Agriculture Reference
In-Depth Information
101 g CO
2
e m
-2
, whereas the No-till system exhibited net mitigation: -14 g CO
2
e
m
-2
. The Early Successional system was the most mitigating, at -387 g CO
2
e m
-2
.
Closer inspection reveals the basis for these differences: Although N
2
O production
and nitrogen fertilizer manufacture were the two greatest sources of global warm-
ing impact in the annual cropping systems, the soil carbon storage in the No-till
system more than offset the CO
2
e cost of no-till N
2
O and fertilizer manufacture.
And because the Biologically Based system sequestered carbon at an even greater
rate and without the added cost of nitrogen fertilizer, the net mitigation was stron-
ger still (Fig. 2.4; Gelfand and Robertson 2015, Chapter 12 in this volume).
Most of the substantial mitigation capacity of Early Successional fields is
derived from their high rate of soil carbon storage, which will diminish over time.
At the KBS LTER site, the carbon stored annually in Mid-successional soils was
~10% of that in Early Successional soils, and no net soil carbon storage occurred
in the mature Deciduous Forest. As a result, the net CO
2
e balance of the mature
forest is close to 0 g CO
2
e m
−2
, with CH
4
oxidation offsetting most of the CO
2
e
cost of natural N
2
O emissions (Fig. 2.4). Interesting, too, is the recovery of CH
4
oxidation during succession. Methane oxidation rates are typically decimated
when natural vegetation is converted to agriculture (Del Grosso et al. 2000); that
oxidation in the Mid-successional system is more than midway between that of
the Early Successional system and that of the mature Deciduous Forest suggests
an 80- to 100-year recovery phase. Recent evidence from the KBS LTER site
suggests that methanotrophic bacterial diversity plays a role in CH
4
oxidation
differences (Fig. 2.5; Levine et al. 2011).
In addition, if harvested biomass is used to produce energy that would other-
wise be provided by fossil fuels, the net global warming impact of a system will
be further reduced by avoided CO
2
emissions from the fossil fuels displaced by the
biomass-derived energy. Sometimes—as with corn grain in conventional systems—
the displacement is minor or even nonexistent because of the fossil fuel used to
produce the biomass (Farrell et al. 2006) and the potential to incur carbon costs else-
where by clearing land to replace that removed from food production (Searchinger
et al. 2008). In contrast to the energy provided by corn grain is the energy provided
by cellulosic biomass produced in the Early Successional system. Gelfand et al.
(2013) calculated that harvesting successional vegetation for cellulosic biofuel could
provide ~850 g CO
2
e m
−2
of greenhouse gas mitigation annually. Extrapolated yields
to marginal lands across 10 U.S. Midwest states using finescale (0.4-ha) modeling
yielded a potential climate benefit of ~44 MMT CO
2
yr
-1
. However, such near-term
benefits also depend on the methods used to establish the biofuel crop; killing the
existing vegetation and replanting with purpose-grown feedstocks, such as switch-
grass or miscanthus, can create substantial carbon debt (Fargione et al. 2008) that can
take decades to repay (Gelfand et al. 2011); the debt is even greater if the replanted
crop requires tillage (Ruan and Robertson 2013).
The provision of greenhouse gas mitigation is a service clearly within the capac-
ity of modern cropping systems to provide. Various management practices have dif-
fering effects, sometimes in opposition (consider, e.g., no-till energy savings vs. the
carbon cost of additional herbicides) and at other times synergistic (consider that
leguminous cover crops in the Biologically Based system not only increased soil
