1.3 Ecological assessment of floodplain characteristics
1.3.5 Soil microbial biomass characterization
The productivity of an ecosystem is governed directly by the nutrients present in a soil system, the process of humification and nutrient mineralization takes place in soil progressively (Nannipieri et al., 1996; Insam, 1996; Sinsabaugh et al., 1991, Caldwell, 2005). Among the active pool of nutrients present in the soil, the most important ones are Carbon, Nitrogen, Phosphorus and Sulphur. The biogeochemical cycles circulating these elements make the nutrients available to the living organisms through the producer plants. The labile components of soil, commonly the minerals are fixed and made available to the living organisms through degradation processes by the organisms
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existing in the soil mostly importantly bacteria, fungi and actinomycetes (Waldrop et al., 2000). Almost all ecological processes required to sustain life through dissimilative reactions to gain energy by decomposing organic matter and nutrient cycling are dependent on microorganisms. Biodegradation of organic matter in soil may refer to microbial transformation (Nannipieri, 1996). Nutrients in the soil are mineralized by microorganisms and a minor but significant quantity is retained by the microorganisms as microbial biomass (Figure 1.1). Microbial biomass manages the fertility of soil and help in tracing contaminants in soil (Hargreaves et al., 2003; Broos et al., 2007).
Around 2-10% of the nutrient is confined as microbial biomass (Kujur et al., 2012).
Microbial biomass has played an essential role in the field of agriculture, it functions as an indicator of nutrient content in the soil (Griffiths et al., 2012; Kujur et al., 2012). It has several purposes including stabilization of soil aggregates and acting indicator of pollutants in soil (Brookes, 2007).
Figure 1.1 Diagrammatic representation of the processes nutrient mineralization and formation of microbial biomass (adapted fromNagaoka et al., 2014)
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Soil microbial estimation is carried out by classical method of chloroform- fumigation-incubation (CFI) proposed by Jenkinson and Powlson (1976). Chloroform act as a fumigant and biocide releasing the cellular component of soil microorganisms not solubilize non-microbial soil organic matter (Vance et al., 1987). The fumigated soil is extracted with K2SO4 followed by treatment on an oscillating shaker for 30 min or 24 hours. The extract is filtered and analysed for CNP concentrations. The difference between the values of CNP in fumigated and non-fumigated soil is microbial biomass content. Vance and others (1987) demonstrated the deviance of values among variables and variance among a defined set of variables for microbial biomass estimation based on which a correction factor of 2.64 for microbial biomass carbon and 0.45 for microbial biomass nitrogen and phosphorus respectively. Other methods of soil microbial biomass estimation include microwave irradiation to destroy microbial cells in short time period and most of the researchers showed a positive approach towards microwave irradiation method comparable to CFI method (Hendricks et al. 1988; Islam et al., 1998). Soil microbial estimation is advantageous over soil microbial characterization because this method includes both the culturable and unculturable microorganism present in soil and releases the nutrient components of the entire soil microflora by decomposing the microbial organic matter only.
Researchers follow organic or inorganic amendment technologies in soil to determine the direct involvement of soil organic matter in increasing the soil microbial biomass aided by enzymatic activities (Goyal et al. 1999). Influence of inorganic fertilizers and organic amendments on soil organic matter and soil microbial properties
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under tropical conditions. Previous studies indicate that enzyme activities directly correlate to such amendments but the increase or decrease of soil microbial biomass in undefined (Albiach et al., 2000; Araújo et al., 2010). Brookes (2001) explained that microbial biomass turnover responds positively to fertilizer inputs in soil, the nutrients are taken up by the microorganisms, released gradually and taken up more efficiently by the crops in agricultural fields. Microbial biomass and significant rates of bacterial productivity in soil is low governed by the geochemical parameters (Trasar-Cepeda et al., 1997). Alongi (1988) suggested that high microbial biomass in Mangrove soils in Australia depends on the presence of bacterial consumers under tidal influence. In case of high bacterial growth followed by their consumption by protozoans, the nutrient (Carbon) level may be high in microbial biomass and hence less available for higher organisms. Microbial biomass depends on litter decomposition in a particular area, forest ecosystems logically have high soil microbial biomass followed by grasslands.
Grasslands studied in temperate region showed accumulation of soil microbial biomass in undisturbed high litter grasslands as compared to grassland having low metabolic quotients in terms of low litter diversity (Bardgett et al., 1999). Apart from organic amendment activities and influence of environmental parameters, major factor affecting the soil microbial biomass is the land use activities (Yao et al. 2000). Celik (2005) reports that forest and grassland soils in highlands of southern Mediterranean Turkey showed change in soil physical properties as result of crop management activities.
Tillage results in significant fluctuations in soil microbial biomass and soil productivity as the cultivation disintegrates the soil aggregates exposing the previously inaccessible organic matter to microbial attack and hence accelerates the decomposition and mineralization of organic matter in soil (Franzluebbers, 1999). Other factors as natural
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calamities like too influence soil microbial biomass (Jia et al., 2005). Forest fires are intended to increase the nutrient levels but the intensity of heat kills most of the microorganisms in the topsoil and somewhat subsurface soils imparting a sterilizing effect. Recurrence of microbial biomass depends on plant recolonisation (Certini 2005).
However Fu and others (2012) found that a short-term warming experiment in alpine meadow on the Tibetan Plateau had no obvious effect on the decrease or increase of soil microbial biomass. Natural process like forest succession in Europe showed that microbial biomass was promoted by forest succession in abandoned arable sites upto a level of mature forest (Susyan et al., 2011). Long term succession on post-mining similarly showed increase in the concentration of microbial biomass in soil (Frouz et al., 2005). Microbial biomass also accumulates in municipal solid wastes as reported in Erriadh city of Beja (Fourti et al., 2011).
The CNP stoichiometry, the complexity and spatial heterogeneity have been broadly discussed by Cleveland and Liptzin (2011), it was observed that a Redfield like tend to exist in soil as well soil microbial biomass. Analytical results revealed the element concentrations of CNP in individual phylogenetic groups within the soil microbial community on average shows atomic C:N:P ratios in soil in the ratio of 186:13:1 and the soil microbial biomass in the ratio of 60:7:1 at a global scale. The ratio of soil microbial biomass to soil organic matter is strongly influenced by agricultural activities. Emmerling and others (2001) found that there was a gradual increase in the ratio of soil microbial biomass to soil organic matter on shifting from conventional agricultural practices to integrated agricultural management system.
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Similar observations of increase of increase of soil microbial towards long-term organic fertilization were observed by Esperschutz and others (2007).
Soil microbial biomass has been adopted as important tool is soil productivity analysis, significantly correlated to the soil physicochemical and biological parameters.
Role of microbial biomass in nutrient mineralization is often studied and established as function of soil enzymatic activities in an agroecosystem and urban ecosystems.