Agriculture Reference
In-Depth Information
Moreover, the right mixture of grasses could provide habitat for beneficial insects
as well as for birds and other wildlife, providing additional environmental benefits
especially if the marginal land were otherwise degraded due to prior management.
Using land-use databases and the EPIC model (Zhang et al. 2010) to scale KBS
results for the fertilized Early Successional community to a 10-state U.S. North
Central region, Gelfand et al. (2013) estimated that marginal lands could produce
at least 21 × 10
9
L of biofuel annually, or about 25% of the 80 billion L 2022 target
legislated for advanced biofuels by the U.S. Energy Independence and Security Act
of 2007.
The GWI of Land-Use Conversion for Biofuel Production
In 2014, about 10 million ha of former U.S. cropland were enrolled in the USDA
Conservation Reserve Program (CRP) (USDA-FSA 2014). Converting these con-
servation plantings—most commonly in grassland vegetation—back to cropland
risks the release of substantial amounts of stored soil organic C, effectively creating
a C debt that models suggest could wipe out the benefits of up to 48 years of sub-
sequent grain-based feedstock production (Fargione et al. 2008). Actual measure-
ments of C debt following conversion, however, are not yet available, and theory
suggests that the debt could be significantly less than this with careful management.
In 2009 three KBS fields enrolled in the CRP program since 1987 were con-
verted from long-term brome grass (
Bromus inermis
) to no-till soybean as a recom-
mended break crop prior to growing various cellulosic feedstocks. The advantage
of soybeans as a break crop is that glyphosate-tolerant soybeans can be sprayed
multiple times during the growing season to kill any remnants of the preexisting
vegetation (brome grass, in this case). A CO
2
eddy covariance tower was placed in
each field and in an unconverted CRP reference field (Zenone et al. 2011). Eddy
covariance towers measure net ecosystem CO
2
flux by observing CO
2
concentra-
tions and the movement of air between the atmosphere and the plant canopy at
intervals of one-tenth of a second, allowing estimation of CO
2
fluxes that are then
summed over a 30-minute period to provide half-hour snapshots of net ecosystem C
gain and loss. Summing the half-hour snapshots over days and weeks provides, ulti-
mately, the annual NEP of the studied ecosystem. In this way, total soil C change
can be inferred long before it can be measured directly with soil sampling.
Figure 12.6A shows seasonal patterns of NEP in the converted and reference
CRP systems during the year of conversion. Net Ecosystem Productivity was nega-
tive in both systems at the beginning of the year, reflecting net emissions of CO
2
as soil respiration exceeded wintertime photosynthesis by brome grass, which was
nil. The negative fluxes turned positive beginning in the spring (around Day 100) as
brome grass CO
2
fixation began to exceed total respiration. The CRP reference sys-
tem continued to gain CO
2
until ca. Day 220, when brome grass senescence in the
fall led to reduced photosynthesis, and respiration again dominated the CO
2
flux. By
the end of the year, however, the cumulative NEP was still positive (above the ori-
gin in Fig. 12.6A), indicating net sequestration of CO
2
within the ecosystem. In the
CRP converted system, on the other hand, an herbicide application around Day 120
interrupted CO
2
fixation by the brome grass, and the system continued to lose more
