Agriculture Reference
In-Depth Information
Water management might also be manipulated to lessen emissions. Currently
direct seeding and other water-conserving practices are being adopted in many
parts of Asia in response to shortages of water and labour (Guerra
et al
., 1998). A
single, well-timed period of drainage in the early season can decrease emissions
by 50% without compromising yield (Sass
et al
., 1992; Neue, 1997; Wassmann
et al
., 2000c). However the timing is critical so that entrapped CH
4
that would
otherwise be oxidized is not released, and N is not lost from the soil through
nitrification-denitrification, especially if conditions are such that nitrous oxide
forms.
8.2 NITROGEN OXIDES
8.2.1 GLOBAL BUDGET
Nitrous oxide
(
N
2
O
)
is an important greenhouse gas with a radiative forcing
effect 310 times that of CO
2
and a lifetime in the troposphere of approxi-
mately 120 years. Part of the N
2
O is converted to NO in the stratosphere, and
so contributes to depletion of ozone. Nitric oxide (NO) is very reactive in the
atmosphere and has a lifetime of only 1-10 days. It contributes to acidification
and to reactions leading to the formation of ozone in the troposphere, and so also
to global warming.
Table 8.4a shows estimates of the global nitrous oxide budget from different
sources and sinks (Prather
et al
., 2001) and Table 8.4b estimates for NO
x
(
NO
+
NO
2
)
.AsforCH
4
, there are substantial emissions from natural sources, par-
ticularly the ocean and humid tropical forests for N
2
O and lightning and soil
processes for NO
x
. However anthropogenic sources account for 40% of the
total emission in both cases. For NO
x
, this is largely from road transport and
power plants, but for N
2
O 60% is from agricultural soils. The post-industrial
increase in N
2
O abundance in the atmosphere is smaller than that of CH
4
-314 ppb
(molar mixing ratio in the troposphere) compared with 270 ppb in 1750-and
the current percentage increase (0.25% year
−
1
) is smaller than that of CH
4
.
However because of its large radiative forcing effect, the increase is highly sig-
nificant.
Irrigated ricefields are not expected to be major sources of N
2
Oifthefields
are kept continuously submerged during the growing season (Buresh and Austin,
1988; De Datta, 1995; Hou
et al
., 2000). Rates of nitrification of NH
4
+
in rice-
fields and subsequent denitrification can be substantial (Chapter 5). In general,
conditions are sufficiently reducing and the availability of organic substrate suf-
ficiently large that denitrification proceeds as far as N
2
with little intermediary
N
2
O produced en route. However, under fluctuating water regimes, as in rice
systems in which the soil is deliberately drained in the middle of the season,
conditions may be ideal for N
2
O emission. This is often done to remove toxic
products of anaerobic metabolism or simply to save water. A common practice
