Agriculture Reference
In-Depth Information
Table 12.2. Global Warming Impacts for the first decade (1989-1999) of the MCSE.
a
System
Global Warming Impact (GWI)
b
(g CO
2
e m
−2
yr
−1
)
Soil C
N Fertilizer
Lime
Fuel
N
2
O
CH
4
Net GWI
Annual Crops (corn-soybean-wheat rotation)
Conventional
0
27
23
16
52
-4
114
No-till
-110
27
34
12
56
-5
14
Reduced Input
-40
9
19
20
60
-5
63
Biologically Based
-29
0
0
19
56
-5
41
Perennial Crops
Alfalfa
-161
0
80
8
59
-6
-20
Poplar
-117
5
0
2
10
-5
-105
Successional Communities
Early Successional
-220
0
0
0
15
-6
-211
Mid-successional
-32
0
0
0
16
-15
-31
Mown Grassland
(never tilled)
0
0
0
0
18
-17
1
Deciduous Forest
0
0
0
0
21
-25
-4
a
See Table 12.1 for a description of systems. All systems are replicated (
n
= 3-6).
b
Net GWI is determined as the sum of GWI components: soil carbon (C) sequestration, agronomic inputs of nitrogen
(N) fertilizer, lime and fuel, and GHG exchanges of nitrous oxide (N
2
O) and methane (CH
4
) with the atmosphere.
Units are carbon dioxide equivalents (CO
2
e; g m
−1
yr
−1
) based on IPCC conversion factors (IPCC 2007). Negative
values indicate net climate change mitigation potential.
Source:
Robertson et al. (2000).
Net GWIs over a 9-year period (Table 12.2) ranged from 114 g CO
2
e m
−2
yr
−1
(net
emission) in the conventionally managed corn-soybean-wheat rotation to -211 g
CO
2
e m
−2
yr
−1
(net mitigation) in the Early Successional community abandoned
from agriculture 9 years earlier. Net GWIs also differed substantially among the
annual cropping systems: net GWI was low in the No-till system (14 g CO
2
e m
−2
yr
−1
) and intermediate in the Reduced Input and Biologically Based systems (63
and 41 g CO
2
e m
−2
yr
−1
, respectively), suggesting the potential for substantial miti-
gation relative to the Conventional management.
Close analysis shows the source of these differences. While in all the annual
crops, N
2
O production was the largest single source of GWI, in the No-till system
soil C storage more than offset the GWI of N
2
O emissions, although additional con-
tributions from N fertilizer manufacture, lime (calcium and magnesium carbonate)
application, and fuel use kept GWI in the No-till system positive (Table 12.2). And
although not enough C was stored in the Reduced Input and Biologically Based
systems to offset N
2
O production, savings from lower N fertilizer and lime use
helped to reduce their net GWI to about half that of the Conventional system.
The hybrid Poplar system's combination of low N
2
O emissions and enhanced
soil C accumulation over 9 years resulted in a substantial mitigation capacity
of -105 g CO
2
e m
−2
yr
−1
(Table 12.2). Although Alfalfa, the other perennial system
