80 K. Bharati et al. Agriculture, Ecosystems and Environment 79 2000 73–83
Table 7 Variation in the readily mineralizable carbon RMC content of a flooded alluvial soil planted to rice under the influence of Azolla and urea
a
Treatment Readily mineralizable carbon
m
g g
− 1
dry soil Days after transplantation
10 20
30 40
50 60
70 Mean
Control no N 1259
b
1533
b
2501
a
821
a
309
a
763
a
821
a
1144 Urea 60 kg N
2631
a
785
d
1262
b
568
b
444
a
624
ab
722
a
1005 Azolla incorporated 30 kg N + urea 30 kg N
2128
a
1771
a
2387
a
508
b
312
a
934
a
927
a
1281 Azolla dual cropping 30 kg N + urea 30 kg N
2127
a
1537
b
1425
b
461
b
497
a
823
a
842
a
1102 Azolla incorporated 30 kg N + dual cropping 30 kg N
2027
a
1088
c
2273
b
888
a
308
a
496
b
876
a
1137
a
Mean of three replicate observations. In a column, means followed by a common letter are not significantly different at p 0.05 by Duncan’s Multiple Range test DMRT.
Table 8 Variation in the ninhydrin reactive nitrogen NRN content of a flooded alluvial soil planted to rice under the influence of Azolla and urea
a
Treatment
m
g ninhydrin reactive N g
− 1
dry soil Days after transplantation
10 20
40 50
70 Mean
Control no N 2.00
c
1.53
a
2.80
b
3.51
ab
1.75
a
2.32 Urea 60 kg N
6.30
b
1.55
a
5.51
a
4.38
a
1.11
a
3.77 Azolla incorporated 30 kg N + urea 30 kg N
7.85
a
1.23
a
4.74
a
3.16
b
1.05
a
3.61 Azolla dual cropping 30 kg N + urea 30 kg N
6.61
ab
1.19
a
5.18
a
2.44
c
0.62
a
3.21 Azolla incorporated 30 kg N + dual cropping 30 kg N
7.37
a
1.38
a
5.17
a
4.14
a
0.74
a
3.76
a
Mean of three replicate observations. In a column, means followed by a common letter are not significantly different at p 0.05 by Duncan’s Multiple Range test DMRT.
20.62 kg CH
4
Mg
− 1
grain yield was recorded in the plots where Azolla was grown as dual crop.
4. Discussion
4.1. Methane flux Application of organic substrates, including green
manure, often increased the CH
4
flux from flooded
Table 9 Variations in select parameters of rice plants and cumulative CH
4
efflux from a flooded rice paddy under the influence of Azolla and urea
a
Treatment Grain yield
Straw yield Harvest
Cumulative kg CH
4
Mg
− 1
Mg ha
− 1
Mg ha
− 1
index CH
4
kg ha
− 1
grain yield Control no N
3.58
a
3.75
a
48.84 94.94
a
26.52 Urea 60 kg N
4.58
b
5.18
b
46.93 155.28
c
33.90 Azolla incorporated 30 kg N + urea 30 kg N
4.38
b
4.93
b
47.05 149.37
c
34.10 Azolla dual cropping 30 kg N + urea 30 kg N
4.33
b
4.94
b
46.71 89.29
a
20.62 Azolla incorporated 30 kg N + dual cropping 30 kg N
4.24
b
4.78
b
47.01 105.64
b
24.92
a
Average of three replicate observations. In a column, means followed by a common letter are not significantly different at p 0.05 by Duncan’s Multiple Range test DMRT.
rice paddy Yagi and Minami, 1990; Denier van der Gon and Neue, 1995; Wassmann et al., 1996. Azolla,
which is used to supplement the N requirement of the growing rice crop, is either incorporated in flooded
paddy as green manure or grown as a dual crop. In the present study, CH
4
emission was low in all the plots during the first 2 weeks after transplantation with the
exception of Azolla incorporated plots. Fresh Azolla is easily degradable and upon decomposition, adds to the
soil pool of fermentable substrates for methanogenic
K. Bharati et al. Agriculture, Ecosystems and Environment 79 2000 73–83 81
consortia. Its incorporation as green manure Treat- ment III resulted in a high CH
4
flux during the first 20 DAT, which also coincided with the most active period
of Azolla decomposition Watanabe et al., 1989. Soil organic matter, both native and applied, is the main
source of CH
4
emitted from the paddy soils during the initial stages of crop growth Neue et al., 1997.
The situation was markedly different in treatments IV and V where Azolla was grown as dual crop, ei-
ther in conjunction with urea or following incorpora- tion of Azolla as green manure. In both the treatments,
CH
4
flux was low, possibly because the growing Azolla crop had a moderating effect on the CH
4
flux from flooded soil as compared to the incorporated
Azolla. Application of urea stimulated CH
4
flux from flooded fields by causing increased plant growth and
metabolic activity that perhaps contributed to a higher CH
4
efflux. In an earlier greenhouse study, applica- tion of urea stimulated CH
4
production and emission through rice plants Banik et al., 1996.
A consistently higher CH
4
flux was observed in Azolla incorporated plots as compared to no N control
plots. This difference in CH
4
emission among treat- ments was maintained upto 60 days. A second emis-
sion peak of CH
4
flux was observed in all the plots, albeit with varying degree depending upon the treat-
ment, during maturity period of the crop. The increase in emission during this period could be attributed to
C supplied by root lysis or exudation from rice after flowering Schutz et al., 1989; Lindau et al., 1991.
However, the initial priming effect caused by Azolla incorporation was persistent even at later stages.
4.2. Redox status and oxygen diffusion Redox status of a flooded soil is an indirect indi-
cator of CH
4
flux pattern from rice ecosystem Wang et al., 1993 and soils with lower redox potential
are usually associated with high CH
4
flux. Follow- ing flooding, soil Eh decreases in a thermodynamic
sequence and the progress of soil reduction is con- trolled by the relative abundance of electron donors
and electron acceptors in the soil. The main electron donor in flooded rice soils is readily decomposable
organic matter and in soils where Azolla was incor- porated, the enhanced reduction is probably due to
the ready availability of organic matter from decom- posing Azolla. Interestingly, soil from treatments with
dual crop of Azolla registered a higher redox potential leading to low CH
4
flux. Lowering of the Eh is negatively influenced by the
diffusion of oxygen to the surface soil layer. High DO
2
in the floodwater might retard CH
4
emission from rice field by promoting CH
4
oxidation at the soil–water interface Hanson and Hanson, 1996. Further, leach-
ing of oxygenated water by percolation in flooded rice fields would inhibit methanogenesis by keeping
the soil in a more oxidized state as well as stimu- late CH
4
oxidation in the reduced soil layer Kimura et al., 1992. The mean DO
2
concentration was higher in field plots with a dual crop of Azolla indicating the
role of Azolla in enriching the standing water with oxygen. In dual cropping of Azolla, the growing fern
forms a mat above the standing water, but unlike many other aquatic plants is not particularly known to re-
lease oxygen through its floating roots Ashton and Walmsley, 1976. It is possible that the oxygen re-
leased during active photosynthesis by Azolla in the standing water maintains more oxidized conditions af-
fecting the emission of CH
4
from the flooded soil. In an earlier report, CH
4
emission from a flooded field was low during the active growth of algae and this was
attributed to the release of oxygen from the algal mat during photosynthesis Wang et al., 1995.
4.3. Other soil and plant parameters The
a
-naphthylamine oxidase activity of rice roots, another index of the oxidation status of the rhizosphere
region Ota, 1970, has been correlated well with CH
4
efflux from different cultivars Satpathy et al., 1998, growth stages Adhya et al., 1994 and even diurnal
variation in CH
4
flux Satpathy et al., 1997 from rice plants. In this study,
a
-naphthylamine oxidase activity of the root base was high in field plots with dual growth
of Azolla indicating higher oxidation status in such field plots. Thus, low CH
4
flux from field plots with dual cropping of Azolla could be a combination of
higher redox potential, high dissolved oxygen in the standing water and higher
a
-naphthylamine oxidase activity indicating an overall higher oxidation status.
Higher rate of soil organic matter decomposition is known to accelerate CH
4
production by supplying large amount of substrates for methanogenesis. The
RMC content of the soil, an indicator of available sub- strate for methanogenesis Mishra et al., 1997 was
82 K. Bharati et al. Agriculture, Ecosystems and Environment 79 2000 73–83
high in Azolla-incorporated plots which also exhibited higher CH
4
flux. NRN, an index of free amino and amide-N released from dead and decaying microbial
cells, is another indicator of available substrates for microbial activity Joergensen and Brookes, 1990.
NRN content of the rhizosphere soil was high in almost all the amended soils as compared to no N
control and had a statistically significant positive re- lationship with CH
4
emission under the influence of Azolla. Thus, RMC and NRN which are indicators of
substrate availability had direct or indirect effect on CH
4
efflux from Azolla-incorporated plots. Azolla is used as a green manure or biofertilizer to
supplement the N demand of the rice crop and can partially replace the costly chemical N fertilizer under
conditions of sustainable agriculture. In the present study, application of Azolla either alone or in combi-
nation with urea resulted in a significant increase in grain yield that was statistically at par with that of urea
alone. In an earlier study, similar grain yield increase was recorded due to Azolla application Watanabe
et al., 1989. What was interesting is that lowest value of CH
4
release per ton of rice yield 20.62 kg CH
4
Mg
− 1
grain was obtained in plots were Azolla was grown as a dual crop. Thus, a dual crop of Azolla
results in a lowering of CH
4
efflux while increasing the rice yield.
5. Conclusion
