Environmental Engineering Reference
In-Depth Information
the last 50 years, ecology has produced a wealth of new understanding, much of which has great
relevance for bioenergy and the ecological systems used to produce it. In this section, we briefly
describe a few of these insights, focusing on those most germane to bioenergy. It is our belief that
for any system to be ecologically sustainable, we need a substantial understanding of how each of
the following processes support and are affected by a given bioenergy production system.
6.2.1 k
Ey
t
ErmS
and
p
rocESSES
6.2.1.1 nutrient cycling
The compounds that form the cells of all living things are primarily composed of carbon, nitrogen,
phosphorus, sulfur, and water. Over time, each of these essential nutrients circulates through the
local ecosystem and wider biosphere. The rates at which these nutrients move through living and
nonliving ecosystem components can be influenced by human activities. Well-documented instances
of anthropogenic disruptions in nutrient cycles include increased nitrogen deposition from industrial
agriculture (resulting in eutrophication of many aquatic ecosystems) and increased atmospheric
carbon dioxide (CO
2
) from combustion of fossil fuels. One of the primary attractions of bioenergy
is its potential to reduce fossil fuel carbon emissions by closing the carbon cycle with sequestration
by green plants instead of the net carbon pollution from coal, oil, and natural gas. However, land-
use changes associated with intensified management such as monoculture agriculture and plantation
forestry can have enormous effects on other key cycles such as water circulation and the mainte-
nance of soil fertility. The CO
2
and other GHGs released as a direct result of conversion of natural
vegetation to bioenergy has been called “the carbon debt” to be “paid back” by reductions in GHG
emissions by bioenergy as compared with the fossil fuels being displaced (Fargione et al. 2008).
However, some recent analyses suggest that such initial debt can be so high that it can take centuries
or longer to make up for carbon losses from ecosystem conversion (Fargione et al. 2008).
6.2.1.2 Biomass and trophic structure
For all terrestrial and most marine ecosystems, solar radiation transformed by green plants into
sugars produces the living tissues that support life. The total amount of living and dead biomass
is greatest at lower trophic levels and declines as one moves up trophic levels from green plant
producers, to primary consumers like herbivorous animals, to primary predators like wolves and
hawks. This is sometimes called “a pyramid of biomass” because a large amount of primary plant
production (i.e., biomass) is needed to support relatively less biomass of herbivores. It is also the
reason why bioenergy systems focus on using plants as a carbon source when carbon is also present
in rabbits, deer, and bison; growing bison as a biofuel feedstock would be highly inefficient and ethi-
cally questionable. On average, there is an approximately 90% reduction in biomass as one moves
up one trophic level, so that 10,000 kg of grass could support 1000 kg of grasshoppers, which could
support 100 kg of insectivorous songbirds. For bird-eating hawks then, such a system would support
only 10 kg, about the weight of ten red-tailed hawks (
Buteo jamaicensis
). For bioenergy systems to
meet ecological definitions of sustainability, they must operate with recognition that a dynamic bal-
ance must be maintained between trophic levels including soils and plants, and the animals, fungi,
and bacteria that depend on continued manufacture and recycling of biomass into the ecosystem.
For these reasons, the amount of biomass that can be sustainably removed is often not well known.
6.2.1.3 vegetation diversity and structure
Diversity begets diversity is a simple rule of thumb in ecology. This means that higher diversity at
lower trophic levels usually supports higher diversity at the next level up and so on. Thus a temperate
forest in North America has relatively few tree species and supports relatively few leaf-chewing cat-
erpillars and a relatively simple bird community that feeds on these caterpillars. In contrast, a similar
size patch of rainforest in Brazil may have a hundred times the tree species, a thousand times the
