Agriculture Reference
In-Depth Information
Table 12.5. Estimation of the GHG cost of producing 1 kg of
soybean seeds using three different approaches.
Approach
GHG Cost
(kg CO
2
e kg
−1
seeds)
U.S. dollar value
a
0.25
Energy content
b
2.62
Direct estimation
c
0.31
a
From West and Marland (2002).
b
Assumes all energy for soybean production is derived from fossil diesel with an energy
content of 36.4 MJ L
−1
; the energy content of soybean seeds is 23.8 MJ kg
−1
; CO
2
emission from burning fossil diesel is 2.67 kg CO
2
L
−1
.
c
Based on CO
2
e emissions from irrigated soybean production (239.9 kg C ha
−1
yr
−1
; West
can also affect the amount of NO
3
−
driven from the root zone into surface water
and groundwater systems (Gehl et al. 2005), where it can be denitrified to N
2
O
(Beaulieu et al. 2011).
Nitrous Oxide and Methane Fluxes
Soil N
2
O emissions are directly related to soil N availability and therefore to
N fertilization and N fixation. In KBS LTER systems, those with high soil N
availability—either from fertilizer inputs (e.g., in the Conventional and No-till
systems) or from N fixed by leguminous cover crops (e.g., by red clover in the
Reduced Input and Biologically Based systems) or by the primary crops them-
selves (e.g., soybean and alfalfa)—showed higher N
2
O emissions than did sys-
tems with lower N inputs and availability (Fig. 12.3). This is a common finding
in the N
2
O literature (see Robertson and Vitousek 2009, Millar et al. 2010); in
fact, global GHG inventories for agricultural N
2
O emissions are largely based
on a simple percentage of national fertilizer N inputs (IPCC 2006). Higher N
2
O
emissions in crops with lower N availability (i.e., wheat vs. soybeans, Fig. 12.3)
suggest, however, that not only N availability but also specific crop (i.e., rotation
type) may have an effect.
Nitrous oxide emissions appear to be especially high where N fertilization
exceeds crop N requirements. McSwiney and Robertson (2005) found a nonlin-
ear, exponentially increasing N
2
O emission rate from KBS soils in continuous
corn as fertilization levels increased beyond the point required for maximum
yield. Others have since found similar responses (Grant et al. 2006, Ma et al.
2010, Millar et al. 2010, Hoben et al. 2011), suggesting that mitigation efforts
directed toward more precise fertilizer use may have greater payoffs than those
estimated by inventory methods based on a simple percentage of inputs. Millar
et al. (2010, 2012, 2013) incorporated this relationship into C market incentives
that can compensate farmers for more conservative N fertilizer use, which in the-
ory is a promising way to promote fertilizer conservation in general, with both
climate and water quality (Hamilton 2015, Chapter 11 in this volume) benefits.
