Agriculture Reference
In-Depth Information
evaluated, also had substantial soil C accumulation, much of this was offset by
agricultural lime applications and high N
2
O emissions—both related to alfalfa's
N fixation capacity. Nitrogen fixation provides inorganic N to nitrifying bacteria,
which in turn provide nitrate (NO
3
−
) to denitrifiers, and both nitrifiers and denitri-
fiers produce N
2
O (Ostrom et al. 2010). Nitrifiers also produce acidity, increasing
the need for lime application. As a result, alfalfa possessed only a modest net miti-
gation capacity in spite of high rates of soil C sequestration.
Results from the full-cost analysis of GWI in the MCSE (Table 12.2) also sug-
gest an eventual diminution of the Early Successional community's strong miti-
gation potential. Older successional communities had a substantially higher net
GWI (though still negative), primarily because of lower soil C accumulation (Table
12.2). For example, in the late successional Deciduous Forest net soil C accumula-
tion was nil, and although CH
4
oxidation was significant, it was largely offset by
N
2
O emissions, leading to an overall GWI close to zero.
Gelfand et al. (2013) extended the GWI analysis of the MCSE by an additional
decade, and although results showed similar trends, there were two important dif-
ferences (Table 12.3). First, Hamilton et al. (2007) found that lime contributions to
GWI are likely far less than calculated earlier due to how lime is dissolved in these
soils. Dissolution by strong acids such as nitric (HNO
3
) leads to immediate CO
2
release—as was assumed in the earlier analysis. However, dissolution by carbonic
acid (H
2
CO
3
)—a weak acid existing in equilibrium with dissolved CO
2
—leads
to net CO
2
capture by the soil solution and its hydrologic export as bicarbonate
(HCO
3
−
), which resides in the groundwater system for long periods. Thus, the net
GWI in KBS soils, where dissolution by the two reactions tends to occur in about
equal proportions, is likely nil. And second, a more recent and deeper soil C sam-
pling (Syswerda et al. 2011) showed that soil C sequestered by the hybrid Poplar
system was largely lost during reestablishment after harvesting, when for ~2 years
soils were warmer and moister as a result of greater insolation and reduced tran-
spiration due to lack of canopy cover. These results revise but do not substantially
alter the original study's conclusion that different cropping practices contribute dif-
ferentially to a given cropping system's GWI, and they illustrate how a long-term
systems approach is necessary to fully partition the benefits and liabilities of spe-
cific management systems.
Biofuel and Energy Flux Considerations
Neither Robertson et al. (2000) nor others (e.g., Mosier et al. 2004) considered
the end use of the biomass produced by cropping systems in their calculations of
GWI—all harvested biomass was assumed to be oxidized to CO
2
, thereby provid-
ing no further mitigation capacity. If, on the other hand, harvested biomass is used
for energy that would otherwise be provided by fossil fuel, then an additional miti-
gation credit must be added to the GWI of the cropping system that produced it,
so long as additional GHGs are not produced elsewhere by land cleared to offset a
potential loss of food production (Searchinger et al. 2008, 2009). For example, the
MCSE Poplar system discussed above would gain an additional mitigation credit
of ~319 g CO
2
e m
−2
yr
−1
were those trees grown on previously unforested land not
