and that use manipulated earthworm communities, stable isotopes (
C
13
and
N), and, when possible, extend for several years or use a chronosequence to explore effects on
nutrient cycling over longer time scales. To investigate the effects of earthworms on microbial activity
and turnover, microbial biomass could be labeled with
15
C-labeled glucose and
N-labeled ammonium
13
15
