**3.1 Soil microorganisms**

Microorganisms are small microscopic organisms that cannot be seen with naked eyes. They inhabit animal intestine, food, soil, water and other different environment. They belong to any of the following group of organisms: bacteria, fungi, protozoa, micro-algae and viruses [26]. The largest proportion of the earth's biodiversity has been reported to be microorganisms and they play an integral role in the processes of ecosystem thereby providing functions that eventually sustain all forms of life [27, 28].

Soil microbial biomass is the living component of soil organic matter. Soils with high organic substances tend to have a higher microbial biomass contents as well as their activities since organic matters are the preferred energy source for microbes. The surface horizon of the soil has the highest microbial activities when compared to the deeper horizon [29]. The soil microbial biomass helps in the enzymatic transformation of soil organic matter into humus, carbon and other nutrients which are utilized by microorganisms for their own growth [30]. Soil microbial biomass and its enzymatic activities are strongly influenced by seasonal changes in soil temperature, moisture and available residue [31]. Soil enzymes may originate from animals, plants or microbes and can either exist in bound or free form within

the soil. Soil enzymes play a vital role in the biochemical functioning of soils [32] including nutrients cycling [33], soil structure maintenance [34] and decomposition of organic residue [35]. The activities of soil enzymes are controlled by many factors such as soil microbial community [36], soil physio-chemical properties [37], vegetation type [38] and ecological disturbances [39]. Prior to the utilization of complex organic matter by microorganisms as their source of energy, they produce a quite number of extra cellular enzymes in order to decompose them [40]. Soil enzymes are specific in the types of reactions they participate. For example, a starch hydrolyzing enzyme known as amylase hydrolyses 1-4D glucosidic linkage of amylase and amylopectin and consist of -amylase and β-amylase. -amylase is synthesized by animals, plants and microorganisms while β-amylase is primarily synthesize by plants [41]. To a large extent, soil microbial activities is dependent on the quantity of available carbon and this is shown by dehydrogenase activity [29]. Dehydrogenase is involved in the biological oxidation of soil organic matter, and also responsible in oxidizing organic matter by transferring hydrogens and electrons from substrates to acceptors [42]. Phosphatase originate from root exudates and microorganisms, it cleaves the phosphate from organic substrates and also involved in P cycle in soil [43]. It has been evidently suggested by Ushio *et al.* [44, 45] that plant species significantly have more direct impacts on the composition of soil microbial community and their activities in addition to soil physicochemical properties. Plants' rhizosphere has been reported by Vyas and Gupta [46] to have profound effect on microbial population and activities. From the study of Islam and Borthakur [47], increase in microbial biomass and enzyme activities indicates high rate of release of nutrients by rice crops which aid microbial activities.
