Agriculture Reference
In-Depth Information
Table 3.2
Greenhouse gas emissions from agriculture
Total emissions 1990
(MtCO
2
e)
Total emissions 2005
(MtCO
2
e)
Total emissions 2030
(projection) (MtCO
2
e)
Type of emission
Agricultural soils (N
2
O)
1,804 (30.5 %)
1,984 (32.5 %)
2,666 (36.5 %)
Enteric fermentation (CH
4
)
1,755 (29.6 %)
1,864 (30.5 %)
2,289 (31.3 %)
Rice cultivation (CH
4
)
670 (11.3 %)
710 (11.6 %)
739 (10.1 %)
Manure management (CH
4
, N
2
O)
408 (6.9 %)
389 (6.4 %)
455 (6.2 %)
Other emissions (CH
4
, N
2
O)
1,283 (21.7 %)
1,164 (19.0 %)
1,164 (15.9 %)
Total non-CO
2
emissions
5,920 (100 %)
6,111 (100 %)
7,313 (100 %)
Between 1990 and 2005, N
2
O emissions from
agricultural soil management have increased
10 %, from 1,804 to 1,984 MtCO
2
e, which
corresponds to 32.5 % of total agricultural
emissions (Table
3.2
).
Underlying this trend are increases in crop
production and fertilizer use and other nitro-
gen sources such as crop residues.
The emissions of N
2
O that result from
anthropogenic N inputs or N mineralization
occur through both (a) direct pathway (i.e.,
directly from the soils to which the N is added/
released) and (b) indirect pathways: (1) following
volatilization of NH
3
and NOx from managed
soils and from fossil fuel combustion and bio-
mass burning and the subsequent redeposition of
these gases and their products NH
4
+
and NO
3
−
to
soils and waters and (2) after leaching and runoff
of N, mainly as NO
3
−
, from managed soils.
In most soils, an increase in available N
enhances nitrifi cation and denitrifi cation rates
which then increase the production of
N
2
O. Increases in available N can occur through
human-induced N additions or change of land-
use and/or management practices that mineralize
soil organic N.
From 2005 to 2030, N
2
O emissions from agri-
cultural soils are projected to increase 34 %,
from 1,984 to 2,666 MtCO
2
e. This projection
assumes continued increases in fertilizer
usage. Over the projection period, emissions
are expected to increase in all regions (US-EPA
2011
).
3.2.2.1 Inventory of Nitrous Oxide
Emissions from Managed Soils
Nitrous oxide is produced naturally in soils
through the processes of nitrifi cation and denitri-
fi cation. Nitrifi cation is the aerobic microbial
oxidation of ammonium to nitrate, and denitrifi -
cation is the anaerobic microbial reduction of
nitrate to nitrogen gas (N
2
). Nitrous oxide is a
gaseous intermediate in the reaction sequence of
denitrifi cation and a by-product of nitrifi cation
that leaks from microbial cells into the soil and
ultimately into the atmosphere. One of the main
controlling factors in this reaction is the
availability of inorganic N in the soil. This
methodology, therefore, estimates N
2
O emissions
using human-induced net N additions to soils
(e.g., synthetic or organic fertilizers, deposited
manure, crop residues, sewage sludge) or of min-
eralization of N in soil organic matter following
drainage/management of organic soils or cultiva-
tion/land-use change on mineral soils.
3.2.3
Enteric Fermentation (CH
4
)
Normal digestive processes in animals result in CH
4
emissions. Enteric fermentation refers to a fermen-
tation process whereby microbes in an animal's
digestive system ferment food. CH
4
is produced as
a by-product and can be exhaled by the animal.
Domesticated ruminants such as cattle, buffalo,
sheep, goats, and camels account for the majority of
CH
4
emissions in this sector (US-EPA
2011
).
Global CH
4
emissions from enteric fermentation
increased by 6 % between 1990 and 2005, from
1,755 to 1,864 MtCO
2
e, corresponding to 30.5 %
of total agricultural emissions (Table
3.2
).
From 2005 to 2030, CH
4
emissions from enteric
fermentation are projected to increase 23 %,
from 1,864 to 2,289 MtCO
2
e (US-EPA
2011
).
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