**2. Role of dehydrogenase activity in the soil environment**

There are lots of enzymes in soil the environment, such as Oxidoreductases, Hydrolases, Isomerases, Lyases and Ligases. Each of them play key biochemical functions in the overall process of material and energy conversion (Gu et al., 2009).

Soil dehydrogenases (EC 1.1.1.) are the major representatives of the Oxidoreductase enzymes class (Gu et al., 2009). Among all enzymes in the soil environment, dehydrogenases are one of the most important, and are used as an indicator of overall soil microbial activity (Quilchano & Marañon, 2002; Gu et al., 2009; Salazar et al., 2011), because they occur intracellular in all living microbial cells (Moeskops et al., 2010; Zhao et al., 2010; Yuan & Yue, 2012). Moreover, they are tightly linked with microbial oxidoreduction processes (Moeskops et al., 2010). What is important dehydrogenases do not accumulate extracellular in the soil.

Dehydrogenases play a significant role in the biological oxidation of soil organic matter (OM) by transferring hydrogen from organic substrates to inorganic acceptors (Zhang et al., 2010). Many specific dehydrogenases transfer hydrogen to either nicotinamide adenine dinucleotide or nicotinamide adenine dinucleotide phosphate (Subhani et al., 2001). Throughout mentioned co-enzymes hydrogen atoms are involved in the reductive processes of biosynthesis. Due to this fact, the overall DHA of a soil depends on the activities of various dehydrogenases, which are fundamental part of the enzyme system of all living microorganisms, like enzymes of the respiratory metabolism, the citrate cycle, and N metabolism (Subhani et al., 2011). Thus, DHA serves as an indicator of the microbiological redox-systems and could be considered a good and adequate measure of microbial oxidative activities in soil.

Brzezińska et al. (2001) found that active dehydrogenases can utilize both O2 and other compounds as terminal electron acceptors, although anaerobic microorganisms produce most dehydrogenases. Therefore, DHA reflects metabolic ability of the soil and its activity is considered to be proportional to the biomass of the microorganisms in soil. However, the relationships between an individual biochemical property of soil DHA and the total microbial activity is not always obvious, especially in the case of complex systems like soils, where the microorganisms and processes involved in the degradation of the organic compounds are highly diverse (Salazar et al., 2011).
