Agriculture Reference
In-Depth Information
energy inputs insignificant even though they are often the component of greatest
interest. Energy in human labor and machinery manufacture (Hülsbergen et  al.
2001) could similarly be included, but these inputs do not significantly differ among
production-scale farming systems regardless of the final products (Pimentel and
Patzek 2005). Thus, if one is calculating an energy balance to determine the most
energy-efficient system, only significant sources of manageable energy need be
included, that is, energy inputs that differ and are affected by various management
options. Such comparisons assume that differences outside the farm gate are neg-
ligible, that is, that the energy costs of labor inputs, farm implements, and storing
or transporting crop yields are identical or sufficiently similar to be an insignificant
part of the overall system budget. This makes analyses more tractable, as measure-
ments of fluxes and pools at the farm scale are relatively straightforward.
Thus, the choice of system boundaries should be explicit and based on the needs
of the study. As for nutrient budgets or biogeochemical cycles (Robertson 1982),
boundaries should be expanded only as far as necessary to encompass the fluxes
relevant to the question under study. In a comparative analysis of biofuel cropping
systems, for example, it make sense to expand the boundary to include the cost of
transporting harvested grain and cellulosic biomass, as does inclusion of the fate of
grain ethanol end-products such as dry distillers grain.
Components of GHG Balances in Cropping Systems
The primary purpose of an agricultural GHG balance is to track the exchanges
of GHGs between cropping systems and the atmosphere. Figure 12.1 summarizes
major fluxes between these two pools. The cropping system contains three main
compartments:  agricultural inputs that cost CO 2 e to manufacture and transport,
GHG production and consumption by soil microbes, and CO 2 captured by the crop-
ping system and ultimately emitted in consumption of the harvested biomass. All
three compartments are interrelated and influenced by management decisions.
The GWI of a given system can be studied using a mass-balance approach,
which accounts for fluxes into and out of the system and provides estimates of
change in the pool of interest—ultimately resulting in GHG exchanges (expressed
as CO 2 e) with the atmosphere:
dX
dt
( )
( )
=
Flux InXt FluxOutX t
where X is the pool of interest, and Flux In and Flux Out are the sum of all mea-
sured and estimated fluxes into or out of the studied system over a given time period
t . Although t is usually annualized, when processes involve different time scales, it
is important that t be appropriately normalized, such as over the length of a rotation.
A comparison of a 1-year continuous corn rotation to a 3-year corn-soybean-wheat
rotation, for example, should be performed over at least one 3-year period to capture
different crop effects, and preferably more in order to capture climatic variation.
The same is true for other periodic management practices as well; for example, if
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