Agriculture Reference
In-Depth Information
be expected. Current efforts to make water use in rice production more efficient
will undoubtedly increase N
2
O emissions unless steps are taken to avoid this.
Rice can also be an indirect source of N
2
O (and NO) emissions via deposition
of volatilized ammonia on natural ecosystems, particularly wet tropical forests,
which are one of the main 'natural' sources of N
2
O(Table8.4a).
8.2.2 PROCESSES GOVERNING NITROUS AND NITRIC OXIDE
EMISSIONS FROM RICE
Emissions of nitric and nitrous oxides are the result of microbial nitrification and
denitrification in soils, controlled principally by soil water and mineral N contents,
labile organic carbon, and temperature. Nitric oxide is a direct intermediate of
both nitrification
NH
4
+
−−−→
NH
2
OH
−−−→
NO
−−−→
NO
2
−
−−−→
NO
3
−
and denitrification,
NO
3
−
−−−→
NO
2
−
−−−→
NO
−−−→
N
2
O
−−−→
N
2
It is thought that little net NO is produced in denitrification, it being readily
reduced to N
2
O, and nitrification is therefore the main source of NO (Anderson
and Levine, 1986; Skiba
et al
., 1993). Nitrous oxide is also produced in both
nitrification and denitrification. At low O
2
concentrations in otherwise aerobic
soil, small amounts of N
2
O are formed as a by-product of nitrification, N
2
O not
itself being reduced to NO
2
−
. In denitrification, the proportion of N
2
O produced
relative to N
2
increases as the availability of O
2
increases and that of carbon
decreases (Tiedje, 1988). In general only a small fraction of the N nitrified or
denitrified in these pathways is released as NO or N
2
O. The emission is therefore
sensitive to the amount of mineral N in the system, which is driven principally by
additions of nitrogen fertilizers and deposition of nitrogen from the atmosphere.
As discussed in Chapter 5, in submerged soils nitrification occurs in aerobic
sites at the floodwater-soil and root-soil interfaces. Denitrification occurs upon
diffusion of the NO
3
−
to the anaerobic bulk soil. Denitrification is favoured over
dissimilatory reduction to NH
4
+
(
NO
3
−
→
NO
2
−
→
NH
4
+
)
because of the large
ratio of available carbon to electron acceptors in submerged soils. Denitrification
is likely to proceed completely to N
2
with little accumulation of N
2
O because of
the very large sink and therefore steep concentration gradient of O
2
, and because
carbon is less likely to be limiting (Section 5.1).
However this will not be the case when a submerged soil is drained and air
enters, leading to gradients of oxidation from the surfaces of soil cracks towards
the anaerobic interiors of soil clods. Now conditions may be ripe for production
of nitrous and nitric oxides.
Hence there is a fine balance between conditions favouring methanogenesis and
those favouring nitrous oxide production. This is nicely illustrated in Figure 8.6,
