Throughout integrated metabolic activity of soil microbes.

Throughout the globe, the primary focus has been
shifted on agricultural soils for the amelioration of an ever increasing carbon
dioxide levels in the environment. In agroecosystems, the management practices
undertaken, helps us to determine whether the soil will serve as a source for
atmospheric CO2 by virtue of a higher relative output than input or
a sink where the relative input is higher (Singh et al 2009). In this context,
the mineralization of soil organic matter along with the respiration of roots
and the soil microbes contributes a substantial amount of the net carbon
dioxide emission. The rate of soil CO2 efflux affects the amount of
C loss from the soil.

The measurement of the quantity of C loss as a
result of mineralization and cellular respiration is carried out by the process
of soil CO2 flux, which is essentially, the sum of autotrophic root
respiration along with the heterotrophic cellular respiration of the soil
organisms and microbes (Lloyd and Taylor, 1994). The latter process, also
called basal soil respiration is an in
vitro process under the temperature-specific controlled laboratory
conditions, where the plant roots and the visible soil fauna are sorted out of
the sample, thus giving an estimation of the microbial activity in soil. The in situ measurements of soil respiration,
on the other hand, gives us an estimate of the cumulative impact of CO2
release as well as the actual mineralisation rate from any ecosystem (Borken et
al 2002). Panikov, (2005) have considered basal soil respiration to be the
integrated metabolic activity of soil microbes. Estimation of the basal
respiration is necessary for the measurement of the amount of carbon released
into the atmosphere from the agricultural systems and gives an insight into the
functioning of the microbial biomass.

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Substrate induced respiration on the other hand measures
the ability of the microbes to quickly assimilate the available nutrients, and is
measured by adding predefined quantities of easily available substrates like
glucose in the soil. Anderson and Domsch, (1978) had given a conversion factor
of 40.04 to convert the rate of substrate induced soil respiration into the microbial
biomass. However Blagodatskyaya and Kuzyakov, (2013) are of opinion that the
substrate induced rates of respiration signifies the respiration of active and
potentially active microorganisms in soil, when not converted to the SIR
biomass. They have classified substrate induced respiration for two groups of
microbes, those capable of immediate growth on addition of an utilizable
substrate, and the one that switches from a dormant to active state in a short
span of time. Microbes respire at the initial SIR level for different lengths of
time, depending on the lag phase required for adjusting to the condition of the
soil (Nordgren et al., 1988), and then start to increase exponentially. This
growth continues until the carbon and other nutrients, limit further growth and
a stationary phase is reached.

The basal and the substrate induced
soil respiration, are often used as probable indicators for the study of
changes in soil structure as well as function (Bastida et al., 2008). Nutrient
cycling is considered to be an essential component, not only for the primary
production but the long-term ecosystem dynamics as well (Doran and Parkin,
1996). In addition, the soil microbial activities including basal soil
respiration and the substrate induced soil respiration, have been proved to be
powerful indicators of soil quality (Bending et al., 2004; Gaofei et al., 2010;
Bastida et al., 2008) as well as estimating biological processes in
agroecosystems. The soil microbial respiration moreover, is a sensitive
indicator of soil management practices in agricultural lands, the use of
fertilizers and the crop rotation (Gaofei et al., 2010; Bending et al., 2004).

A large volume of literature is
present on the role of basal soil respiration and the substrate induced
respiration, on the dynamics of microbial biomass in the dry tropical region.
However, the data related to the moist humid tropical conditions of eastern
India is scarce and not properly documented, specially the effect of crop
rotation at different growth stages of the kharif (rainy) and rabi (winter) crop
cycle have not been studied till date, as evident from the lack of available
reports (Tripathi et al 2012). Farmers in this area generally rely on the
traditional rice based cropping systems, those evolved through years of
experience. So this chapter aims at investigating the role of basal soil
respiration and substrate induced respiration in different rice based cropping
systems in a moist tropical humid condition. The specific objectives included
are the study of the following involving grassland and various rice-based crop
sequence, (i) the effect of crop rotation on the levels of basal and substrate
induced soil respiration and (ii) the temporal variations in basal and
substrate induced soil respiration levels.

2.
Material and methods

Soil
sampling and processing

Basal soil respiration and
substrate induced respiration were estimated at the vegetative and the maturity
stages in each of rainy and winter crop cycle and also once at summer fallow.
In total 5 samplings were done during the annual cycle. Soil samples were collected
from 0-15 cm depth in completely randomized design from 5 spots per replicate
(4 replicates per site), mixed and sieved through a 2mm mesh screen. Visible
plant debris was removed and the field moist samples were preconditioned by
spreading the soil in a thin layer over a sheet of polythene.

Measurement of Basal Soil
Respiration

 

                The
basal
soil respiration was measured by the method described by Alef (1995). Three
portions of 10 g field moist soil were taken in three separate incubation
flasks and to this was added 1 ml of water. 5 ml of 0.1 M NaOH solution was
pipetted into a glass vial and the same was kept hanging into the flask with
the help of a metal wire, and the setup was made airtight with the help of
stopper. The same procedure was followed for blank which without soil. The
samples as well as blank were incubated at 22oC for 24 hours in
incubator. The vial containing NaOH solution was taken out of the flask after
24 hours, and the contents were quantitatively transferred to a 50 ml conical
flask with 10 ml distilled water. 5 ml of BaCl2 solution and 3 drops
of phenolphthalein indicator solution were added to the NaOH solution, which
was then titrated with 0.05 M HCl until the colour changed from pink to colourless.

Calculation

Basal
Soil Respiration (?g CO2 -C g-1 oven dry soil h-1
at 22oC) was calculated by using the equation:-                                                 

where, Vo is volume of HCl used for
titration of blank, V is volume of HCl used for titration of sample, S is the
strength of the HCl in normality, M is the weight of soil (g), dwt is the oven
dry weight of 1 g sample, the molecular weight and atomic weight of CO2
and C are 44 and 12 respectively, the equivalent weight of CO2 is 22,
t is the time of incubation in hour.

Measurement of substrate
induced respiration

The substrate induced soil respiration
was measured by the method described by Alef (1995). Three portions of 10 g field
moist soil were taken in three separate incubation flasks and to this was added
1 ml of 5% glucose solution. 5 ml of 0.1 M NaOH solution was pipetted into a glass
vial and the same was kept hanging into the flask with the help of a metal wire,
and the setup was made airtight with the help of stopper. The same procedure
was followed for blank which without soil. The samples as well as blank were
incubated at 22oC for 5 hours. After 5 hours incubation, the vial
containing NaOH solution was taken out of the flask and the contents were quantitatively
transferred to a 50 ml conical flask with 10 ml distilled water. 5 ml of BaCl2
solution and 3 drops of phenolphthalein indicator solution were added to NaOH
solution, quickly without any time delay, which was then titrated with 0.05 M
HCl until the colour changed from pink to colourless.

Substrate Induced Soil Respiration (?g CO2 -C g-1
oven dry soil h-1 at 22oC) was calculated by using the
following equation:-                                                  

where, Vo is volume of HCl used for
titration of blank, V is volume of HCl used for titration of sample, S is the
strength of the HCl in normality, M is the weight of soil (g), dwt is the oven
dry weight of 1 g sample, the molecular weight and atomic weight of CO2
and C are 44 and 12 respectively, the equivalent weight of CO2 is 22,
t is the time of incubation in hour.

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.

3.
Results:

Basal
Soil Respiration

The levels of basal soil respiration
varied significantly among all the cropping sequences and grassland and also
through the two crop cycle and summer fallow and ranged from 2.20 to 5.71 ?g CO2 -C g-1h-1.
The levels of basal soil respiration were minimum in the
rainy season, with increase in the winter season, finally peaking at the summer
fallow stage for all the ecosystems including the grassland (Fig 1). Within
both the crop cycles i.e. rainy season and the winter season crop cycles, the
levels of basal soil respiration increased from the vegetative to the maturity
stage in all the crop sequences. The level of basal soil respiration was
highest in the grassland and was followed in the decreasing order by the
rice-wheat rotation, the rice-rice rotation and rice-fallow crop sequence
during the vegetative and maturity stage of both the rainy season and the
winter and the winter season crop period, and also during the summer fallow.
The mean annual level of basal soil respiration was highest in the grassland
(5.05 ?g CO2 -C g-1h-1)
which decreased significantly due to cultivation.   Among the cropping sequences rice-wheat
(3.94 ?g CO2 -C g-1h-1) was comparable to the
rice-rice (3.72 ?g CO2 -C g-1h-1) rotation,
whereas the rice-fallow (2.62 ?g CO2 -C g-1h-1)
rotation was significantly lower than both the rice-wheat and the rice-rice
crop rotational sequence (table 1).

Substrate induced respiration

The levels of
substrate induced respiration followed a similar trend to that of the basal
soil respiration with levels being recorded the minimum during the rainy
season, increase in the winter, while the maximum was recorded in the summer
season. Within both the crop cycles also, the levels of substrate induced
respiration followed the same trend as that of the basal soil respiration with
an increase from the vegetative to the maturity stage (fig 2). The
mean annual level of the substrate induced soil respiration was found maximum
in the grassland (18.95 ?g CO2
-C g-1h-1 at 22oC) (table 2) which decreased
significantly due to cultivation. Among the cropping sequences the rice-wheat
crop rotation (15.83 ?g
CO2 -C g-1h-1 at 22oC) was
significantly higher than the rice-rice crop rotation (13.10 ?g CO2
-C g-1h-1 at 22oC) and the rice-fallow
rotation (9.94 ?g CO2 -C g-1h-1 at 22oC)
was significantly lower than the rice-wheat and rice-rice rotation. The levels
of substrate induced respiration were on an average 3-4 times higher than the
basal soil respiration for all the cropping sequences including the grassland.

4. Discussion

4.2 Impact
of crop rotation on basal respiration

Soil respiration is the process by which fixed
carbon (photosynthesis) is released in the atmosphere and has two principal
contributors; the plant root respiration and microbial counterpart (Anderson
and Domsch, 1985). Basal soil respiration or the one without any form of
substrate addition depends mainly on the physiological state of the microbes as
well as their maintenance requirements (Blagodatskaya and Kuzyakov, 2013). Rong
et al., (2015) have reported that perennial pasture has greater soil respiration
than croplands in a study of the major land-use types in agro-pastoral region
of northern China. In our study, the trends of the basal soil respiration of
the cropping systems were similar to the microbial biomass content for the
rainy and the winter season. Ananyeva et al (2008) have reported decreased
basal respiration in arable soils as a result of dramatic reduction in
microbial activity, compared to their natural analogues. However, in summer,
the basal soil respiration significantly increased across all the cropping
systems. In dry summer, due to acute water shortage, the energy is channelled
for cell maintenance in place of biomass growth, which in turn increased the
basal respiration level. Soil moisture is responsible for the variability in
soil respiration (Gupta and Singh, 1981). Nannipieri et al., (1990) have
reported that the available C in soil has a larger impact on the levels of
basal soil respiration than the microbial biomass. The microbial biomass in the
soils of natural ecosystems can exist in dormant, less active forms. It might
be the basal respiration of dormant populations that has acquired the ability
to overcome the harsh environmental conditions caused by water and nutrient
shortage (Ohya et al., 1988). In our study, among the rice based cropping
sequences, the rice-wheat crop rotation showed the maximum level of basal
respiration in summer which indicated that the population of microorganisms are
in less active form, where maintenance of biomass is the primary requisite as
opposed to the cell growth and division (Alexander, 1977).

Impact
of crop rotation on substrate induced respiration

The potential respirometric activity of
the microorganisms in soil is the most widely used parameter for the determination
of microbial activity in soil (Ananyeva et al 2008). Blagodatskaya and kuzyakov
(2013) have defined substrate induced respiration as the activity of
glucose-responsive microorganisms in soil. Paul and Clark, (1996) have
underlined the theory of respiration kinetics, where a source of labile C
increases the microbial SIR rate by three to four times the soil’s native CO2
release (Anderson and Domsch, 1978). In the present study also, the rate of
increase in substrate induced respiration over the basal soil respiration is
approximately 3-4 times in all the cropping systems (table 2). Tekley et al (2006)
have studied the substrate induced respiration in the tropical land use systems
of Ethiopia consisting of forests and agricultural farms. They have reported
higher substrate induced respiration in agricultural plots compared to forests,
although the cumulative percentage of respired C was higher in forests and
explained their findings on the basis of the availability of easily extractable
nutrients in the farmland for the microbiota along with a higher gluscose
responsive biomass was the reason behind such trend. Contrary to the above
observation, in this study, the substrate induced respiration was maximum in
grassland compared to the cropping systems. This may be attributed to the fact
that in grassland, due to leaf litter fall, mineralization of the available
nutrients takes place which in turn help in build up of the soil organic matter
(Singh et al 2006). This process helped the glucose responsive microbes to
respire at a higher rate in the grassland. The observation of rice wheat crop
rotation being the highest in substrate induced respiration among rice based
rotational strategy may be explained by the availability of different root
residues that has served as nutrient source on their decomposition. Rapid
growth of microorganisms take place in the presence of easily available C
source (Fontaine et al 2003).

Temporal
variations in basal and substrate induced respirations

The rice fields are maintained as
waterlogged systems during the vegetative stage of crop growth for both the
rainy and the winter crop. This flooded soil develops a thin oxidized layer of
a few millimeters at the water soil interface. According to Gaunt et al (1995)
the oxidized region represents a zone of positive redox potential while the
reduced region has low negative redox potential. The water phase only supports
10-5 times slower diffusion of oxygen than the air phase which is
insufficient to meet the oxygen demand for the growing microorganisms. This has
led to the least levels of basal and substrate induced respiration at the
vegetative stage of rice crop. The saturation of water in soil leads to reduced
oxygen levels, creating an environment for the growth of such microorganisms
that can proliferate in oxygen deficient environments only (Kimura, 2000),
decreasing the basal respiration rates. As the rice crop reached maturity
stage, the waterlogged condition was replaced by relatively drier soil
condition which supports the aerobic environment, thus increasing both the
basal and the substrate induced respirations.

During summer season maximum levels of
the basal and substrate induced respiration compared to the rainy and rabi
winter season crops, can be attributed to desiccation in the clayey Gangetic
alluvial soil (Tripathi et al 2007). Due to the disruption of the Gangetic
alluvial soil of this region in summer, a low residual microbial biomass in
moisture stressed condition have incorporated a large portion of their energy
in maintaining cellular functions and metabolic processes (respiration) leading
to higher CO2 levels, as active growth is not possible in such harsh
conditions. Our results are in accordance with Sugihara et al (2012) where
lower soil respiration is reported in the rainy season and higher in the dry
season. However in the dry tropical conditions, higher soil CO2 flux
was reported (Singh et al 2009). The relatively warm conditions along with
plenty of available moisture during the rainy season have resulted in the
decomposition of organic matter in dry tropics, in turn increasing soil
respiration to a maximum.

During winter, decrease in the basal
soil respiration rates was reported by Lou et al (2004) in soil. A trend of increase
in basal and substrate induced respiration in winter from the rainy season may
be attributed to the availability of nutrients from the decomposition of rice
residues of the rainy season crop.

Conclusion

Basal soil respiration and substrate induced
respiration changed significantly among all the rice based cropping sequences
and through the various crop growth stages. Grassland soil recorded maximum
basal and substrate induced respiration compared to agroecosystems. Rice-wheat
and rice-rice crop rotation showed an increase in the substrate induced
respiration compared to the rice fallow rotation, due to the addition of easily
available nutrients in the form of fertilizers. Further research is needed to
identify whether the fertilizer addition, directly alters the plant growth and the
microbial composition in soil. Thus crop rotation strategy will be helpful in
maintaining soil fertility and long term sustainability in humid tropical
conditions of south Bengal.  

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