Agriculture, Ecosystems and Environment 79 2000 73–83
Influence of incorporation or dual cropping of Azolla on methane emission from a flooded alluvial soil planted to rice in eastern India
K. Bharati, S.R. Mohanty, D.P. Singh, V.R. Rao, T.K. Adhya
∗
Laboratory of Soil Microbiology, Division of Soil Science and Microbiology, Central Rice Research Institute, Cuttack - 753006, India Received 15 December 1998; received in revised form 17 August 1999; accepted 8 November 1999
Abstract
Green manures are widely used in rice production and may influence methane efflux CH
4
. Influence of application of Azolla A. caroliniana Wild., a widely used biofertilizer for rice Oryza sativa L., on CH
4
efflux from a flooded alluvial soil planted to rice, and select soil and plant variables were investigated in a field experiment at Cuttack, India. Azolla was
either incorporated as green manure at the beginning of the experiment or grown as dual crop in the standing water along with the rice crop. Dual cropping of Azolla equivalent to 30 kg N ha
− 1
in conjunction with urea 30 kg N ha
− 1
effected lowest CH
4
flux 89.29 kg CH
4
ha
− 1
. Cumulative CH
4
flux followed the order of urea Azolla incorporated + urea Azolla incorporated + dual crop no N control urea + Azolla dual crop. Growing Azolla had a moderating effect on CH
4
efflux from flooded soil through an increase in the dissolved oxygen concentration at the soil–floodwater interface. Among the
different soil and plant variables studied, soil redox potential, dissolved oxygen concentration at the soil–floodwater interface and
a
-naphthylamine oxidase activity of root base exhibited significant negative relationship with CH
4
flux. In addition, Fe
2+
and ninhydrin reactive nitrogen NRN contents of the flooded soil exhibited significant positive relationship with CH
4
flux. Results indicated that, dual cropping of Azolla in conjunction with urea considerably reduced CH
4
efflux without affecting the rice yields and can be used as a practical mitigation option for minimizing CH
4
flux from flooded paddy. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Methane efflux; Flooded soil; Azolla application; Rice plants; Grain yield
1. Introduction
Methane CH
4
, the most abundant gaseous hydro- carbon in the atmosphere, is an important greenhouse
gas that may account for approximately 15–20 of the total current increase in global warming Rodhe,
1990. Flooded rice paddy has been identified as one of the important sources of anthropogenic CH
4
with estimates of annual emission ranging between 47
∗
Corresponding author. Tel.: +91-671-642445; fax: +91-671-641744.
E-mail address: crriinfoori.nic.in T.K. Adhya
and 60 Tg per year, representing 8.5–10.9 of total emission from all sources Crutzen, 1995; Houghton
et al., 1995. With the intensification of rice culti- vation to meet the needs for rising population, CH
4
emission from this important ecosystem is likely to increase Anastasi et al., 1992. However, a reduction
of 15–20 would result in stabilization of the CH
4
concentration in the atmosphere to that of 1990 levels Watson et al., 1995.
Flooded soils planted to rice are conducive to the production and emission of CH
4
due to the presence of methanogenic bacteria that utilize readily decompos-
able organic compounds under anaerobic soil condi-
0167-880900 – see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 8 8 0 9 9 9 0 0 1 4 8 - 6
74 K. Bharati et al. Agriculture, Ecosystems and Environment 79 2000 73–83
tion. Both CH
4
production and emission from flooded rice soils are strongly influenced by several soil pro-
cesses including changes in soil redox status and pH, dynamics of substrate and nutrient availability and
textural stratification Bouwman, 1990. In addition, common cultivation practices such as application of
agrochemicals also affect CH
4
efflux from flooded rice soils Neue et al., 1997. However, the relationship
between fertilizer application and CH
4
efflux from flooded rice system is far from clear and available lit-
erature on the effect of fertilizers on CH
4
emission is often contradictory Minami, 1995. While organic
matter amendment generally increases CH
4
emission Wassmann et al., 1996; Neue et al., 1997, CH
4
ef- flux is also strongly influenced by the type, method
and rate of application of chemical fertilizer. Although urea remains the preferred chemical
N-fertilizer for rice cultivation Vlek and Byrnes, 1986, several organic sources including partially de-
composed and fresh organic matter and biofertilizers are widely used for maintaining the soil fertility and
sustained high yield in tropical rice fields Venkatara- man, 1984. Azolla, a free-floating aquatic fern having
symbiotic association with the N
2
-fixing cyanobac- terial symbiont Anabaena Azollae Stras., can fix
30–60 kg N ha
− 1
in 30 days. It is either incorporated as green manure at the beginning of the cropping
season or grown as a dual crop along with rice, in the standing water of flooded fields. The fern is used
to a great extent in China Liu and Zheng, 1992, India Singh and Singh, 1997, Bangladesh Islam
et al., 1984 and Vietnam Lumpkin and Plucknett, 1982 as an important biological source to improve
Table 1 Summary table of various experimental treatments on Azolla application at the Central Rice Research Institute, Cuttack, India
Treatment Treatment
Amendments Total N application
number details
kg N ha
− 1
Azolla application Urea amendment
I No N control
– –
II Urea-N
– Urea to provide 60 kg N ha
− 1
60 III
Azolla incorporation + urea Incorporated as green manure
at transplantation to provide 30 kg N ha
− 1
Urea to provide 30 kg N ha
− 1
60 IV
Urea + Azolla dual cropping Dual cropping to provide
30 kg N ha
− 1
Urea to provide 30 kg N ha
− 1
60 V
Azolla incorporation + Azolla dual cropping
Incorporated as green manure at transplantation to provide
30 kg N ha
− 1
+ dual cropping
to provide 30 kg N ha
− 1
– 60
the N balance of rice fields. The nitrogen fixed by the cyanobacterial symbiont is either released upon decay
of the incorporated Azolla Mian and Stewart, 1985 or leached into the standing water from the growing
Azolla Rains and Talley, 1979 and is available for uptake by the rice crop.
The objective of the study was to evaluate the effects of applying Azolla as green manure or dual cropping
it on CH
4
efflux from flooded alluvial soil planted to rice. In addition, the alterations in select soil and plant
parameters in Azolla applied soil and their relationship with CH
4
emission were investigated.
2. Materials and methods