International Journal of Horticulture 2015, Vol.5, No.3, 1
-
12
10
PE metribuzin 0.7 kg ha
-1
, PE pendimethalin 1.0 kg
ha
-1
, PE atrazine 0.75 kg ha
-1
, PE oxyfluorfen 0.3 kg
ha
-1
and PE oxadiargyl 0.25 kg ha
-1
recorded
significantly lower microbial population immediately
after herbicide spraying (3 DAP) than the treatments
not having any herbicides and thereafter increased.
Among the pre-emergence herbicides, pendimethalin
1.0 kg ha
-1
recorded numerically lower bacteria, fungi
and actinomycetes population at 3 DAP and thereafter
increased. At 25 DAP, the microbial population has
decreased in those treatments where POE glyphosate
1.03 (T
12
) and 1.54 kg ha
-1
were applied than that in
other treatments. However the decline in the
population was not significant. At 50 DAP, those
treatment which have received POE herbicide
viz
.,
fenoxaprop 67 g ha
-1
and metsulfuron 4 g ha
-1
, the
microbial population was distinctly lower than hand
weeding and unweeded check. But at 75 DAP, the
microbial population was regained in those treatments
which have received herbicides and became
comparable with that in hand weeding and unweeded
check during both the years of study.
Herbicides applied in crop fields for weed control are
likely to have effects on the microorganisms in the
rhizosphere of crops, weed and in soil (Lewis et al., 1988).
Application of herbicide in agriculture may have
effects on biological equilibrium following the
changes in soil environment. Herbicides have the
potential to influence the population and functions
with a diverse range of soil microbes and thus
influence the soil fertility (Lupwayi et al., 2004).
Significant reduction of microbial population recorded
in PE pendimethalin 1.00 kg ha
-1
might be due to the
fact that these microbes utilized the chemical as their
food source (Sarkar
et al
. (2005). Similar findings were
made earlier by Das
et al
. (2003)
who reported that,
initial suppression of microbial count on 5
th
day of
pendimethalin imposed plots, thereafter microbial
population were observed growing normally after 25
days of application.
Those treatments which have received POE
fenoxaprop 67 g ha
-1
and metsulfuron 4 g ha
-1
on 45
DAP recorded lower microbial population at 50 DAP
which might be due to effect on microbial
dehydrogenase and nitrification activities (Junnila et
al., 1994). Further, Anil Gupta et al
.
(2011) who had
reported that application of fenoxaprop reduce the
microbial population due to depressive effect of
herbicides on mycorrhizae may be due to many
reasons, they are known to repress cell division as a
consequence of their disturbing nucleic acid
metabolism and protein synthesis (Audus, 1976), but
an inhibition of photosynthetic activity by herbicides
has also been reported (Diaz et al., 1989). It is known
that the carbon requirements of mycorrhizal fungi
must be supplied by the host photosynthetate (Smith,
1980). Any factor which modifies the photosynthetic
products available for distribution might affect VA
mycorrhizal development (Daft and El-Giahmi, 1978).
Thus, it is to be expected that herbicides could affect
VA mycorrhizal infection through their effect on plant
photosynthesis. Slight reduction of bacteria, fungi and
actinomycetes population was recorded in POE
glyphosate 1.03 (T
12
) and 1.54 kg ha
-1
on 25 DAP (T
13
) +
HWon 45 and 75 DAP and recovered within 45 days after
herbicide application. Weaver et al. (2007) who had
reported that glyphosate had only small and transient
effects on the soil microbial community, even when
applied at greater than field rates.
PE metribuzin 0.7 kg ha
-1
(T
3
), PE pendimethalin 1.0
kg ha
-1
(T
6
), PE atrazine 0.75 kg ha
-1
(T
9
) each
integrated with straw mulch 10 t ha
-1
on 10 DAP
followed by HW on 75 DAP
recorded more number of
bacteria, fungi and actinomycetes at 50 DAP due to
the addition of the biomass/organic matter to the soil
during decomposition of organic material. Similar
findings were earlier observed with the works of
Gayathry (2002) and De Roy et al. (2006) wherein
promotion of soil micro flora was observed due to
addition of biomass to soil. There was no reduction in
the microbial population with hand weeding and
unweeded check.
Irrespective of the various pre and post emergence
herbicide application, sudden decrease of soil
microbial population was noticed at the same day of
chemical application in the experimental field.
Thereafter, at 15 and 25 DAP, microbial population
was gradually increased. This might be due to the reason
that certain groups of micro-organisms (primary
population) start to decompose herbicides a few days
after their arrival. Several scientists reported that
herbicides may be a source of nutrition for microbes
(Cook and Hutter, 1981), in which case they
significantly influence microbial growth and
multiplication. Radosevich et al. (1995) also reported
that soil microbiological population uses herbicides
