Agriculture Reference
In-Depth Information
40
40
CH
4
N
2
O
30
30
20
20
10
10
0
0
−
10
−
10
−
20
−
20
−
30
−
30
−
40
−
40
452
387
333
289
280
169 145
E
H
(mV)
−
56
−
189
−
266
−
279
Figure 8.6
Fluxes of CH
4
and N
2
O from ricefields during cropping and fallows plotted
against the corresponding soil
E
H
(Hou
et al
., 2000). Reproduced by permission of Soil
Sci. Soc. Am.
which shows emissions of CH
4
and N
2
O from a ricefield in northern China
measured from March to December for 2 years, plotted against the corresponding
soil redox potentials,
E
H
(Hou
et al
., 2000). It shows that emissions of CH
4
and
N
2
O were strongly correlated with
E
H
. Significant CH
4
emission only occurred at
E
H
<
−
200 mV.
The results suggest the possibility of using management practices to maintain the
redox potential in a range where both N
2
OandCH
4
emissions are low.
100mV, whereas N
2
O emission was only significant at
E
H
>
+
8.2.3 DIFFERENCES BETWEEN RICE PRODUCTION SYSTEMS
Bronson
et al
. (1997a,b) made continuous measurements of CH
4
and N
2
Oemis-
sions from ricefields over a period that included two dry season and one wet
season irrigated rice crops and the two intervening fallow periods. The soil
was clayey and poorly drained. Figure 8.7 shows that during the growing sea-
sons, N
2
O fluxes were generally barely detectable although small emissions
(
≤
3
.
5mgNm
2
day
−
1
)
occurred after N fertilizer applications. Methane fluxes,
on the other hand, were substantial throughout the rice-growing seasons. The
total emission of CH
4
over the season decreased three- to four-fold when N
was supplied as
(
NH
4
)
2
SO
4
rather than urea at 200 kg N ha
−
1
, but emission of
N
2
O was 2.5-fold greater with
(
NH
4
)
2
SO
4
. Mid-season drainage suppressed CH
4
emission by
60%, but markedly increased N
2
O emissions.
Figure 8.8 shows the results for the fallow periods. These lasted 5 to 11
weeks and were weedy. The soil was generally aerobic, and moderate amounts of
NO
3
−
accumulated
(
7-20kg N ha
−
1
)
. Moderately high, continuous N
2
Oemis-
sions occurred, apparently during nitrification of mineralized organic N in the
topsoil and possibly also during denitrification in the wet subsoil. The flux of
N
2
O was greatest immediately after rainfall and after the field was flooded for
rice at the end of the fallow, as a result of denitrification of accumulated NO
3
−
.
≤
