MSc

Weed competition is one of the major factors limiting rice production in
the tropics. Due to the scarcity and high cost of labour, weed management with
herbicides is widely practiced. The problem associated with herbicides is the
persistence of their residues in soil which interact with microorganisms thereby
altering the microbial diversity and soil enzyme activity. Hence, the present
investigation entitled “Effect of herbicides and chemical characteristics of soil on
microbial biomass carbon and enzyme activity” was undertaken at College of
Horticulture, Vellanikkara during 2016-2017. The objectives were: (i) to
determine the impact of pre and post emergence herbicides on microbial biomass
carbon and the soil enzymes namely dehydrogenase, urease, and phosphatase and
(ii) to study the influence of chemical characteristics of the soil on microbial
biomass carbon and enzyme activity.
Representative soil samples were collected during March-April, 2016 from
four sites viz., rice field of Agricultural Research Station, Mannuthy (medium
organic matter soil with a history of herbicide application: S1 Control), non-
cropped area of Agricultural Research Station, Mannuthy (medium organic matter
soil without a history of herbicide application: S1 Absolute control), rice field of
Kole land, Alappad (high organic matter soil with a history of herbicide
application: S2 Control), and non- cropped area of Kole land, Alappad (high
organic matter soil without history of herbicide application: S2 Absolute control).
Physical, chemical, and biological characterization of the soil samples were done
before starting the experiment. Pot culture experiment with rice variety Jyothi was
conducted in the kharif season of 2016 with twelve treatments and six replications
in factorial CRD (four herbicides under each soil type + four controls). The
herbicides
included
pendimethalin,
oxyfluorfen,
bispyribac-sodium,
and
cyhalofop-butyl.
Pre emergence herbicides (pendimethalin and oxyfluorfen) were applied
at six days after sowing (DAS) and post emergence herbicides (bispyribac-sodium
and cyhalofop-butyl) at 16 DAS. Soil samples were analysed at six intervals viz.,
on the day of herbicide application, but two hour before, then at 7, 15, 30, and 60
days after herbicide application (DAHA) and at harvest so as to evaluate the
changes in the chemical and biological properties.
Data on analysis of soil samples before the period of experimentation
revealed that soils from rice field of Kole land recorded physical, chemical, and
biological characteristics which are more favourable for the growth of microflora.
The chemical characteristics like pH, electrical conductivity, and organic
carbon followed a decreasing trend towards the harvest in all the treatments.
Decline in microbial biomass carbon (MBC) was observed upto harvest except on
60 DAHA. Pendimethalin treatment in S1 soil (medium organic matter) registered
the highest per cent reduction in MBC with respect to control at different
intervals. Percentage reduction in MBC due to herbicide application was
comparatively lower in S2 (high organic matter) throughout the period of study.
Activity of dehydrogenase, urease, acid phosphatase and alkaline phosphatase was
higher in S2 compared to S1. In all cases, the enzyme activity increased upto 60
DAHA with slight variations and declined thereafter registering a peak at 60
DAHA. Among the four enzymes studied, urease recorded maximum increase at
60 DAHA followed by acid phosphatase, dehydrogenase and alkaline
phosphatase.
All the vegetative and yield parameters of rice were high in S2 (high
organic matter soil) irrespective of the treatment combinations. Adverse effects of
herbicides on the plant characters were minimal. Whole plant analysis for major
nutrients also exhibited appreciably higher values in S2 compared to S1.
Out of the five biological parameters analysed, microbial biomass carbon
showed the highest variation from control at all the sampling intervals. The
adverse effects of herbicides on MBC and enzyme activity followed the order:
pendimethalin > bispyribac-sodium > oxyfluorfen > cyhalofop-butyl. Organic
carbon had highly significant and positive correlation with all the biotic
components viz., MBC, dehydrogenase, urease, acid and alkaline phosphatase
activity at different sampling intervals consequent to herbicide application.
Further study should be focused on identification of microbial diversity,
effect of rhizosphere on biological activity and analysis of biological parameters
with respect to crop growth stages consequent to herbicide application.