Temporal variation of
microbial biomass C and N

The
lowest levels of microbial biomass during the rainy season, in this study, may
be attributed to the waterlogged conditions present during the rainy season. 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 distinct thin oxidized layer of a few millimeters in thickness at
the water soil interface (Gaunt et al 1995). During the rainy season, at the
vegetative stage, conditions were suitable for the proliferation of less oxygen
requiring microorganisms. 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. 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). Due to waterlogged condition in rice field during rainy season, level of
the microbial biomass was lower initially. As the rice crop reached maturity
stage, the waterlogged condition was replaced by relatively drier soil
condition which supports the aerobic environment. During drying, the portion of microbial biomass killed
is utilized by the surviving microbial biomass, that uses the cellular debris; thus
microbes could assimilate the available nutrients leading to immobilization and
increase in the microbial biomass during the winter season (Kandeler et al
2006). The microorganisms that have adapted themselves to the anaerobic
conditions were maintained till the winter season of rice cultivation (Kimura, 2000).
Moreover during the winter months, comparatively drier conditions with optimum
moisture prevailed and water logged conditions were no longer present for the rest
of the growing season. This in turn led to the proliferation of aerobic
microbes, as compared to the rainy season, that led to the overall increment in
the levels of microbial biomass C and N during the winter. The release of the
nutrients from the rice residues through decomposition after the rainy season
crop might have stimulated the microbial biomass C and N in the subsequent
cropping season (Singh et al 2007). In the present study, higher levels of the
microbial biomass during winter were probably due to the increased rhizospheric
C inputs to the soil.

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Within
the crop cycle, increase in the microbial biomass levels from the vegetative to
the maturity stage was also attributed to the availability of nutrients
throughout the growing season. Franzleubbers et al. (1995) have showed higher
microbial biomass carbon from vegetative to the maturity stage with increased C
deposits in the rhizosphere, before and during the maturity stage of the crop.
Ghoshal and Singh (1995) have stated the possible role of root decomposition
prior to harvest for the contribution of nutrients to the enlarging microbial
biomass in a tropical dryland agroecosystem and have reported increase in
microbial biomass from the vegetative to the maturity stage with a decrease in
the grain forming stage. Ma et al. (2015) have reported the lowest microbial
biomass during the wettest season in a study at the northeastern China. Our
results were in accordance with the above report where the lower microbial
biomass was recorded during the rainy and higher during the winter seasons.

During
the summer season, the clayey Gangetic alluvial soil of the area gets
completely desiccated fragmented and cracks appear which helps the sunlight to reach
to greater depths within the soil. The cracks in the soil also widen with
increasing temperature to greater depths within the soil profile, resulting in
the death of most of the microbes (Tripathi et
al., 2007), that explained the lowest levels of soil microbial biomass
during summer season in the present study. The lowest microbial biomass was
reported by Ma et al (2015) to be in the driest soils. Our study also showed
that the microbial biomass was lowest during the summer which was the driest.

Impact of crop rotation
on Microbial biomass C and N

The
practice of crop rotation has been reported to have both, a stimulatory as well
as an inhibitory effect on microbial biomass depending on the crop used in the
literature. In a study by Chen et al (2015) on rice rotations with upland
crops, rice fallow rotation recorded lower levels of the microbial biomass
carbon than when it is cropped with potato and rye grass in China. In the
present study, efficient nutrient management in rotational rice-wheat cropping
systems have lead to a buildup of microbial biomass C and N over time as
compared to rice fallow rotations. Our results are in accordance with the fact
that greater diversity in rice-wheat crop rotation has stimulated the rhizospheric
microorganisms to assimilate a wide range of nutrients. This has led to an
increase in microbial biomass in rice-wheat rotation where greater microbial
activity in terms of higher microbial biomass carbon and nitrogen is measured
compared to monocultures and rice-fallow rotations. In the rice-rice
monocultures, rapid mineralization of the available nutrients has led to a
lower microbial biomass. Similar reports are available from Tiemann et al., (2015), who have shown soil microbial
biomass C to increase 33% higher in rotational sequences than in monocultures. McDaniel
et al., (2014) have reported an
average increase of 21% microbial biomass in a meta-analysis and increased
microbial activities with a diverse plant input into the soil. Contrary to our
findings, Balota et al. (2003) have
reported the inclusion of soybean in a crop rotation that resulted in a
decrease of the microbial biomass carbon in the 0-5 and 5-10 cm depth in the tropical
soils of southern Brazil. Our results show increased microbial biomass N in
both rice-rice and rice wheat rotations (although not significant) on addition
of easily available nutrients in the form of fertilizer before sowing, during
the winter season. Ocio et al. (1991)
pointed out that the assimilation of the available nutrients usually contributes
to large increases in microbial biomass which may be sustained for a
considerable period of time. Growth of microorganisms can be regulated directly
by plant growth. Greater plant diversity increases microbial growth in soil. In
the present study also, rice-wheat crop rotation strategy has resulted in
increased microbial biomass carbon and nitrogen by virtue of greater plant
diversity.

Role of cultivation in
the levels of microbial biomass C and N

The
highest levels of microbial biomass C and N in the grassland soil compared to
other cropping sequences were probably due to a higher return of plant inputs
to soil. Unlike that of the cultivated plots where a major portion of crops are
harvested out of the ecosystems, plant biomass remained within the system and
hence returned to the soil (Ren et al 2018). Higher levels of root debris,
exudates along with a higher amount of diverse litter, root biomass has
resulted in an increase in the levels of microbial biomass C and N in grassland
compared to agricultural cropping systems (Nguyen and Marschner, 2017). Our
results are in accordance with Accoe et
al. (2002), who have reported greater microbial biomass in continuous
grasslands than in maize cropping. A higher microbial biomass C and biomass N
have been reported by Robertson et al.
(1993), in grasslands compared to crop systems. A higher nutrient availability,
coupled with greater crop residues has increased the levels of soil microbial
biomass C and microbial biomass N in our present study. In our study, decrease
in the levels of microbial biomass C and N from grassland to cropland are in
accordance with a lot of available reports (Gupta and Germida 1988; Smith and
Paul 1990; Groffman et al 1993). He et al (1997) along with Piao et al (2000)
have explained the reduction on the basis of more nutrient availability and
greater vegetation cover in the grassland soil in comparison to agroecosystems.
In the present study, a higher level of microbial biomass in grassland was due
to a greater return of plant inputs into the soil, in form of litter.

Impact of crop rotation
on Microbial Biomass C: N ratio

The
ratio of microbial biomass carbon and nitrogen are used to describe the
structure as well as the state of microbial communities in soil. A higher ratio
is indicative of a greater proportion of fungi whereas bacterial predominance
is suggested in lower values (Campbell et
al., 1991). The ratio of microbial biomass C: N is often considered as a
robust indicator of productivity in rice fields (Li et al 2015). A wider
microbial biomass C: N ratio indicates the buildup of the fungal communities
relative to that of bacterial populations, thus giving an estimate of ecosystem
recovery by a greater retention of soil nutrients (Arunachalam and Pandey,
2003). The fact that fungi-dominant soils have a high soil microbial biomass
C:N ratios is attributed to the fact that the C:N ratios in fungi is 8:1-29:1,
whereas in bacterial populations, the corresponding values range between
4:1-8:1 (Paul and Eldor 2007). Anderson and Domsch, (1980) have reported the
average value to be 6.7 Joergensen (1995) gave ratio values ranging from 5.2 in
agricultural soil to 20.8 in forest soil after a study comprising of 82
samples. These ratios are often used as a reasonable indicator of ecosystem
recovery, lower the value, quicker is the build-up of microbial population. In
the present study, the annual mean of the ratio was maximum in grassland and
minimum in rice-rice rotation (figure 3). Although the difference in the ratio
was not significant, yet it gives an insight into the readily available labile
pool of soil organic matter which the microbes use for their growth and
proliferation.

 

 

Impact of crop rotation
on microbial quotient

The
role of microbial biomass as a fraction of organic carbon pool is well documented
in literature. The microbial biomass presented as a percentage of soil organic
carbon is generally given by many, as a reasonable estimate of the quantity of
carbon already incorporated in the microbial cellular system, in turn
indicating substrate availability and organic matter dynamics. The higher the
ratio the greater is the quality of easily decomposable organic matter compared
to the passive pool. Our results are in accordance with Saviozzi et al. (2001) who have reported a higher
microbial biomass C/ organic carbon in cultivated soils in comparison with that
of the grassland by virtue of a higher return of maize stock (crop) residue
into the soil. Moore et al., (2000)
have reported that multi cropping systems show higher values than mono cropping
ones with an average of about 2- 3%. Fauci and Dick (1994) have reported an
increase of 1-7% in microbial quotient N. Srivastava and Singh (1989) gave a
range of 1.9-3.3% for the microbial quotient C in cropland soils of the
tropical drylands. Similar variations in the range of 1-6% was observed for
microbial quotient N. Cropping systems having similar properties show higher
proportion of decomposable carbon compared to stable humus when the ratios are
higher (Anderson and Domsch, 1989). In our study, rice-rice and rice-wheat crop
rotations, having higher ratios indicated easily available carbon compared to
rice-fallow rotation which in turn will help in buildup of soil organic matter.

Conclusion

Crop
rotations have a significant impact on the levels of soil microbial biomass. The
soil microbial biomass C and N changes among all the rice based cropping
systems and through the various crop growth seasons. The transformation of
organic matter in soil along with the mineralization of nutrients regulating
plant productivity is regulated primarily by the soil microbial biomass.
Grassland soil recorded higher microbial biomass C and N than agroecosystems. Rice-fallow
crop rotation system had the least values of both microbial biomass C and N. Moisture
content of the soil also played a significant role in the regulation of the
seasonal variation in the levels of soil microbial biomass. On the basis of
higher microbial biomass, rice-wheat crop rotation strategy may be recommended
in the proper management and improved crop productivity such that long term
sustainability and soil fertility is achieved in humid tropical conditions of
south Bengal.