Vol. 8, Special Issue 10, Part N (2024)

Diversity of microbial populations, various diversity indices (PLFA, Shannan index,etc) and microbial biomass carbon (MBC), are neccessary to understand the effects of different nutrient management practices under rice ecosystem. Rice fields differ in many ways from other agricultural lands and natural terrestrial ecosystems. Rice fields are waterlogged for most of the time, which enhances algal photosynthesis and thus the fertility of the soil. Moreover, water-logged soils are typically anoxic, which limits the growth and activity of aerobic microorganisms, including bacteria, fungi protitsts etc, but favors anaerobic protists able to tolerate the temporary oxic soil conditions. The prolonged hydration also influences the ability of microeukaryotes to decompose plant residues in rice field soils. 
In spite of several studies for estimating thefluxes and appraising microbial pools, the composition and functionsof soil microbiota are still mostly undiscovered. Most of the soil microorganisms are still unknown,only a few have been isolated, cultured and identified, and directlyrelated to their function in agroecosystems. Currently, several techniques,viz., analysis of DNA and phospholipid fatty acids (PLFAs)as well as cultivation on Biolog plates are available to facilitate thecommunity-level characterization of soil microorganisms (Widmer et al., 2001; Wang et al., 2011).
Diversity richness, Shannon index and Evenness index indicated that bacterial functional diversity increases with increase in the application of inorganic Nitrogen (N) fertilizer, but decreased again with excessive N application. The application of N fertilizer significantly increased MBC and the main microbial groups, including bacteria, fungi, actinomycetes, Gram-positive bacteria, and Gram-negative bacteria but varied in different growth stages. Application of organic manures, especially rice straw, resulted in larger overall bacterial populations than in control or chemical fertilizer treatments (Zhang et al., 2012). Organic manures significantly increased soil biological activity and microbial population by providing C source, high content of organic C due to application of rice straw along with chemical fertilizers, can provide a longer lasting C source for soil microorganisms. They provide required carbon, nitrogen and energy for microbial growth and reproduction. Organic amendments along with low amount of chemical fertilizer have a great potential to enhance paddy soil microbial biomass and microbial activity Zhang et.al., 2012). Over the period of the experiment of two years, organic amendments along with low quantity of mineral fertilizer appreciably increased Azospirillum and PSB population as compared to the mineral fertilizers alone.