The natural ability of soil to support optimum growth and yield of crops depends not only on its physical and chemical properties but also on the intensity of biological processes. Soil microbes mediate the biochemical transformations of organic matter that underpin essential ecosystem functions, including decomposition, mineralization of plant-available nutrients, and nutrient retention. Organic production relies on these microbially-derived ecosystem functions and, thus, maybe a model system for the ecological intensification of agriculture . Soil fertility, to a great extent, is controlled by different biochemical activities of the microflora, especially in the immediate surroundings of the roots and the rhizosphere, which under the influence of roots, carry a particularly dense population of microorganisms. Microorganisms can alter the degree of nutrient supply to higher plants through decomposition of organic compounds, immobilization of available nutrients, mineralization of nutrients or by promoting solubilization of fixed or insoluble mineral forms. The quantity and quality of soil organic matter and carbon and nitrogen inputs are the overriding controls on soil microbial biomass and activity. Thus, distinct organic amendments (e.g. manure, leguminous cover crops, and composted materials) can stimulate microbial biomass differently through increased labile organic matter .
Soil enzymes are a group of enzymes present in soil and are continuously playing an important role in maintaining soil ecology, physical and chemical properties, fertility and health. These enzymes also play a key role in biochemical functions in the overall process of organic matter decomposition in the soil system. They are important in catalyzing several vital reactions necessary for the life processes of microorganisms in soil and stabilization of soil structure, decomposition of organic wastes, organic matter formation and nutrient cycling and hence, play an important role in agriculture. The enzymatic activities of soil catalyze the biochemical activities performed by bacteria and thereby indicates the potential of soil to permit the basic biochemical processes necessary for maintaining soil fertility. Organic management increases overall enzyme activity , but activities of specific enzymes may change depending on the composition of the amendments and the relative availability of nutrients, as well as other factors, such as soil type and its unique characteristics . Dehydrogenase activity estimates overall microbial activity due to its presence in all the microorganisms . Phosphatase activity is related to the phosphorus cycle in soil. At the same time, higher phosphatase activity increases significantly in typical purple soil irrigated by biogas slurry in China . Therefore, while studying the effect of different levels and sources of fertilization on soil fertility, attention must be focused on the activity of the microflora and enzymatic activity towards soil biological quality.
In rice (Oryza sativa ) among the various practices, optimizing the use of manures and fertilizers is one of the important strategies for increasing productivity of rice. The contribution of organic manures is to be judged not only in terms of nutrient contribution, but also by their role in building up nutrient reserves in soil and increasing organic matter level of soil which ultimately improves physical, chemical and biological properties of soil. Using organic sources like farm yard manure, vermicompost, green manure combination with inorganic fertilizer deserves priority for sustained production and better on farm resource recycling and utilization. Organic manure provides nutrients, growth promoting substances, as well as they are microbial enriched products that play a vital role in organic rice production. In the present investigation, an attempt was made to examine the influence of different levels and sources of fertilization on dynamics of soil chemical properties, microbial biomass and enzyme activities under anaerobic rice cultivation.