Soil food webs in agricultural ecosystems - Microbial Ecology in Sustainable Agroecosystems - page 84

compared to the bacterial pathway. Nutrient availability and retention are either tied
to the activities of the fungal pathway or are a function of the combined activities of
the bacterial and fungal pathways and how synchronous these activities are with
plant growth.
4. Disturbances associated with agricultural practices alter the energetic organization of
the soil food web in ways that affect its stability. What the theoretical exercises do
is link these changes to asymptotic stability by suggesting that the observed devia-
tions in the activities of organisms, energy channels, and soil processes induced by
tillage are destabilizing, less so for the minimum tillage practices that emulate natural
systems.
What the modeling, field manipulations, and laboratory experiments suggest is that it
is neither the size of the interactions nor the magnitude of the nutrient flow that determines
the stability of an ecosystem. Rather, it is the patterning of the interactions among species
and the distribution of nutrients within the community that govern its stability. The com-
mon feature that disturbances have on ecosystems, be they natural or human induced, is
to change the densities of organisms, the species composition of communities, and hence
the patterning of interaction and nutrient flows within them. It is through these changes
that the stability of the system is impacted. In short, models have shown us that it is the
energetic organization of communities that forms the basis of ecosystem structure and
asymptotic stability. Empirical studies have shown us that different management practices
alter the energetic organization of communities to various degrees. Aligning management
practices to promote the stable patterns that emerge from theory seems palpable.
To this point, we have used food webs as a tool to study agricultural systems under the
guises of long-term asymptotic stability and steady-state assumptions. Arguably, many
natural systems and agricultural systems may not possess these properties or, if they do,
may be operating within the short-term transient state (i.e., a dynamic state in response to
a perturbation) rather than the long-term asymptotic state. Recent discussions on transient
dynamics and behavior offer an interesting twist to this discussion that affects the way in
which we think about systems and management. Hastings (2001, 2004, 2010) proposed that
many ecological systems may not exhibit traditional asymptotic dynamic behavior but
rather are operating within a transient dynamic state in which the asymptotic behavior is
uncertain. The disruption of soils through tillage and annual amendments of fertilizers,
water, or herbicides may not be sustainable in a traditional dynamic systems context, but
neither is the natural system. In other words, it is unclear whether the natural system
would be able to persist without the regular disruption and resetting of activity afforded
by the seasons.
With this in mind, the call to “Let the soil work for us” and underpinnings of sus-
tainable agricultural practices are not centered on creating a persistent stable system.
Rather, the focus is more aligned with conserving resources that are necessary to sus-
tain the system in a perpetual transient state. Here, a sustainable practice would manage
the system in a perpetual transient dynamic state, maximizing production and mini-
mizing the loss and export of plant-limiting nutrients. In real terms, this translates to
adopting practices that promote the placement of residues at the soil surface and that
minimize the disruption of soils in ways that restore a balance in the activities of organ-
isms within the bacterial and fungal energy channels found in natural systems. Soil
food webs provide us a means of studying and assessing agricultural practices to opti-
mize these aims.