The Energetics of Agroecosystems - Agroecology: The Ecology of Sustainable Food Systems - page 255

18 The Energetics of

Agroecosystems

Energy is the lifeblood of ecosystems and of the biosphere
as a whole. At the most fundamental level, what ecosys-
tems do is capture and transform energy.
Energy is constantly flowing through ecosystems in
one direction. It enters as solar energy and is converted
by photosynthesizing organisms (plants and algae) into
potential energy, which is stored in the chemical bonds of
organic molecules, or biomass. Whenever this potential
energy is harvested by organisms to do work (e.g., grow,
move, reproduce), much of it is transformed into heat
energy that is no longer available for further work or
transformation — it is lost from the ecosystem.
Agriculture, in essence, is the human manipulation of
the capture and flow of energy in ecosystems. Humans
use agroecosystems to convert solar energy into particular
forms of biomass — forms that can be used as food, feed,
fiber, and fuel.
All agroecosystems — from the simple, localized
plantings and harvests of the earliest agriculture to the
intensively altered agroecosystems of today — require an
input of energy from their human stewards in addition to
that provided by the sun. This input is necessary in part
because of the heavy removal of energy from agroecosys-
tems in the form of harvested material. But it is also
necessary because an agroecosystem must to some extent
deviate from, and be in opposition to, natural processes.
Humans must intervene in a variety of ways — manage
noncrop plants and herbivores, irrigate, cultivate soil, and
so on — and doing so requires work.
The agricultural “modernization” of the last several
decades has been largely a process of putting ever-greater
amounts of energy into agriculture in order to increase
yields. But most of this additional energy input comes
directly or indirectly from nonrenewable fossil fuels.
Moreover, the return on the energy investment in conven-
tional agriculture is not very favorable: for many crops,
we invest more energy than we get back as food. Our
energy-intensive form of agriculture, therefore, cannot be
sustained into the future without fundamental changes.

Energy is most commonly defined as the ability to do
work. Work occurs when a force acts over some distance.
When energy is actually doing work it is called kinetic
energy. There is kinetic energy, for example, in a swinging
hoe and a moving plow, and also in the light waves coming
from the sun. Another form of energy is potential energy,
which is energy at rest yet capable of doing work. When
kinetic energy is doing work, some of it can be stored as
potential energy. The energy in the chemical bonds of
biomass is a form of potential energy.
In the physical world and in ecosystems, energy is
constantly moving from one place to another and changing
forms. Two laws of thermodynamics describe how this
occurs. According to the first law of thermodynamics,
energy is neither created nor destroyed regardless of what
transfers or transformations occur. Energy changes from
one form to another as it moves from one place to another
or is used to do work, and all of it can be accounted for.
For example, the heat energy and light energy created by
the burning of wood (plus the potential energy of the
remaining products) is equal to the potential energy of the
unburned wood and oxygen.
The second law of thermodynamics states that when
energy is transferred or transformed, part of the energy is
converted to a form that cannot be passed on any further
and is not available to do work. This degraded form of
energy is heat, which is simply the disorganized move-
ment of molecules. The second law of thermodynamics
means that there is always a tendency toward greater dis-
order, or entropy. To counter entropy — to create order,
in other words — energy must be expended.
The operation of the second law can be clearly seen in
a natural ecosystem: as energy is transferred from one
organism to another in the form of food, a large part of that
energy is degraded to heat through metabolic activity, with
a net increase in entropy. In another sense, biological sys-
tems don't appear to conform to the second law because
they are able to create order out of disorder. They are only
able to do this, however, because of the constant input of
energy from outside the system in the form of solar energy.
Analysis of energy flows in any system requires mea-
suring energy use. Many units are available for this pur-
pose. In this chapter, we will use kilocalories (kcal) as the
preferred unit because it is best oriented to linking human
nutrition with energy inputs in food production. Other
units and their equivalents are listed in Table 18.1.

ENERGY AND THE LAWS OF
THERMODYNAMICS

An examination of the energy flows and inputs in agricul-
ture requires a basic understanding of energy and the
physical laws that govern it. First of all, what is energy?

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