PhD

Biologically and biochemically mediated processes in soils are
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fundamental to the functioning of terrestial ecosystem. Understanding the
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underlying biological processes in tandem with identification of indicators of
ecosystem stress is inevitable to provide strategies and approaches to promote
long term sustainability of the fragile ecosystem. All members of the food
chain are dependent on the soil as a source of nutrients, support and sustenance
besides their reliance on soil as a sink for terrestial wastes. Now there is growing
evidence to believe that soil biological parameters can hold potential as early
and sensitive indices of soil degradation or restoration. In this context, soil
enzymes play an active role in the microbial ecology and transformations
brought about by catalysing enumerable reactions in soils. The activity of soil
enzymes thus hold potential as soil quality indicators though they are sensitive
to temporal changes due to environmental and management factors. Thus the
long term effects of soil management techniques such as crop rotation, liming,
fertilization and pesticide applications on the biological regimes need detailed
investigations. Hence, with the overriding objective of projecting the effects
of long term fertilization, application of soil amendments and agrochemicals
on enzyme activities in relation to soil fertility, the present investigation was
carried out.

Analysis of soil samples collected from the experimental plots of five
permanent manurial trials distributed in the coastal, midland and mid-upland
regions of Kerala reveal drastic differences in the activities of five major soil
enzymes, viz., urease, phosphatase, protease, dehydrogenase and cellulase. It
is obvious from the study of permanent manurial plots of the Rice Research
Station, Kayamkulam that the activity with respect to urease, protease and
cellulase is low compared to the corresponding samples collected from the
long term experimental plots of Pattambi, Balaramapuram and Chethackal.
The activity of phosphatase and dehydrogenase reported in this case was
moderate to high compared to Pattambi and Balaramapuram. In the case of
protease and cellulase the highest activity was represented by samples collected
from the Central Experimental Station of the Rubber Research Institute of
India, Chethackal. In general, the highest activity with respect to all enzymes
studied have been reported in the intercropping experiment at the Central
Experimental Station, Chethackal, representing the hilly terrain and the lowest
values were seen for the permanent manurial experiment at Balaramapuram.
In the permanent manurial experiments, activities of the enzymes were
found to be higher in plots receiving treatments involving combinations of
organic manures and chemical fertilizers. It is evident from the study that the
total quantity of organic amendments added to the soil per se could not sustain
a higher microbial activity coupled with a higher enzyme activity, but required
supplemental addition of mineral nutrients through chemical fertilizers also.
Further, the role of supplemental addition of chemical fertilizers in the
enhancement of enzyme activities was very clear from the stastically significant
results obtained for the permanent manurial experiment at Pattambi.

Significant differences in the enzyme activities have been observed in
the case of the permanent manurial experiment at Balaramapuram consequent
to the main and interaction effects of chemical fertilizers at higher levels of
addition especially for urease, phosphatase and dehydrogenase. Hence it is
clear from the study that the application of mineral fertilizers over a basal
dose of organics could enhance the microbial activity of the soil to significantly
higher levels yielding higher enzyme activities even in a highly weathered
low activity clay soil like the red loam soil of Balaramapuram.
Enzyme activities observed were the highest under the rubber based
cropping system at Chethackal followed by the rice based experiments at
Pattambi and Kayamkulam while the lowest activity was noticed at
Balaramapuram. Of the physico-chemical characteristics studied through path
co-efficient analysis and regression analysis, OC, CEC, ~03' available N,
available P, available K and respiratory activity were found to influence the
enzyme activities to the maximum extent.
Pot culture experiments to study the effect of amendments and
agrochemicals on enzyme activities revealed significant effects due to
treatments. In all these cases, the maximum inhibitory effect on enzyme
activities in general, were observed consequent to the application of antibiotics
followed by fungicides, insecticides and herbicides. Immediately after the
application of treatments there was a decline in the enzyme activity during the
active tillering, the activity gradually resumed during the panicle initiation
and then decreased asymptotically towards the harvest stage. Thus it is evident
from the present study that considerable suppression of the enzyme activity is


observed consequent to the application of agrochemicals though it is rendered
relatively harmless over a period of time within the life span of the crop.
However, the long term effect of this application need to be investigated in
detail.
Among the various amendments used in the present study, treatment
involving the application of organic amendment as vermicompost in
combination with lime and fertilizers was found to be significantly superior.
All the five enzymes thus registered the highest value in this treatment
irrespective of the stage of the crop.