Environmental Engineering Reference
In-Depth Information
SOC varied across sites and ranged from −0.6 to 4.3 Mg C/ha per year for the 0- to 30-cm depth.
For example, on four farms in Nebraska, change in SOC for the 0- to 120-cm depth averaged 2.9 Mg
C/ha per year. The variation in SOC change reiterated the importance of long-term environmental
monitoring sites in major agro-ecoregions.
In addition to increasing soil C, growing switchgrass may increase wildlife habitat, increase
landscape and biological diversity, increase farm revenues, and return marginal farmland to
production (Sanderson et al. 1996; McLaughlin and Walsh 1998; McLaughlin et al. 2002; Roth et al.
2005). Not harvesting some switchgrass each year would increase the habitat value for grassland
bird species that require tall, dense vegetation structure (Roth et al. 2005).
22.6.2 E
nErgy
B
alancE
Energy produced from renewable carbon sources is held to a different standard than energy
produced from fossil fuels, in that renewable fuels must have highly-positive energy values. The
energy efficiency and sustainability of ethanol produced from grains and cellulosics has been
evaluated using net energy value (NEV), net energy yield (NEY), and the ratio of the biofuel output
to petroleum input [petroleum energy ratio (PER)] (Schmer et al. 2008) An energy model using
estimated agricultural inputs and simulated biomass yields predicted switchgrass could produce
greater than 700% more output than input energy (Farrell et al. 2006). These modeled results were
validated in a multifarm, field-scale research in the central and northern Great Plains, United States.
Switchgrass fields on ten farms in Nebraska, South Dakota, and North Dakota produced 540% more
renewable fuel (NEV) than nonrenewable fuel consumed over a 5-year period (Schmer et al. 2008).
The estimated on-farm NEY was 60 GJ/ha per year (Schmer et al. 2008), which was 93% greater
than human-made prairies and 652% greater than low-input switchgrass grown in small plots in
Minnesota (Tilman et al. 2006). The 10 farms and five production years had a PER of 13.1 MJ
of ethanol for every MJ of petroleum input, and produced 93% more ethanol per ha than human-
made prairies and 471% more ethanol per ha than low-input switchgrass in Minnesota (Schmer
et al. 2008). Average greenhouse gas (GHG) emissions from switchgrass-based ethanol in this study
were 94% lower than estimated GHG emissions from gasoline (Schmer et al. 2008). In simulated
production trials in Wisconsin, switchgrass produced the most net energy, followed by an alfalfa-
corn rotation and then continuous corn (Vadas et al. 2008). Producing switchgrass for bioenergy is
an energetically positive and environmentally sustainable production system for the central Great
Plains and Midwest.
The implementation of switchgrass-based bioenergy production systems will require the
conversion of marginal land from annual row crops to switchgrass production and could exceed
10% in some regions depending on the yield potential of the switchgrass strains. In a 5-year study in
Nebraska, the potential ethanol yield of switchgrass averaged 3474 L/ha and was equal to or greater
than the potential ethanol yield of no-till corn (grain + stover) on a dry-land site with marginal soils
(Varvel et al. 2008). Removing an average of 51% of the corn stover each year reduced subsequent
corn grain yield, stover yield, and total biomass yield. Growing switchgrass on these marginal sites
will likely enhance ecosystem services more rapidly and significantly than on more productive sites.
22.6.3 E
conomicS
Switchgrass is an economically feasible source for cellulosic ethanol. A field-scale study using
known farm inputs and actual harvested switchgrass yields conducted on 10 farms over 5 years
in Nebraska, South Dakota, and North Dakota determined switchgrass could be delivered at the
farm gate for $54/Mg (Perrin et al. 2008). They concluded that the development of new cultivars
improved production practices, and an expanded market for switchgrass may reduce the farm-gate
cost by as much as 20% (Perrin et al. 2008). Large quantities of switchgrass could be delivered
