Agriculture Reference
In-Depth Information
unprecedented demands on croplands globally; in the United States, agricultural
biomass needs are expected to approach 700 Tg (Perlack et al. 2005, NRC 2009),
which could take as much as 90 million ha (222 million acres) of additional crop-
land (CAST 2011, Robertson et al. 2011)—about half as much U.S. land as we
use today for all annual crops. Impacts on the biogeochemistry (Robertson et all
2011) and biodiversity (Fletcher et al. 2011) of agricultural landscapes are likely to
be correspondingly high. The climate change implications of these impacts make
it all the more important that policy and landowner decisions be based on accurate
GWI assessments.
Gelfand et al. (2013) used 20 years of observations from the MCSE to analyze
the life-cycle C balances of systems that could potentially be harvested for use as
biofuel feedstocks. For the two annual crop systems evaluated—the Conventional
and No-till systems—they assumed grain was used for grain-based ethanol (corn,
wheat) or biodiesel (soybean), and that 60% of wheat straw was used for cellulosic
ethanol. No residue was removed from the corn or soybean portions of the rotations
in order to protect existing SOM stores (NRC 2009). Three perennial cropping
systems provided biomass for cellulosic ethanol—Alfalfa, Poplar, and the Early
Successional community, which was either fertilized or unfertilized.
Resulting GHG balances (Fig. 12.5B) show a negative (net mitigating) GWI for
all biofuel cropping systems. Fossil fuel offset credits were greatest in the Alfalfa
and fertilized Early Successional communities and lowest in the more intensively
managed systems. The differences were related to both yield and management. For
an example, high yields of the No-till system were balanced by relatively high
management inputs, which decreased total fossil fuel offset credits. On the other
hand, cellulosic biomass produced in the less productive Early Successional sys-
tem lacked significant management inputs and therefore provided more fossil fuel
offset credits (Fig. 12.5A). Credits for the Early Successional community would
be substantially higher were technology developed to improve harvest efficiency
for perennial grasses, now only 55% (Monti et al. 2009). Nevertheless, the Early
Successional community still exhibited the highest net mitigation potential with a
GWI of about -851 g CO
2
e m
−2
yr
−1
, while the more productive No-till system was
only fourth, with a net GWI of -397 g CO
2
e m
−2
yr
−1
. Alfalfa was intermediate to
these with a mitigation potential of about -605 g CO
2
e m
−2
y
−1
because of the high
GWI cost of increased N
2
O emissions and lower SOC accumulation (Fig. 12.5B).
The net mitigation potential of the Poplar system was low, owing to the lack of
net soil organic C gain over its rotation including the subsequent break period.
Fertilizing the Early Successional community increased its productivity and thus
its fossil fuel offset by ~35%, though net GWI remained basically unchanged due
to the greater CO
2
e cost of the fertilizer N and increased soil N
2
O emissions associ-
ated with fertilization. Nevertheless, by increasing productivity with no net change
in GWI, N fertilization would reduce the amount of land needed to produce a given
amount of biofuel feedstock.
The boundary of this analysis includes the full life cycle of biofuel and fos-
sil fuel production. Expanding the boundary to include indirect land-use effects
could change GWIs significantly for the worse. More specifically, the GWI of
these systems will be significantly less mitigating if biofuel crops were to displace
