PhD

The studies on 'Methane ermssion from wetland rice fields of Kerala'
revealed that the methane efflux from the Lateritic soils of Kerala was generally
low, to the tune of 1.64 mg m-2 h{1 during the first crop season (June to September)
and 3.5 mg m-2 hr-I during the second crop season (October to January). Temporal
including diumal and seasonal as well as spatial variations were detected in methane
emission from the rice fields. The methane emission during the second crop season
was relatively higher and the emission pattem revealed evening peaks. The methane
efflux showed progressive increase from the tillering stage to the booting stage in
both crop seasons. The factors such as soil temperature, water temperature, redox
potential and solar radiation did not show any individual influence on the methane
emission.
The application of organic manures such as rice straw, glyricidia and farm
yard manure @ 5 t ha-I produced significant and positive influence on methane
emission probably due to the enhanced availability of carbon substrate for methane
production. In this respect, rice straw and glyricidia showed a better influence
compared to farm yard manure.
The positive influence of nitrogen has been detected in the present study,
with the two sources of nitrogen fertiliser, namely, urea and factomphos, showing
variations in their influence on n:ethane efflux. Application of urea contributed to a
significantly higher methane emission than factomphos during the second crop
season.
Incorporation of organic manures and application of nitrogen positively and
significantly influenced the grain yield and straw yield of rice during the second
crop season. A higher grain yield was observed during the first crop season
compared to second crop season. The ancillary data on biometric and yield
contributing characters also indicated variable influence of the sources of organic
manure and nitrogen fertilisers during the two crop seasons.
In a pot culture experiment in Lateritic soil using 14C labelled rice straw,
daincha, sunhemp and glyricidia, it was observed that the decomposition of added

• residues started immediately after their incorporation. The process of decomposition
consisted of an initial rapid declining phase followed by a slow and steady phase.
The presence of rice crop was not found to influence the pattem of decomposition,
The green manures exhibited a higher decomposition rate than rice straw due
to their low C : N ratio and 10\\' lignin content. The highest rate of decay among the
green manures was exhibited by glyricidia, followed by daincha and sunhemp.
While 60 per cent of the added organic carbon from sunhernp was retained in the
soil in the first five days after incorporation, 50 per cent was retained from daincha
and only 40 per cent was retained from glyricidia. In the mean while, about 85
per cent of the added organic carbon from rice straw was retained in the soil in the
first five davs.
The incubation study on the decomposition pattern of rice straw and daincha
in different rice soils of Kerala employing radio tracer technique revealed that the
decomposition rate of daincha was faster than rice straw in all the soil types.
A veraged over the seven soils, 60 per cent of the added organic carbon was lost
from daincha within the first two days, while it was only 15 per cent in the case of
rice straw.
In general the Kari soil retained the highest amount of organic carbon (34
per cent) added through rice straw followed by the Kole soil (31 per cent) and the
lowest amount was retained by the Karappadam soil (14 per cent). The other soils
namely Lateritic, Onattukara, Pokkali and Chittoor black exhibited intermediate
retention of organic carbon ranging from 20 to 28 per cent. The half life of labelled
rice straw estimated for different soils ranged from 64 days to 80 days.
The highest amount of the added organic carbon from daincha was retained
in the Pokkali soil (15 per cent) followed by the Lateritic soil (14 per cent), the
Onattukkara soil (12 per cent) and the Kari soil (11 per cent). The lowest amount
was retained in the Kole soil (4 per cent) while the Karappadam and Chittoor black
soil retained 9 per cent and 7 per cent respectively.