Agriculture Reference
In-Depth Information
with well aerated manure storage systems pro-
ducing higher emissions. The emission of N
2
O
from manure requires an anaerobic environ-
ment preceded by aerobic conditions as it is
caused by nitrification and denitrification of N.
Nitrification is the aerobic process whereby
ammonium is oxidized to nitrate; denitrification
is the anaerobic process whereby nitrites and
nitrates are transformed to N
2
O and N
2
gas.
The IPCC (2006) tier 1 method of estimat-
ing direct N
2
O emissions involves estimating the
total amount of N excreted by livestock species by
category for each type of manure management
system and multiplying by an emission factor
for that type of manure management system.
The tier 2 method follows the same calculation
as tier 1 but includes the use of country-specific
data for some or all of these variables, while a
tier 3 method would attempt to estimate amount,
timing and spatial distribution of livestock excre-
tions as well as their interaction with simulated
soil processes (e.g. soil moisture and nitrogen) to
calculate N
2
O emissions.
Nitrogen is also lost indirectly from manure
by volatilization (e.g. NH
3
, nitrogen oxides (NO
x
))
or to ground or surface water by leaching and
runoff. This N is also subject to nitrification and
denitrification after loss from the farm, produc-
ing 'indirect' N
2
O emissions. The IPCC (2006)
methodology calculates volatilized N from an
estimate of the amount of N excreted, the pro-
portion of manure managed in each manure
management system and the fraction of volatil-
ized N. The tier 1 method is applied using default
N excretion data, default manure management
system data and default fractions of N lost from
manure through volatilization and leaching.
The tier 2 and 3 approaches, as indicated before,
use more elaborated or advanced approaches
of estimating some or all of the variables.
soils separately from indirect emissions. Direct
N
2
O emissions from managed soils are estimated
using three emission factors, with default values
used in the tier 1 approach and country-specific
emission factors that account for differences in
land cover, soil type, climatic conditions or man-
agement practices used in the tier 2 approach. The
first emission factor is for the amount of N
2
O
emitted from N in soils due to application of inor-
ganic fertilizer and manure, decomposition of
crop residues, and mineralization of native soil
organic matter. The second emission factor is for
the amount of N
2
O emitted from organic soils, and
the third factor estimates the amount of N
2
O
emitted from urine and faeces N deposited by
grazing animals on pasture. Indirect N
2
O emis-
sions are emitted off-farm from N lost via runoff,
leaching and volatilization. These emissions are
estimated from the assumed fractions of N lost
from manure, residues and fertilizer.
Carbon dioxide
Carbon dioxide is usually considered cycled
through the atmosphere: CO
2
is taken up by
plants and converted to carbohydrates, plants
are consumed by animals, C in manure is
returned to the soil to decompose. Thus, the CO
2
from animal respiration (i.e. biogenic CO
2
) is not
usually included in LCA of GHG emissions from
farms. The level of organic C in agricultural soils
reflects the balance between inputs and remov-
als. Carbon can be released from or sequestered
by soil depending upon management practices.
For example, practices such as the application of
limestone, dolomite or urea fertilizers lead to CO
2
emissions. In addition, tillage of soil leads to a
loss of C because organic matter decomposition
is accelerated and harvest of the crop results in
less C returning to the soil. Some practices that
increase organic matter and C in soils include
reducing tillage, restoring grasslands, planting
perennial crops and eliminating fallowing of
land (Janzen
et al
., 1998).
Most LCAs of ruminant production systems
assume soil C is at steady state (i.e. neither gain-
ing nor releasing C). In soils not at steady state,
CO
2
emissions and removals from soil C change
can dominate the GHG balance of the farm.
Beauchemin
et al
. (2011) showed that when
grassland was newly seeded on previously cropped
Soil N
2
O
As with manure, N
2
O is also produced in soils
through the processes of nitrification and denitri-
fication. The main factor affecting N
2
O soil emis-
sions is the availability of inorganic N in the soil.
Direct N
2
O emissions result from the application
of N inputs to soils, and indirect N
2
O emissions
from the re-deposition of N arising from volatiliza-
tion, leaching and runoff. The IPCC methodology
estimates direct emissions of N
2
O from managed
