Soil food webs in agricultural ecosystems - Microbial Ecology in Sustainable Agroecosystems - page 82

the seasonal rhythm in the activities of the energy channels and processes. What we see
in many intensively managed systems are pulses of microbial activity during times of
plant senescence or after the crop has been harvested, resulting in the decomposition of
residues and the losses of plant-available nutrients through leaching (Elliott et al., 1984;
Hendrix et al., 1986; Moore and de Ruiter, 1991).
The models and experimentation have offered some insight into the mechanisms at
play and the long-term consequences as they relate to sustainability. Field manipulations
point to mechanisms that include the placement of crop residues, the bioavailability of
labile versus resistant soil carbon, the changes in aggregate structure, the relative sizes of
habitable pore space, water availability, and the toxicity of different groups to pesticides,
confirming this conclusion. The more intensive management practices alter the environ-
ment and placement of detritus in ways that favor the activities of bacteria and consumers
of bacteria over fungi and their consumers.
The models highlight the importance of body sizes, physiologies, and life histories
of the soil biota and their resistance and resilience to disturbances. Coleman (1994) noted
that organisms within the bacterial pathway tend to be smaller in size and more energeti-
cally efficient and to possess shorter life histories than their counterparts in the fungal
pathway (see Table 4.5 ) . These differences affect the turnover rates of populations, and
thus the mineralization rates can explain the enhanced activity of the bacterial energy
channel in tilled systems. These traits can also explain differences in resistance and resil-
ience of the pathways to disturbances (Moore et al., 1993). The fungal energy channel,
dominated to a large extent by arthropods, is particularly vulnerable to the physical mix-
ing of soils during tillage. Add to this the use of insecticides that target arthropods and
we can explain in part the demise of the fungal energy channel that is comprised of many
arthropod taxa.
The resilience of the channels in response to disturbances can explain the changes in
the timing of the activities of the energy channels. Investigations of wetting/drying cycles
and freezing/thawing cycles of soils revealed that the organisms within the bacterial
channel are more resilient than those within the fungal energy channels (Allen-Morley
and Coleman, 1989; Hunt et al., 1989). Models of food chains parameterized to resemble
the bacterial energy channel have shorter return times compared to ones that resemble
a fungal energy channel (Moore et al., 2004; Rooney et al., 2006). These results can be
explained in large part by differences in the reproductive and turnover rates of organisms
within the respective channels. Taken together, if we assume that both energy channels
were equally affected by a tillage event with the same proportionate decline in biomass,
the models would predict that the bacterial energy channel would rebound at a faster rate
than the fungal pathway.
These studies demonstrated that intensive agricultural practices have disproportion-
ately high negative effects on the organisms within the fungal pathway and conversely
have high positive effects on organisms within the bacterial pathway. This coupled with
the different capacities of the energy channels to respond to disturbance can explain the
observed shifts in activity of the channels and the processes they mediate. Our models of
coupled bacterial and fungal pathways and whole food webs provide more realistic assess-
ment of how changes in the relative sizes of energy flows or shifts from one pathway to
another might affect food stability ( Figure 4.10 ) .