Microbial (through immobilization) of various nutrients. Jenkinson and

Microbial
biomass is the most labile pool of the soil organic matter and regulates
nutrient availability in soils. The soil microbial biomass is described as the
living portion of soil organic matter without the live root fractions and
organisms larger than 5 x 10-15m3. Bacteria, fungi,
antinomycetes, rotifers, and protozoa are the major soil organisms that constitute
the soil microbial biomass. It has been recognized both as a transforming agent
of the added and native organic matter, as a source (through mineralization)
and a sink (through immobilization) of various nutrients. Jenkinson and Ladd
(1981) thus correctly compares the microbial biomass with the “eye of needle”
through which all the dead nutrients had to pass in the process of their
breakdown to simple inorganic salts, as the mineralization of the key elements
have kinetics which require microbial biomass as a reactive agent.

The
microbial biomass constitutes the active fraction of soil organic matter, with
a rapid turnover rate of 1-5 years, in the Century model by Parton et al. (1987). The CENTURY model
visualizes five organic matter pools in soil of which three represents the soil
organic matter fractions a) an active fraction (1-5 year turnover), b) slow
fraction composed of weakly associated carbon and refractory materials (20-40
year turnover) and c) a passive fraction with a mean residence time of 200-1500
years. The Roth C Model as proposed by Coleman and Jenkinson (1999) also
considers a microbial biomass pool with a mean residence time of 1.5 years. The
active fraction in the Century model and the labile microbial biomass pool of
Roth C model represents a dynamic portion of the organic matter pool.

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The
soil microbial biomass has been used as an index of soil fertility, any
increase in its level is considered as an improvement in soil health. Due to
rapid turnover rate, the soil microbial biomass is sensitive to variation in
cropping systems, giving an early indication of changes in soil fertility and
productivity (McDaniel and Grandy, 2016).

The
naturally occurring grasslands are often converted to agricultural lands, for
the purpose of food and fodder. The changes that occur as a consequence of this
conversion from grassland to cropland have been reported to affect the dynamics
of the soil microbial biomass carbon and nitrogen (Yang et al 2008; Tripathi
and Singh, 2009). Chen et al (2010) has reported a decline of 53% in the levels
of microbial biomass C and 72% in the levels of microbial biomass N, in a 50
year old field after cultivation compared to native grassland.

In
agricultural systems, the crops that are grown have a potential to affect the soil
microbial biomass carbon and nitrogen (Curl and Truelove, 1986). Generally,
studies regarding microbes and their role in soil fertility have been limited
to the rhizospheric soil and only a few studies have documented the effect of
crop rotations on changes  in microbial
biomass dynamics in the bulk soil, which better reflect the potential effects on
subsequent crops in the long term scenario (Larkin 2003). Larkin and Honeycutt
(2006) have also proposed that the pattern of cropping systems were responsible
for distinct differences in the soil microbial biomass.

Microbial
biomass C and N when expressed as a percentage of organic carbon and total
nitrogen gives the microbial quotient of a particular ecosystem. This parameter
gives an insight on the quantities of nutrients available for the microbial
biomass and their dynamics in soil (Sparling, 1992). The quantity of soil
organic C and N present in the microbial biomass gives a reflection into the
quantum of C and N immobilized into the microbial standing crop (Anderson &
Domsch 1989). The linkage and interaction between the soil organic carbon and
the microbial biomass (Insam & Domsch 1988) also serves as an index for the
monitoring of soil management practices (Smith & Paul 1990).

The
changes in microbial biomass with varying temperature and moisture levels
provide useful information regarding the relationship between biomass carbon
and nitrogen as well as their impacts on the cycling and availability of carbon
and nitrogen in soil. Studies in the past by Lynch and Panting 1980, Biederbeck
et al. 1984, McGill et al. 1986; have reported the seasonal variations of
microbial biomass carbon and nitrogen in different management regimes.
Microbial biomass carbon and nitrogen are reported to be influenced by the
changes in season, through the variations in microclimate (Behera and Sahani,
2003). The changes in the soil microbial biomass with season, are crucial in
controlling the turnover of essential nutrients like carbon (C), nitrogen (N),
phosphorus (P), and sulphur (S), in turn, regulating nutrient availability for
plant uptake (He et al., 1997; Chen et
al., 2003).

A
large amount of literature is available on the dynamics of microbial biomass
carbon and nitrogen in the dry tropical condition (Singh et al 2007, Singh et
al 2016, Singh and Ghoshal 2014). But however, little information is available
with regard to microbial biomass carbon and nitrogen for the moist humid
conditions of eastern India (Tripathi et al 2012). Moreover, reports on the
effect of crop rotation on soil microbial biomass at different stages of the
crop cycle in this region are scanty. So this chapter aims at investigating the
dynamics of soil microbial biomass C and N in different rice based cropping
systems in a moist tropical humid condition. The specific objectives included
are the measurement of: (i) the temporal variations in microbial biomass carbon
and nitrogen levels, (ii) the effect of crop rotation on the levels of
microbial biomass carbon and nitrogen, (iii) the effect of conversion of a
grassland to cropland on the levels of microbial biomass carbon and nitrogen,
(iv) the effect of crop rotation on the microbial biomass C: N ratio, and (v) the
effect of crop rotation on the microbial quotient.

 

MATERIAL AND METHODS

Soil sampling and their
analysis

Soil
samples were collected at the seeding and the maturity stages in both the rainy
and the winter crop cycle and also once at the summer fallow from 0-10 cm soil
depth using soil corer having diameter 3 cm and height 10 cm. In total 5
samples were taken during the annual cycle. The soil was sampled randomly from
5 spots in each replicate (4 replicates per site), mixed and sieved through a 2
mm mesh screen. The visible plant debris was removed.

The
microbial biomass carbon and nitrogen were estimated by the chloroform
fumigation extraction method (Vance et al 1987, Brookes et al 1985). The field
moist samples were pre-conditioned by spreading the soil in a thin layer over a
sheet of polythene. The soil moisture was adjusted to 40% of the field water
holding capacity by spraying double distilled water uniformly over the soil
which was then covered with polythene overnight. The soil was transferred in
polythene bags and kept for 7 days at room temperature (250C) in an
airtight container with 100% humidity (20 ml distilled water in vial) and 20 ml
of 10% alkali taken in another vial. The role of alkali was to remove the CO2
released by the microorganisms. The container was opened for 30 minutes
every day for aeration. After 7 days, 25 g of the preconditioned soil samples
were taken in two conical flasks. One was saturated with 10-15 ml of purified liquid
chloroform, in a 500 ml conical flask, kept overnight for the fumigation
process to kill the microbes and treated as fumigated soil. The other flask was
left as such and treated as the non-fumigated soil.  Both the non-fumigated and the fumigated
samples were extracted with 100 ml K2SO4, shaked for 30
minutes on an oscillating shaker at 200 r.p.m. and filtered through Whatman no.
1 filter paper. The filtrate solution was used for estimation of microbial
biomass C and N by chloroform fumigation-extraction method (Vance et al. 1987, Brookes et al 1985).

For
the measurement of soil microbial biomass carbon, 2 ml of 0.4(N) K2Cr2O7
& 15 ml of acid mixture (H2SO4 : H3PO4
:: 2:1) were added to 8 ml of the filtered extract in a 250 ml of round bottom
flask fitted with a Liebig condenser. The whole mixture was gently refluxed for
30 minutes cooled and diluted with 25 ml distilled water which was added
through the condenser as a rinse. The residual dichromate was measured by a
back titration with 0.4 (N) FAS using phenanthraline indicator. At the end
point, a sharp colour change from green to buff pink took place. Soil microbial
biomass carbon was estimated using the equation: MBC= 2.64 EC is the
difference between organic C extracted from the K2SO4
extracts of non- fumigated and fumigated soils.

The
microbial biomass nitrogen was estimated by the method described by Brookes et al. (1985). 50 ml of each of the
fumigated and the non-fumigated extract were digested separately with 5g K2SO4,
0.5g CuSO4 and 15 ml conc. H2SO4 taken in digestion
tube in an automated Kelplus N analyzer (Pelican equipments). After digestion,
the samples were cooled and a mixed indicator (methyl red, methylene blue and
ethanol) was added (2 drops), and then distillation was performed in the N
analyzer with 40% sodium hydroxide and 2% boric acid.. The distilled solution
was titrated against 0.1 (N) HCl. The microbial biomass N was estimated using
the equation: MBN=En/0.54, where En is the difference between amount of N
extracted from the K2SO4 extract of fumigated and
non-fumigated soil and 0.54 is the fraction of biomass N extracted after
chloroform fumigation.

Microbial quotient (qCO2)
was calculated as the ratio of MBC to TOC and MBN to TON (Anderson and Domsch
1989) and expressed as ?g of biomass C ?g total organic C-1. The
microbial biomass C: N ratio was calculated by dividing MBC and MBN at
different crop sequences and cropping seasons throughout the annual cycle. The
process of determination of organic C and total N are described in chapter 5.

Result

Variations in soil
Microbial biomass Carbon

The
levels of soil microbial biomass varied considerably among all the crop
sequences and grassland and also through the two crop cycles and summer fallow,
and ranged from 490.5 µgg-1 to 224 µgg-1 and from 96.86 µgg-1
to 38.33 µgg-1 in case of soil microbial biomass C and N
respectively. The levels of soil microbial biomass carbon were the least during
the summer season and highest during the winter season crop periods for all the
cultivated agroecosystems, whereas the levels were higher during the rainy
season in case of grassland (Fig 1, table 1). Within both the crop cycles i.e.
the rainy season and the winter season crop cycles, the levels of soil
microbial biomass C and N increased from the vegetative to the crop maturity stage
in all the crop sequences, except the grassland, where the levels decreased
slightly from the vegetative to the maturity stage of the winter crop season.

The
level of soil microbial biomass carbon was highest in the grassland and was
followed in decreasing order by the rice-wheat, the rice-rice and the
rice-fallow crop rotation sequences during the vegetative and the maturity
stages of both the rainy season and the winter season crop period, and also
during the summer fallow. However the level in grassland was slightly lower as
compared to rice-wheat rotation only during the maturity stage of the winter
crop. The mean annual level of soil microbial biomass carbon was highest in
grassland (464.26µgg-1) which decreased significantly due to
cultivation. Among the cropping sequences, the rice-wheat (400.6 µgg-1)
was significantly higher than rice-rice (360.86 µgg-1), whereas
rice-fallow (292.2 µgg-1) was significantly lower than rice-wheat
and rice-rice crop rotations.

Variations in soil Microbial
Biomass Nitrogen

The
levels of soil microbial biomass nitrogen were the highest during the rainy
season crop periods for all the cultivated agroecosystems as well as the
grassland (Fig 2, table 2). The levels decreased in the winter season and the
least was recorded during the summer season. The level of soil microbial
biomass nitrogen was highest in the grassland and was followed in decreasing
order by the rice-wheat, the rice-rice and the rice-fallow crop rotation
sequences during the vegetative and the maturity stages of both the rainy
season and the winter season crop period, and also during the summer fallow.
The mean annual level of soil microbial biomass nitrogen was highest in
grassland (84.63µgg-1) which decreased significantly due to
cultivation. Among the cropping sequences, the rice-wheat (69.82 µgg-1)
was significantly higher than the rice-rice (63.42 µgg-1), whereas
rice-fallow (52.36 µgg-1) was significantly lower than rice-wheat
and rice-rice crop rotations.

One
way ANOVA of the data for the annual cycle showed the impact of crop rotation
and crop stages on soil microbial biomass carbon and nitrogen. Among the rice
based cropping systems, the level of microbial biomass carbon and nitrogen
significantly increased with the crop rotation compared to grassland. Thus crop
rotation of rice-wheat significantly increased microbial biomass C and N,
following in the decreasing order by rice-rice monocultures and the least in
rice-fallow rotations throughout the annual cycle.

Variations in Microbial
biomass C: N

The
levels of microbial biomass C: N increased from the rainy season crop to the winter
season crop in all the agroecosystems, except grassland, where the levels
slightly decreased in the winter season (5.36 to 5.24), in comparison to the
rainy season. The levels of microbial biomass C: N increased from 4 in the
rainy season to 5.62 in the winter season in rice-fallow rotation, 4.53 to 4.79
in the rice-rice rotation and 4.97 to 5.61 in the rice-wheat rotation. The
levels were maximum in the summer season; 6.31 in the grassland, 6.03 in the rice-fallow
rotation, 5.66 in the rice-rice rotation and 5.93 in the rice-wheat rotation.
The annual mean level was maximum (5.50) in the grassland. Among the rice based
cropping sequences, rice-wheat rotation recorded the maximum value (5.42) followed
in decreasing order by rice-fallow rotation (5.05) and the least in the rice-rice
(4.86) crop rotation (figure 3).

Microbial quotient

The
microbial quotient (MBC/OC) and (MBN/TN) varied significantly among the
cropping systems (table 3). The mean annual level of microbial quotient carbon was
observed maximum in the rice-rice (3.73%) cropping system, followed in decreasing
order in rice-wheat (3.17%) rotation, grassland (2.94%) and the least in the rice-fallow
(2.86%) rotation. The microbial quotient nitrogen was maximum in the rice-rice
(7.99%) rotation, following a decreasing trend in the rice-wheat rotation
(6.06), rice-fallow rotation (6.0) and the least in the grassland (5.38).

Statistical
analyses:

Assigning the soil and crop stages as treatment factors analysis of
variance (ANOVA) was carried out using SPSS 16.0 statistical package. The
factor soil had four levels and the crop stages had 5 levels also. The
replicate had four levels. The least significant difference (LSD) was applied
to evaluate the significance of differences between individual treatment
factors. The treatment means were compared by Duncan’s multiple range tests
(DMRT) at 0.05p.