Agriculture Reference
In-Depth Information
Thornton, 1966) used NSS design. More recent
investigation of NSS design has been conducted
by Goopy
et al
. (2011) to measure CH
4
emission
from sheep for short periods (1-2 h).
For whole-animal CH
4
emissions, a general
disadvantage is the expense of building enclosures
and the animal training to ensure stress is not an
influencing factor in the emission measurement.
Caution must be used when using enclosures for
measuring NH
3
emissions from manure since
NH
3
is readily absorbed by water that may con-
dense on the inner surface of the chamber. This
would cause an under-estimation of NH
3
concen-
tration and therefore NH
3
emissions.
Non-flow-through steady-state (NFT-SS)
A widely used chamber for determining NH
3
emissions from manure-amended soils is the use
of a NFT-SS chamber. The design of this cham-
ber is reported by Marshall and Debell (1980),
which consists of a small volume cylinder cover-
ing the source. The top of the cylinder that is
exposed to the atmosphere has two sponges
treated with an acid that removes ambient NH
3
.
The outer sponge is for the ambient NH
3
while
the inner sponge collects only the NH
3
emitted
into the chamber air space from the manure sur-
face. It follows that the amount of NH
3
absorbed
by the inner sponge over time is assumed to
equal that generated at the surface.
This NFT-SS chamber design was used by
Bittman
et al
. (2005) to determine NH
3
emis-
sions from dairy cow slurry applied to pasture.
They reported that the technique greatly under-
estimated the NH
3
emission relative to a micro-
meteorological technique, a finding they report
that other studies also found. However, the pre-
cision of the chamber was high such that the
treatment effect was similar between techniques.
The advantage of this enclosure design is the
ease of operation, however, the small area of the
enclosures and the short sampling time makes
sampling of variable (spatial and temporal)
emissions difficult.
Non-steady-state (NSS)
The emission of gas (
F
G
; flux in g s
−1
or flux den-
sity in g m
−2
s
−1
) from the NSS enclosure design is
determined as:
cc
t
-
-
cc
t
-
-
V
A
()
()
a
F
=
t
2
t
1
V
or
b
F
=
t
2
t
1
(15.1)
G
G
21
t
21
t
S
where
C
t2
and
C
t1
is the gas concentration (g m
−3
)
at times
t
2 and
t
1 (s),
V
(m
3
) is the inside volume
of the enclosure and
A
S
is the surface area (m
2
).
Equation 15.1a is used for measuring CH
4
emis-
sions from whole animals (ruminants) while Eqn
15.1b is for NH
3
or CH
4
from a manure surface.
For whole-animal CH
4
emissions, the volume of
the animal is subtracted from
V
(Goopy
et al
.,
2011). As well, caution must be exercised to
ensure safe concentrations of CO
2
and O
2
exist
inside the enclosure and that the welfare of the
animal is not jeopardized. This means that the
chamber can only be used for short durations
(
c
.1-2 h depending on the design of the enclo-
sure). This is problematic for determining daily
emissions. For example, Goopy
et al
. (2011)
reported
r
2
values of 0.42-0.48 between 2 and
22 h enteric CH
4
emissions in sheep. In both the
whole animal and manure surface applications,
it is important that the build-up of gases inside
the chambers does not suppress the emission
from the source. The advantage of the NSS design
is the simple construction and portability of the
chambers but the disadvantage is that measure-
ments are not continuous (e.g. for a manure
source the gas concentrations are made periodi-
cally throughout the day for just a few minutes)
and therefore there is a need to fill data gaps to
estimate the 24-h emission.
Flow-through steady-state (FT-SS)
The FT-SS chamber offers an advantage over
NSS chambers since gases are not accumulated
inside the chamber allowing more continuous
measurements over a longer time. Therefore,
unlike the NSS design (for short-term emis-
sions), the FT-SS design can be deployed to cap-
ture daily patterns directly. The
F
G
is calculated
from this chamber design as:
()
(
)
a
Ff
=
CC
-
or
G
O
I
(15.2)
f
A
(
)
()
b
F
=
CC
-
G
O
I
S
where
f
is the flow-through rate (m
3
s
−1
), and
C
O
and
C
I
are the exhaust and intake concentration
(g m
−3
), respectively. For whole-animal CH
4
emis-
sions, Eqn 15.2a is used to express the emission
in units of g day
−1
. When the technique is used
for CH
4
or NH
3
emissions from a manure surface,
