Agriculture Reference
In-Depth Information
Table 3.1
Sources of direct and indirect agricultural
GHGs
CO
2
e
(million tons)
Sources of agricultural GHGs
Methane from cattle enteric
fermentation (CH
4
)
1,792
Fig. 3.1
Main GHGs (CO
2
, CH
4
, and N
2
O) released from
agriculture (
o
oxygen,
h
hydrogen,
c
carbon,
n
nitrogen)
Manures (CH
4
+ N
2
O)
413
Nitrous oxide from fertilized soils
(N
2
O)
2,128
Fertilizer production (CO
2
+ N
2
O)
410
Biomass burning (CH
4
+ N
2
O)
672
3.1
Global Agricultural
Emissions
Rice production (CH
4
)
616
Farm machinery (seeding tilling,
spraying, harvest) (CO
2
)
158
Irrigation (CO
2
)
369
Agriculture is one of the main causes of the
increase in greenhouse gases observed over the
past 250 years (IPCC
2007
). Agriculture contrib-
utes to greenhouse gas increases through land use
in four main ways:
CO
2
releases linked to deforestation.
Methane releases from rice cultivation.
Methane releases from enteric fermentation in
cattle.
Nitrous oxide releases from fertilizer
application.
Agriculture releases to the atmosphere signifi -
cant amounts of CO
2
, CH
4
, and N
2
O (Fig.
3.1
)
(IPCC
2001
). CO
2
is released largely from micro-
bial decay or burning of plant litter and soil
organic matter (Smith
2004
). CH
4
is produced
when organic materials decompose in oxygen-
deprived conditions, notably from fermentative
digestion by ruminant livestock, from stored
manures, and from rice grown under fl ooded con-
ditions (Mosier et al.
1998
). N
2
O is generated by
the microbial transformation of nitrogen in soils
and manures and is often enhanced where avail-
able nitrogen (N) exceeds plant requirements,
especially under wet conditions (Oenema et al.
2005
).
The atmospheric concentration of both meth-
ane (1,774 ppb) and nitrous oxide (319 ppb) has
increased markedly world over as a result of
human activities. The observed increase in con-
centrations of methane and nitrous oxide is pre-
dominantly due to agriculture and use of fossil
fuel. Globally, agriculture contributes about 60 %
of nitrous oxide and 50 % of methane emissions.
Agricultural methane and nitrous oxide emissions
Pesticide production
72
Land conversion to agriculture
5,900
increased by 17 % from 1990 to 2005 (Smith
et al.
2007
). The three major sources of global
methane and nitrous oxide emissions from the
agriculture sector are soil (38 % of CH
4
+ N
2
O),
rice production (11 % of CH
4
), and biomass
burning (12 % of CH
4
+ N
2
O).
Agriculture accounted for an estimated emis-
sion of 5.1-6.1 Gt CO
2
e/year in 2005 (10-12 %
of total global anthropogenic emissions of
GHGs). CH
4
contributes 3.3 Gt CO
2
e/year and
N
2
O 2.8 Gt CO
2
e/year. Despite large annual
exchanges of CO
2
between the atmosphere and
agricultural lands, the net fl ux is estimated to be
approximately balanced, with CO
2
emissions
around 0.04 Gt CO
2
/year only (low agreement,
limited evidence). Together, these agricultural
processes comprise 54 % of methane emissions,
roughly 80 % of nitrous oxide emissions, and
virtually all carbon dioxide emissions tied to land
use (Table
3.1
) (IPCC
2007
).
In 2005, agriculture (crop and livestock)
directly accounted for 13.5 % of global GHG
emissions. Also, agriculture is a major driver of
deforestation, which roughly accounts for an
additional 17 % of global GHG emissions (IPCC
2007
).
The main direct sources of GHG emissions in
the agricultural sector are not only carbon diox-
ide (CO
2
). Agriculture is a source of nitrous oxide
(N
2
O), accounting for 58 % of total emissions,
mostly by soils and through the application of
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