Agriculture Reference
In-Depth Information
Figure 12.1
. Conceptual diagram of Global Warming Impact (GWI) components in agri-
cultural cropping systems. Arrows indicate the flux of carbon dioxide (CO
2
), methane
(CH
4
), and nitrous oxide (N
2
O) between cropping systems and the atmosphere. Atmospheric
exchanges are CO
2
unless noted otherwise.
a no-till system is plowed every few years to solve a management problem or reap
mineralizable N benefits, then
t
needs to span one or more of these tillage cycles.
Agricultural Inputs
Management decisions have a strong influence on the magnitude of CO
2
e emissions
associated with agricultural inputs including seed production, agrochemicals, and
fuel use in farm operations. For example, the MCSE Conventional system, which
is tilled, emits 35% more CO
2
from fuel use than does the No-till system (Fig.
12.2). Although the No-till system lacks soil preparation, the additional herbicides
and energy required at planting (because the soil is more resistant than had it been
plowed) partly offset the CO
2
e savings associated with reduced fuel use by not
tilling (Fig. 12.2). Likewise, synthetic N fertilizer can be a large source of CO
2
e
because of CO
2
emitted during its manufacture (Table 12.2), but this cost is avoided
in alfalfa, which acquires its N from the atmosphere through biological N fixa-
tion. However, this savings is almost entirely offset by the CO
2
e costs of alfalfa's
increased agricultural lime and potassium (K) requirements. Thus, overall CO
2
e
emissions of the Alfalfa system are ~60% of those of the No-till and Conventional
systems, despite the absence of N fertilizer use (Fig. 12.2).
The CO
2
e cost of producing agricultural lime (0.04 g CO
2
e kg
−1
; West and
Marland 2002) is independent of its fate. As noted earlier, Hamilton et al. (2007)
estimate that CO
2
emissions from agricultural lime applied to KBS soils are fully
offset by CO
2
capture when at least 50% of the lime is dissolved by carbonic acid
rather than by a strong mineral acid. Nitric acid in agricultural soils is largely
produced by nitrifying bacteria that produce 2 moles of H
+
for every mole of
ammonium oxidized to NO
3
−
(Robertson and Groffman 2015), and this can be
