Environmental Engineering Reference
In-Depth Information
We will examine various bioenergy alternatives and assess key tradeoffs associated with each. In
this chapter, we address the following questions:
• What are the current dominant agricultural and forestry systems used to grow plants for
bioenergy?
• In what ways do these managed systems interact with the physical and biological systems
that support life on earth?
• Can managed bioenergy production systems be designed in ways that preserve the ecosys-
tem services furnished by intact ecological systems?
Addressing such questions at this time is of paramount importance because bioenergy pro-
duction is expanding worldwide, and several nations have adopted policies that will promote the
continued growth of the share of energy that originates from plants (Commission of European
Communities 2005). The scope of bioenergy systems that we discuss in this chapter include all
plant-based energies, including direct electricity generation from burning plant material and
conversion of plant material to liquid biofuels such as ethanol and biodiesel. This spans a wide
range of target plants from traditional food crops like corn and sugar cane, to grasses such as
Panicum virgatum
(switchgrass) and miscanthus
(
Miscanthus
spp.), to woody species including
Pinus
spp. (pines),
Eucalyptus
spp. (eucalypts), hybrid and other
Populus
spp. (poplars), and even
numerous species of algae. What all plant-based bioenergy systems have in common is a reliance
on photosynthesis as the biochemical means of gathering carbon into a convenient form for use
a s energ y.
Green plants have been sequestering carbon for at least 3 billion years. Our current modern
industrial world has grown and become utterly dependent on burning fossil fuels, which are the
direct or indirect products of photosynthesis accumulated over many millions of years. A second
characteristic of current plant-based bioenergy systems is the intensification of production systems
compared with the ecosystems they replace. Like food-based agricultural systems, the focus for
these systems has been on maximizing short-term per-hectare plant productivity. Simply put, this
means removing all plants but the target species, which is then planted in a systematic array and
typically maintained with high inputs of water, fertilizer, herbicides, and insecticides. Fossil fuels
are often used at virtually every stage of production and distribution as well as in the manufacture
of the diverse agricultural inputs.
It is largely because of the high emissions of GHGs during land conversion or during produc-
tion and distribution of target plants that some bioenergy systems compare unfavorably to petrol
(Fargione et al. 2008; Charles 2009). In fact, some recent analyses suggest that if GHG reduction
were the primary goal, conversion of cropland to forest would generate higher carbon sequestration
rates than the avoided emissions from the use of ethanol and biodiesel generated from any of the
common feedstocks (e.g., sugarcane, corn, sugarbeets, rapeseed, and woody biomass) (Righelato
and Spracklen 2007). Clearly, refinements and standardization of life-cycle assessment methods
will help clarify the true GHG costs and benefits associated with first- and second-generation
biofuels (FAO 2008; Searchinger et al. 2008; Purdon et al. 2009). Until such analyses are available
and widely accepted, several well-tested principles from ecological science can be used to guide
bioenergy toward more sustainable models.
6.2 deFInInG ecoloGIcal sustaInaBIlIty
Ecology is the branch of science concerned with understanding how the biological and physical
world interacts and function. Ecology is concerned with many spatial scales from microscopic to
global: examining processes such as energy flows through ecosystems, biogeochemical cycling; the
role of plants, animals, fungi, and microorganisms in supporting these cycles; and the ways that
landscape pattern and composition influence biodiversity. During the last century and particularly
