Microbial Population Dynamics of Soil Under Traditional Agroforestry Systems in Northeast India
Awadhesh Kumar Shukla
depth wise variation in bacterial and fungal populations was studied in
three arecanut-based traditional agroforestry systems practiced by two
ethnic communities viz., the Kalitas and Nyishis. Arecanut palm was abundantly
found in Harmutty and this site also recorded greater species diversity.
Soil was sandy loam in all sites. Soil organic C, total N and available
P decreased with increasing soil depth. Bacterial population was highest
during spring and that of fungi during autumn. Nonetheless, the highest
microbial counts were recorded in the topsoil (0-10 cm) layer except during
the rainy season when the population was greater in the subsurface (10-20
cm) layer. Altogether, 29 soil micro-fungal forms were recorded from three
sites. Aspergillus and Penicillium were the abundant genera
in the sites. Soil organic C and total N concentration had correlation
with the microbial colony forming units. It was noticed that plant residues,
added organic matter, vegetation, plant species composition and soil mineral
nutrients altered the microbial population as well as their species composition
under traditional agroforestry system.
Soil is a complex ecosystem, delimited by physico-chemical parameters
that hold enormous numbers of diverse living organisms. Nevertheless,
microbes are the least unstated mechanism of soil by both agronomists
and the soil practitioners on the farm. Several soil organisms offer benefits
to crop growing in an ecosystem, but are not well understood. The soil
microbes decompose the plant and animal residues entering the soil and
convert them into soil organic matter, which influences on soil physical,
chemical and biological properties and on creating a complimentary medium
for biological reactions and life support in the soil environment. Nonetheless,
enhanced site-specific diversity typically results in higher levels of
belowground microbial diversity and production (Olson et al., 2000).
Large quantities of readily decomposable organic matter are added to
agricultural soils every year as crop residues or animal wastes and have
a significant outcome on soil microbial commotion. The plant species growing
on the soil also equally influence the population and species composition
of the soil fungi (Hackl et al., 2000). Fungi and bacteria play
a focal role in nutrient cycling by regulating soil biological activity
(Arunachalam et al., 1997). However, the rate at which organic
matter is decomposed by the microbes is interrelated to the chemical composition
of the substrate as well as environmental conditions.
Multi-strata agroforestry systems are extensive in the humid and sub-humid
tropics of the world, counting lowland and mountainous regions (Nair and
Muschler, 1993) that posses major challenge for
study of plant-soil interactions. There have been a number of studies
on the distribution of forest soil microfungi in terrestrial ecosystem.
Some studies dealt with the influence of plant community (Chung et
al., 2007; Carney and Matson, 2006), some with depth effects (Arunachalam
et al., 1997) and others attempted to examine seasonal trends (Kennedy
et al., 2005). Conversely, an integrated approach to the dynamics
of microbial population in the traditional agroforestry system in and
around Arunachal Pradesh is deficient, although such a challenge may give
an insight into the microbial population dynamics as influenced by soil
organic matter and nutrient build up. This study deals with the seasonal
and depth wise variations in soil bacterial and fungal populations in
relation to the micro-environmental and soil nutrient variability in one
of the least studied traditional agroforestry systems in the northeast
India. To go about the aforementioned approach, data on the seasonal and
depth wise variations in bacterial and fungal populations in traditional
agroforestry have been consummated and discussed in this paper to derive
some conclusions regarding the influence in relation to micro-environmental
and soil nutrient variability on the microbial population dynamics.
MATERIALS AND METHODS
Study Sites: The study was conducted during 2003 January to 2004
November in traditional agroforestry systems of two villages in Arunachal
Pradesh (27Â°60Â´N latitude and 94Â°21Â´E longitude) namely, Doimukh
and Nirjuli (126 m above MSL) of Papum Pare district and Harmutty village
(120 m above MSL) of North Lakhimpur district of Assam (26Â°46Â´N latitude
and 93Â°50Â´E longitude), bordering Arunachal Pradesh, India. The Kalitas
dominated Harmutty while Nirjuli and Doimukh were inhabited by the Nyishis,
one of the major tribes of this area. Three traditional agroforestry systems
were selected in each of the sites. The average area of the agroforestry
plot varied between 200-400 m2 in Nirjuli and Doimukh and 320-490
m2 in Harmutty. The agroforestry were multi-storied consisting
of trees, shrubs, herbs and climbers (Table 1) and consisted
of species that are chosen according to the requirements of the household
(Tangjang et al., 2004).
The sites are characterized by a climate with most rainfall occurring
during the summer months (May-July) with relatively a little or scanty
during the winter months. Total annual rainfall is typically 1100-1600
mm in all the sites and ambient temperatures average from minimum 12Â°C
to a maximum of 37Â°C in all the study sites experiencing a humid tropical
climate. Soils at the site were sandy loam to loamy sand. Nirjuli and
Doimukh area are composed of newer alluvium (newer terrace deposits) represented
by valley fill deposits comprising of sediments, while Harmutty area is
composed of alluvium belonging to Pleistocene and recent times (Kumar,
1997). Over all, soil was sandy loam in all sites and slightly acidic
||Vegetation and microclimate in the
three agroforestry systems (n = 12)
||Soil properties in the traditional
|SL: Sandy Loam; LS: Loamy Sand; WHC:
Water Holding Capacity; BD: Bulk Density
Soil Sampling and Analysis: Soil from three depths (viz., 0-10,
10-20 and 20-30 cm) were sampled during January, March, May and November
that are the representative months of winter, spring, rainy and autumn
seasons, during when the sampling was done. In each stand, ten replicates
of soil samples were collected aseptically in sterilized polythene bags
using a steel corer (6.5 cm inner diameter) and were used for the isolation
of bacteria and fungi within 24 h. The remaining soil samples were air-dried
and used for the determination of physico-chemical properties (Anderson
and Ingram, 1993). Soil moisture content was determined gravimetrically
by oven drying 10 g of fresh sieved soil for 24 h at 105Â°C.
Isolation of Bacterial and Fungal Population: Soil bacterial population
was estimated by Waksman`s (1952) method using the nutrient agar medium
at 105 dilutions. Fungal population was estimated by dilution
plate method (Johnson and Curl 1972) using Martin`s Rose Bengal agar medium
at 103 dilutions in water. The inoculated Petri-dishes were
incubated at 30Â±1Â°C for 24 h and 25Â±1Â°C for 5 days
for bacteria and fungi respectively. To calculate the populations of bacteria
and fungi, colonies developed on Petri dishes were counted with the help
of digital colony counter and expressed as number of colony forming units
(cfu) g-1 dry soil. Representative isolates of fungi were identified
under microscope with the help of standard manuals (Domsch et al.,
1980; Barnett and Hunter, 1972).
Bacterial and fungal counts were greater in the surface (0-10 cm) layer
of the soil as compared to others (Table 3). Maximum
bacterial population was recorded in spring season in all the sites and
minimum during winter. In contrast, fungal counts were higher during autumn
in all the sites. During rainy season, maximum microbial count was, however,
recorded in the subsoil (10-20 cm) layer. Both bacterial and fungal cfus
were more in Harmutty than in Nirjuli and Doimukh sites. Quantitatively,
bacterial counts were always high as compared to fungal population in
all the sites. In general, similar trend was observed in the distribution
of soil microorganisms in all the sites.
Altogether, 29 forms of fungi were isolated in the three sites. List
of isolated fungi during the study is shown in Table 4.
The relative proportion of different fungal taxonomic groups was almost
identical in all the three sites. Penicillium and Aspergillus
were the most abundant group of species in all the three study sites.
However, some species were restricted to a particular site. Aspergillus
fumigatus, Trichoderma viride and Syncephalus sp. were
exclusively found in Nirjuli, while Trichoderma sp. was encountered
only in Harmutty. Nevertheless, the number of fungal genera encountered
in the three sites was similar.
||Seasonal variations in bacterial cfu
(x105 g-1 dry soil) and fungal cfu (x103
g-1 dry soil) at three soil depths
|W: Winter; S: Summer; R: Spring; A:
||Microfungi isolated from three soil
depths in the three traditional agroforestry systems
|- and +: indicates absence and presence,
Among the soil physico-chemical properties, clay content and soil moisture
show significant positive correlations with bacterial and fungal cfu (<0.05);
water holding capacity and soil pH showed negative correlations, while
organic C and total N showed positive correlations for bacteria and negative
correlations with that of fungi (Table 5).
Generally, topsoil contains high organic matter, which in the presence
of adequate moisture supply is acted upon by the microorganisms to decompose
the complex organic residues into simpler forms; hence, microbial counts
are generally higher in the surface soil layer (Shamir and Steinberger,
2007) as compared to the lower depths. However, the distribution of soil
microbial population is determined by a number of environmental factors
like pH, moisture content and soil organic matter (Kennedy et al.,
2005). Higher bacterial population in the topsoil (0-10 cm) layer during
spring season in present study is in agreement with the observation of
Jha et al. (1992a) who recorded higher populations during spring
and post-rainy seasons. However, peak in bacterial population was recorded
during rainy season may be attributed to favorable soil moisture and temperature
conditions that coincide with greater microbial activity and decomposition.
On the other hand, minimum population counts during winter in the present
study sites may be due to low ambient temperature and greater physiological
water stress which are otherwise critical for the growth and activity
of microbes. Higher fungal population during autumn supported the findings
of other workers (Arunachalam et al., 1997), which perhaps is due
to prevailing favorable moisture and temperature setting during the post-monsoon
periods and also that the litter and other plant residues are decomposed
faster during rainy season and sufficient soil organic matter and humus
accumulates that may have enhanced the colonization of the soil microbes
in subsequent period.
Maximum population in the subsoil (10-20 cm) layer during rainy season
in all the three sites studied corroborates to that of Classen et al.
(2007) who pointed out that during hot summer months, the sub-layer of
soil occasionally harbors more fungal populations caused by temperature
and moisture regimes than the topsoil layer. Higher rate of infiltration
in the loamy sand may also in part have contributed to this phenomenon.
However, Shukla et al. (1989) and Arunachalam et al. (1997)
found negligible differences in fungal population across depths. Low fungal
population during rainy season (Table 3) in the topsoil
may partly be linked to the run-off losses of fungal propagules along
with the plant materials from the hill slope due to heavy rainfall in
the region (Shukla et al., 1989). Further, overall reduction in
microbial population in the lower soil depths was attributed to fewer
amounts of minerals, low oxygen content and increased carbon-dioxide concentration
(Shukla et al., 1989). Consequently, fungal flora noted in the
deeper depths of soil was significantly low in all the three sites. During
winter, low moisture content might have slowed down microbial activity
and organic matter decomposition and thus resulted in a low microbial
A total of 29 soil microfungi were isolated (Table 4).
However, only a few fungal species were found to be dominant and basically
no marked variations in the composition of species were noticed in different
seasons of the year across the sites. The species like Aspergillus,
Botrytis, Geotrichum, Penicillium and Rhizopus
were common to all sites. Some fungal species encountered were rare and
restricted to particular site. Dominance of the genus Penicillium and
Aspergillus in the present study sites may be due to their greater
rate of spore production and dispersal and partly due to their resistance
over extreme environmental conditions (Schimel, 1995). The fungal species
richness recorded in the present study was higher than those (13 species)
reported from subtropical humid forest soils in north-east India (Arunachalam
et al., 1997) and 26, 21 and 27 species, respectively from soils
of valley land, terrace and slopes in this region (Shukla and Mishra,
1992). Conversely, the present species number was slightly lower than
those reported 33 fungal species from subtropical humid forest by Arunachalam
et al. (1997) and was much lower as compared to 41 fungal species
isolated from South Dakota grassland soil (Dennis and Christensen, 1981).
Presence of one or a few dominant species in the present study was however
in agreement with that of Jha et al. (1992b) who pointed out that
for a given community, only a few species are numerically predominant
and may strongly affect the environmental conditions for the other.
The topsoil layer (0-10 cm) had 22 species which was greater than the
subsoil layers (10-20 and 20-30 cm). Dkhar (1983) suggested that fungi
grow slowly in the deeper soil layers due to shortage of mineral nutrients
and compaction of soil along depth. The rate of change in fungal population
was attributed to the type of vegetation growing on a particular area
(Entry and Emmingham, 1996), variation in physico-chemical characteristics
of the soil and environmental complex of the locality (Bossio et al.,
2005). Higher counts of bacterial and fungal population in Harmutty may
be attributed to the dense growth of plants and greater availability of
nutrients on account of greater accumulation of litter and may also be
due to spreading of other biodegradable domestic wastes into the system
by the traditional farmers. Contrarily, low microbial population in Nirjuli
and Doimukh may be because addition of plant remains and household waste
in these sites was comparatively poor that also resulted in low organic
matter content in the soil. Many workers have also recorded a correlation
between fungal species composition and the species composition of the
aboveground vegetation (Chung et al., 2007). Ground floor of Harmutty
site of present study was also dominated by fern species.
Over all, the present study concludes that the population counts of bacteria
and fungi in the soils of traditional agroforestry is influenced by vegetation,
density and species composition. However, the role of macro and micro-climatic
seasonality and soil nutrient status can not be completely ruled out.
It is also understood that the quality of plant residues accumulating
in these homestead agroforestry are furthermore important and may play
a vital role in soil nutrient management within the system through microbial
We thank the villagers of Harmutty, Nirjuli and Doimukh for their cooperation
during the field study. The authors are also thankful to Mr. Moharam Ingti,
Laboratory Assistant for helping in the laboratory studies. This study
is part of a research project funded by the Indian Council of Agricultural
Research (ICAR), New Delhi. The first author also thanks the Council of
Scientific and Industrial Research (CSIR), New Delhi for awarding Junior
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