Agriculture Reference
In-Depth Information
from faeces and urine from enteric fermentation
(Sun
et al
., 2008; Hamilton
et al
., 2010). Therefore,
more definitive work is needed to determine
whether N
2
O emissions are originating from
enteric fermentation processes and what mecha-
nisms are responsible for these possible emissions.
Methane emissions from the large intestine of
pigs is relatively minor, and are estimated to repre-
sent less than 1% of total digestible feed energy
intake (Monteny
et al
., 2001). The larger source of
CH
4
emissions from pig production is manure
management. Total CH
4
emissions from a Dutch
pig production were estimated at 4.8 kg per pig per
year with approximately 30% of those emissions
coming from enteric fermentation and the bal-
ance from manure (Monteny
et al
., 2006). Manure
stored under anaerobic conditions can support
methanogenesis, though the characteristics of
the manure and the environmental conditions
will greatly influence the total CH
4
emission.
When higher concentrations of fermentable
carbohydrates are fed to pigs, there is an increase
in CH
4
emissions from anaerobically stored
manure, as these diets increase the fermentable
substrate available to microbial populations
(Aarnink and Verstegen, 2007). Higher temper-
atures lead to greater metabolic activity in
anaerobically stored manure and a higher rate
of CH
4
emissions (Monteny
et al
., 2006).
Nitrous oxide emissions from pig produc-
tion, as in ruminants, primarily come from
manure, not the animal directly. Denitrification
and nitrification processes in manure and soil
amended with manure are the primary source
of N
2
O emissions. Denitrification is a process
performed by denitrifying bacteria where nitrate
(NO
3
−
) is converted to N
2
and N
2
O is one of the
intermediates produced during this process
(Monteny
et al
., 2001). Nitrification is the con-
version of ammonium (NH
4
+
) to NO
3
−
and N
2
O
emissions are typically minimal during this pro-
cess unless there is very little O
2
available
(Monteny
et al
., 2001). In addition to low O
2
conditions, higher N
2
O emissions will result
when easily degradable C-containing com-
pounds are available (Velthof
et al
., 2003).
ozone (O
3
; the troposphere is the lowest region of
the atmosphere and where the majority of
human activity occurs). Tropospheric O
3
is a
major health concern and component of smog
(Haagen-Smit, 1952). Ozone is one of the six
'criteria' pollutants currently regulated by the
US EPA under the Clean Air Act. In the USA,
there are no federal regulations for ambient VOC
concentrations (the VOC content in consumer
products such as paints is regulated by the EPA
in an effort to improve indoor air quality), but air
districts within the state of California have VOC
emission regulations (EPA, 2011). Emissions of
VOC can be broken down in to two groups: bio-
genic and anthropogenic. Biogenic emissions of
VOC occur from 'natural' sources such as plants
and the ocean (Seinfeld and Pandis, 2006).
Anthropogenic VOC emissions are those that
arise from human activities, and VOC emissions
from animal agriculture fall into this category.
How VOCs, also known as reactive organic gases
(ROG), contribute to O
3
production is repre-
sented by this simplified general equation:
VOCs+NO
x
+sunlight-->O
3
. In this equation,
NO
x
is an abbreviation for oxides of nitrogen,
which includes nitrogen dioxide (NO
2
) and
nitric oxide (NO), and most NO
x
emissions are a
result of the burning of fossil fuels. Sunlight is
a key driver of O
3
formation, and it is no coinci-
dence that areas with abundant sunshine (and
VOCs and NO
x
) such as central and southern
California and the Atlanta metro area often
exceed EPA standards for O
3
(particularly in the
summer months) (EPA, 2012). However, not all
VOCs contribute equally to O
3
formation, and
further examination of the role VOCs play in
O
3
formation reveals why. Ozone is formed
through what is known as the 'primary photol-
ytic cycle'. Three reactions drive this cycle, and
are as follows:
NO
+¾¾+
++¾¾+
+¾¾+
hv
1
NO
O
2
2
OO M
O M
2
3
3
ONO
NOO
3
2
2
where NO
2
is nitrogen dioxide,
hv
is the energy
from a photon, NO is nitric oxide, O is an oxygen
atom, O
2
is molecular oxygen, M is a molecule
(non-specific, but required to stabilize the forma-
tion of O
3
) and O
3
is ozone (Knelson and Lee,
1977). Reaction 2 occurs very fast and Reaction
3 acts to replenish the supply of NO
2
; therefore,
Volatile organic compounds
As previously mentioned, VOCs contribute to the
formation of 'ground level' or 'tropospheric'
