MSc

A field investigation was carried out with four exotic tree species (Acacia
auriculiformis, A. mangium, Casuarina equisetifolia and Swietenia macrophylla)
planted at 2 m × 2 m spacing and of about 30 years age at Kerala Forest Research
Institute sub-centre Nilambur during 2013-2014. The specific objective of the
study was to examine the variations in soil productivity, with special reference to
the beneficial microflora, due to long term occupancy of these trees. The
rhizosphere soils were collected for isolation and enumeration of soil microflora
like actinomycetes, bacteria, fungi, N-fixing bacteria, P-solubilises and Ksolubilising
bacteria
population
at
quarterly
interval
for
a
period
of
one
year.
The

soil
physico-chemical
properties
under
the
trees
were
also
assessed.

It was found that, over the years, the tree species influenced the soil
physico-chemical properties. The lowest bulk density and pH were associated
with tree plots compared to the treeless plots. However, the soil moisture content
was not significantly different. The soil organic carbon, total nitrogen and
exchangeable potassium were significantly higher (2.15%, 0.13% and 80.15 kg
ha
-1
respectively) in S. macrophylla and the lowest (1.38%, 0.07% and 52.34 kg
ha
-1
respectively) in treeless control plot. Available nitrogen and available
phosphorus were significantly higher (71.6 kg ha
-1
and 4.42 kg ha
respectively)
in A. mangium and the lowest (39.05 kg ha
-1
and 4.08 kg ha
-1
-1
) in treeless plot. Tree species greatly influenced the soil microflora population. In general,
microflora population was higher in tree plots than the treeless control. During
the entire period of study, A. auriculiformis had highest mean bacteria, fungi and
nitrogen fixing bacteria population while the highest population of phosphorus
solubilizing microorganism and potash solubilizing bacteria was recorded in A.
mangium. The highest mean population of actinomycetes was associated with C.
equisetifolia. Seasonal variation in microflora population was obvious. Actinomycetes
population was, generally, the lowest during rainy season and the peak population during early summer season and late summer-II. The bacterial population was the
maximum during rainy season and the lowest during early summer season. The
fungal population was more in both the late summer (2013 and 2014) seasons and
the lowest in the winter season. Nitrogen fixing bacteria population was highest in
the late summer-I and found to decrease through the rainy season and winter
season. The populations of phosphate solubilizers and potash solubilizing bacteria
were highest in late summer-I season and lowest in the rainy season (phosphate
solubilizer) and winter season (potash solubilising bacteria).
A. mangium had the maximum height (19.90 m) followed by C.
equisetifolia, A. auriculiformis and the lowest was for S. macrophylla. Diameter
at breast height was also highest in A. mangium (24.31 cm) followed by S.
macrophylla, A. auriculiformis and C. equisetifolia The present study highlighted the influence of tree species on microflora
population. Microflora population was found to be significantly higher in tree
species compared to nearby treeless control plot and was found to be varying
according to seasons. All the tree species have shown higher soil nutrient content
than treeless plot. These four exotic tree species in the present study is seen to
take part actively in the improvement of soil quality and soil health which are the
major determinants of sustainable soil productivity.