Temporal variation ofmicrobial biomass C and NThelowest levels of microbial biomass during the rainy season, in this study, maybe attributed to the waterlogged conditions present during the rainy season. Therice fields are maintained as waterlogged systems during the vegetative stageof crop growth for both the rainy and the winter crop. This flooded soildevelops a distinct thin oxidized layer of a few millimeters in thickness atthe water soil interface (Gaunt et al 1995). During the rainy season, at thevegetative stage, conditions were suitable for the proliferation of less oxygenrequiring microorganisms. According to Gaunt et al (1995) the oxidized regionrepresents a zone of positive redox potential while the reduced region has lownegative redox potential.

The water phase only supports 10-5 timesslower diffusion of oxygen than the air phase which is insufficient to meet theoxygen demand for the growing microorganisms. Saturation of water in soil leadsto reduced oxygen levels, creating an environment for the growth of suchmicroorganisms that can proliferate in oxygen deficient environments only (Kimura,2000). Due to waterlogged condition in rice field during rainy season, level ofthe microbial biomass was lower initially. As the rice crop reached maturitystage, the waterlogged condition was replaced by relatively drier soilcondition which supports the aerobic environment. During drying, the portion of microbial biomass killedis utilized by the surviving microbial biomass, that uses the cellular debris; thusmicrobes could assimilate the available nutrients leading to immobilization andincrease in the microbial biomass during the winter season (Kandeler et al2006). The microorganisms that have adapted themselves to the anaerobicconditions were maintained till the winter season of rice cultivation (Kimura, 2000).Moreover during the winter months, comparatively drier conditions with optimummoisture prevailed and water logged conditions were no longer present for the restof the growing season. This in turn led to the proliferation of aerobicmicrobes, as compared to the rainy season, that led to the overall increment inthe levels of microbial biomass C and N during the winter.

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The release of thenutrients from the rice residues through decomposition after the rainy seasoncrop might have stimulated the microbial biomass C and N in the subsequentcropping season (Singh et al 2007). In the present study, higher levels of themicrobial biomass during winter were probably due to the increased rhizosphericC inputs to the soil. Withinthe crop cycle, increase in the microbial biomass levels from the vegetative tothe maturity stage was also attributed to the availability of nutrientsthroughout the growing season. Franzleubbers et al. (1995) have showed highermicrobial biomass carbon from vegetative to the maturity stage with increased Cdeposits in the rhizosphere, before and during the maturity stage of the crop.Ghoshal and Singh (1995) have stated the possible role of root decompositionprior to harvest for the contribution of nutrients to the enlarging microbialbiomass in a tropical dryland agroecosystem and have reported increase inmicrobial biomass from the vegetative to the maturity stage with a decrease inthe grain forming stage.

Ma et al. (2015) have reported the lowest microbialbiomass during the wettest season in a study at the northeastern China. Ourresults were in accordance with the above report where the lower microbialbiomass was recorded during the rainy and higher during the winter seasons. Duringthe summer season, the clayey Gangetic alluvial soil of the area getscompletely desiccated fragmented and cracks appear which helps the sunlight to reachto greater depths within the soil. The cracks in the soil also widen withincreasing temperature to greater depths within the soil profile, resulting inthe death of most of the microbes (Tripathi etal.

, 2007), that explained the lowest levels of soil microbial biomassduring summer season in the present study. The lowest microbial biomass wasreported by Ma et al (2015) to be in the driest soils. Our study also showedthat the microbial biomass was lowest during the summer which was the driest.Impact of crop rotationon Microbial biomass C and NThepractice of crop rotation has been reported to have both, a stimulatory as wellas an inhibitory effect on microbial biomass depending on the crop used in theliterature. In a study by Chen et al (2015) on rice rotations with uplandcrops, rice fallow rotation recorded lower levels of the microbial biomasscarbon than when it is cropped with potato and rye grass in China. In thepresent study, efficient nutrient management in rotational rice-wheat croppingsystems have lead to a buildup of microbial biomass C and N over time ascompared to rice fallow rotations. Our results are in accordance with the factthat greater diversity in rice-wheat crop rotation has stimulated the rhizosphericmicroorganisms to assimilate a wide range of nutrients.

This has led to anincrease in microbial biomass in rice-wheat rotation where greater microbialactivity in terms of higher microbial biomass carbon and nitrogen is measuredcompared to monocultures and rice-fallow rotations. In the rice-ricemonocultures, rapid mineralization of the available nutrients has led to alower microbial biomass. Similar reports are available from Tiemann et al., (2015), who have shown soil microbialbiomass C to increase 33% higher in rotational sequences than in monocultures.

McDanielet al., (2014) have reported anaverage increase of 21% microbial biomass in a meta-analysis and increasedmicrobial activities with a diverse plant input into the soil. Contrary to ourfindings, Balota et al. (2003) havereported the inclusion of soybean in a crop rotation that resulted in adecrease of the microbial biomass carbon in the 0-5 and 5-10 cm depth in the tropicalsoils of southern Brazil. Our results show increased microbial biomass N inboth rice-rice and rice wheat rotations (although not significant) on additionof easily available nutrients in the form of fertilizer before sowing, duringthe winter season.

Ocio et al. (1991)pointed out that the assimilation of the available nutrients usually contributesto large increases in microbial biomass which may be sustained for aconsiderable period of time. Growth of microorganisms can be regulated directlyby plant growth.

Greater plant diversity increases microbial growth in soil. Inthe present study also, rice-wheat crop rotation strategy has resulted inincreased microbial biomass carbon and nitrogen by virtue of greater plantdiversity.Role of cultivation inthe levels of microbial biomass C and NThehighest levels of microbial biomass C and N in the grassland soil compared toother cropping sequences were probably due to a higher return of plant inputsto soil. Unlike that of the cultivated plots where a major portion of crops areharvested out of the ecosystems, plant biomass remained within the system andhence returned to the soil (Ren et al 2018). Higher levels of root debris,exudates along with a higher amount of diverse litter, root biomass hasresulted in an increase in the levels of microbial biomass C and N in grasslandcompared to agricultural cropping systems (Nguyen and Marschner, 2017).

Ourresults are in accordance with Accoe etal. (2002), who have reported greater microbial biomass in continuousgrasslands than in maize cropping. A higher microbial biomass C and biomass Nhave 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 microbialbiomass C and microbial biomass N in our present study.

In our study, decreasein the levels of microbial biomass C and N from grassland to cropland are inaccordance with a lot of available reports (Gupta and Germida 1988; Smith andPaul 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 andgreater vegetation cover in the grassland soil in comparison to agroecosystems.In the present study, a higher level of microbial biomass in grassland was dueto a greater return of plant inputs into the soil, in form of litter.Impact of crop rotationon Microbial Biomass C: N ratioTheratio of microbial biomass carbon and nitrogen are used to describe thestructure as well as the state of microbial communities in soil.

A higher ratiois indicative of a greater proportion of fungi whereas bacterial predominanceis suggested in lower values (Campbell etal., 1991). The ratio of microbial biomass C: N is often considered as arobust indicator of productivity in rice fields (Li et al 2015). A widermicrobial biomass C: N ratio indicates the buildup of the fungal communitiesrelative to that of bacterial populations, thus giving an estimate of ecosystemrecovery by a greater retention of soil nutrients (Arunachalam and Pandey,2003). The fact that fungi-dominant soils have a high soil microbial biomassC: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 between4:1-8:1 (Paul and Eldor 2007). Anderson and Domsch, (1980) have reported theaverage value to be 6.7 Joergensen (1995) gave ratio values ranging from 5.2 inagricultural soil to 20.

8 in forest soil after a study comprising of 82samples. These ratios are often used as a reasonable indicator of ecosystemrecovery, lower the value, quicker is the build-up of microbial population. Inthe present study, the annual mean of the ratio was maximum in grassland andminimum in rice-rice rotation (figure 3). Although the difference in the ratiowas not significant, yet it gives an insight into the readily available labilepool of soil organic matter which the microbes use for their growth andproliferation.  Impact of crop rotationon microbial quotientTherole of microbial biomass as a fraction of organic carbon pool is well documentedin literature. The microbial biomass presented as a percentage of soil organiccarbon is generally given by many, as a reasonable estimate of the quantity ofcarbon already incorporated in the microbial cellular system, in turnindicating substrate availability and organic matter dynamics.

The higher theratio the greater is the quality of easily decomposable organic matter comparedto the passive pool. Our results are in accordance with Saviozzi et al. (2001) who have reported a highermicrobial biomass C/ organic carbon in cultivated soils in comparison with thatof the grassland by virtue of a higher return of maize stock (crop) residueinto the soil. Moore et al., (2000)have reported that multi cropping systems show higher values than mono croppingones with an average of about 2- 3%.

Fauci and Dick (1994) have reported anincrease of 1-7% in microbial quotient N. Srivastava and Singh (1989) gave arange of 1.9-3.3% for the microbial quotient C in cropland soils of thetropical drylands.

Similar variations in the range of 1-6% was observed formicrobial quotient N. Cropping systems having similar properties show higherproportion of decomposable carbon compared to stable humus when the ratios arehigher (Anderson and Domsch, 1989). In our study, rice-rice and rice-wheat croprotations, having higher ratios indicated easily available carbon compared torice-fallow rotation which in turn will help in buildup of soil organic matter.ConclusionCroprotations have a significant impact on the levels of soil microbial biomass. Thesoil microbial biomass C and N changes among all the rice based croppingsystems and through the various crop growth seasons. The transformation oforganic matter in soil along with the mineralization of nutrients regulatingplant productivity is regulated primarily by the soil microbial biomass.Grassland soil recorded higher microbial biomass C and N than agroecosystems.

Rice-fallowcrop rotation system had the least values of both microbial biomass C and N. Moisturecontent of the soil also played a significant role in the regulation of theseasonal variation in the levels of soil microbial biomass. On the basis ofhigher microbial biomass, rice-wheat crop rotation strategy may be recommendedin the proper management and improved crop productivity such that long termsustainability and soil fertility is achieved in humid tropical conditions ofsouth Bengal.