**6.1 Soil health**

Soil microbes are essential for the proper functioning (i.e., health) of a soil. Yet, there is little information about how to assess the life present in soil to determine if a given soil is healthy or not. Till date, biological measures of soil health have been centered on biological functions, such as respiration or nitrogen mineralization. Soil metagenomics is a promising approach in describing the functional potential of the soil microbial community, which might yield greater insight into the health of a soil than taxonomy-based metrics. Soil health cannot be measured directly, so quality/health of soil is evaluated by indicators. Structural and functional diversity of microbes, presence/absence of important microbial players, microbial activity (respiration, DNA replication and cell division), nutrient cycling, production of cofactors and secondary metabolites, and response to biotic and abiotic stress can be used as biological indicators of soil health. Biological activity of the soil can be known by estimating dehydrogenase enzyme in the soils [75]. β-glucosidase involves in hydrolysis and biodegradation of various β-glucoside present in decomposing plant debris in soil. β-glucosidase is characteristically useful as a soil quality indicator, and may give a reflection of past biological activity, the capacity of soil to stabilize the soil organic matter, and can be used to detect management effect on soils [76]. The amount of these enzymes activity indicates the biological capacity of soil, for the enzymatic conversion of the substrate and also has an important role in the ecology of microorganisms in the ecosystem. The degradation and hydrolysis of chitin is achieved by Chitinase a key enzyme responsible for the same. Its presence in different forms in the ecosystem has demonstrated its effectiveness in the control of soil-borne diseases. Arylsulphatases are responsible for the hydrolysis of sulphate esters in the soil [77] and are secreted by bacteria into the external environment as a response to sulphur limitation. Phosphatases are good indicator of soil fertility and are believed to play critical roles in P cycles [78]. When there is a signal indicating P deficiency in the soil, acid phosphatase secretion from plant roots is increased to enhance the solubilization and remobilization of phosphate, thus influencing the ability of the plant to cope with P-stressed conditions [79].

#### **6.2 Industrial use**

Activity-based screening has the potential to detect entirely novel genes encoding new types and classes of enzymes or to identify the new bioactive compounds. In addition, it is selective for full-length genes and functional gene products. Cellulases has been isolated from various natural environments like soil, rumen, compost soil and many more using metagenomic technique by constructing the metagenomic libraries followed by screening of the biologically active clones.

Cellulases are used in animal feeds for improving the nutritional quality and digestibility, in processing of fruit juices, and in baking, while de-inking of paper is yet another emerging application [80]. Alvarez with his team isolated and characterized a novel cellulase by functional screening of a metagenomic library derived from sugarcane field land soil [65]. Lipases have been found in many species of animals and plants. The enzymes from microbial sources (such as bacteria, yeast and fungi) are currently receiving particular attention because of their actual and potential applications in industry mainly in the detergents, oils and fats, dairy and pharmaceutical industries [80]. A novel metagenomic xylanase has been isolated from compost-soil metagenome that shows alkali stability and thermostability, thus bearing a potential application in paper and pulp industry in pulp bleaching [81]. Vidya with her co-associates isolated a thermostable and calcium-dependent amylase from a soil by constructing and screening soil metagenomic library and suggested its applications in baking and de-starching [82].
