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ABSTRACT

Dehydrogenase enzyme is the most important soil quality as well as overall microbial activities indicator. The main purpose of this article is to clarify the role of intracellular enzyme-dehydrogenase in the soil environment, as well as presentation of soil factors, influencing an enzymatic activity, by either stimulation or inhibition effect on soil dehydrogenase activity (DHA). Several environmental factors, including soil moisture, oxygen availability, oxidation-reduction potential, pH, organic matter content, depth of the soil profile, temperature, season of the year, heavy metal contamination and soil fertilization or pesticide use can affect significantly DHA in the soil environment. The optimal pH range for DHA is between 5.5-5.7. DHA display increasing trend under anaerobic conditions, that suggests the facultative and anaerobic member of soil microbial community become more important in soil respiration processes. Analogically, soil DHA was reported to be negatively correlated with soil water potential, oxygen diffusion rate, redox potential and soil depth (due to spatial stratification of microorganisms abundance).

During biochemical process, bacteria and other microorganisms release enzymes such as dehydrogenase and phosphatase. Soil microorganisms play a key role in cycling of nutrients and its activity is essential in both mineralization and transformation of organic matter in soil ecosystem and by their ability to degrade organic matter, nutrients are released in the available forms for the plant growth. The enzymatic studies give information about nutrient release; organic matter decomposition as well as ecological changes and they control the flow of energy in soil ecosystem. The soil biological and chemical properties also depend on it. The enzymes such as Oxidoreductases, Isomerases, Hydrolases, Lysases play important role in energy conversion. DHA belong to Oxidoreductase enzyme class and they are the indicator of soil microbial activities. Dehydrogenase plays a key role in biological oxidation of soil organic matter by transferring hydrogen from organic substrate to inorganic acceptors as well as they can utilize both O₂ and other compounds as terminal electron acceptors. Anaerobic microorganisms produce most dehydrogenases. DHA signifies the microbial biomass in soil system.

A. Soil factors stimulating dehydrogenase activity.

The most important soil factors stimulating soil DHA are described below.

1.Soil moisture

The DHA activities decrease as water content or moisture or humidity levels decrease in soil environment. Dry soil increases water potential, as well as intracellular enzyme activity slows down (Geisseler et al., 2011).Soil water helps in solubilisation of organic matter content thus increase microbial population. DHA activity is maximum at maximum soil moisture (pF=0) as it is indirectly related with pF value. DHA activity is maximum seen in case of flooded soil as it increase electron transport in soil as well as dry soil has less DHA.

 2. Soil aeration state (redox potential and oxygen diffusion rate)

ODR (oxygen diffusion rate) is the critical factor controlling microbial activities in soil. Water content is inversely related to ODR and redox potential.  The ODR value below (35µg O₂ m²s^-1) is favourable for DHA activity (Stepniewski et al., 2000; Wolinska & Bennicelli, 2010). DHA activity is inversely related with Eh values and it belongs to obligate anaerobes. In case of flooded condition reduction of elements takes place (NO₃^-/NO₂^-, Fe³⁺/Fe²⁺, and Mn⁴⁺/Mn²). DHA is strongly influenced by presence of Fe and Mn in soil.

 3. Organic matter content

Soil organic matter is the most important substrate that acts as an indicator of soil quality as well as microbial activities in soil. SOM has characteristics of nutrient sink and source so that it enhances soil physical, chemical and biological characteristics. The higher OM level in soil provides enough substrate for higher microbial growth as well as enzyme production (Yuan & Yue, 2012).

4. pH

The DHA activities increase with pH increase as acidity suppresses enzymatic activities. It is found that optimum range 7.4-8.5 pH suitable for DHA.

5. Temperature

The DHA increase with increase in temperature until it cross the optimum temperature for microbial growth and range found is between 5-30^oC. The incubation temperature of 37^oC increased DHA activity than other normal temperature (Cassida et al., 1964). For determination of DHA in soil TTC used as it is ineffective at higher temperature.

 6. Season of the year

Changing temperature, wind, rainfall etc affect the DHA activities during one time period of year. DHA activities found highest during autumn season and lower in winter time (Yuan & Yue, 2012) as secretions as polysaccharides etc more in autumn than winter.

 B. Soil factors inhibiting dehydrogenase activity

The most important soil factors inhibiting soil DHA are described below.

 1. Depth of the soil profile

The DHA as well as other microbial activities found more in case of surface soil (0-20cm) of soil and decline above 20cm soil layer. Surface soil provides good aeration, moisture as well as nutrient sources than sub surface layers.

 2. Fertilization and pesticide amendment

Organic fertilization influenced more than inorganic fertilization in case of increase in soil microbial growth and activities. Organic vermicompost, compost application improved soil conditions thus supported maximum DHA activities as well as microbial growth. Inorganic applications of glyphosate, chloropyriphos inhibited DHA (Kadian et. al., 2012).

3. Heavy metals presence

Heavy metals interfere with enzyme-substrate complex formation as well denaturing the enzyme protein or with protein-active groups, affect the synthesis of enzyme microbial cell (Pan and Yu, 2011). They also concluded DHA was significantly lower by 37.8% and by 51.1% in Cd and Pb treatments than control in brown soil. The effect of Cd and Pb combined on DHA were higher, than Cd or Pb alone.

 Conclusions

DHA among other indicators is the most important, bio-sensitive soil quality and fertility indicator. Soil enzymes play important role in microbial processes, organic matter decomposition, nutrient recycling etc. DHA is mineralized by enzymes (i.e. Protease) unless absorbed by clays or humic molecules.  As DHA is associated with living cells the soil environmental factors affect its growth. DHA is negatively related with soil water potential, oxygen diffusion rate and redox potential, depth of soil, heavy metal as well as inorganic fertilizer applications in soil. DHA increased with pH level, organic fertilizer application in surface soil, optimal temperature (5-30⁰C), soil organic matter increase and water content in soil. The soil microbial overview and status can be measured from the activities of DHA in soil environment. Electron transport systems in soil is controlled by DHA and it helps in a lot of ways to improve soil overall condition thus they play a key role in soil ecosystems.

References

 

Casida, L.; Klein, D. & Santoro, T. (1964). Soil Dehydrogenase Activity. Soil Science, 98, pp. 371-376

Geisseler, D.; Horwath, W. & Scow, K. (2011). Soil Moisture and Plant Residue Addition   Interact In Their Effect on Extracellular Enzyme Activity. Pedobiologia, 54, pp. 71-78

Kadian, N.; Malik, A.; Sataya, S. & Dureja, P. (2012). Effect of Organic Amendments on                 Microbial Activity in Chlorpyrifos Contaminated Soil. Journal of Environmental                Management, 95, pp. 199-202

 Pan, J. & Yu, L. (2011). Effects of Cd or/and PbOn Soil Enzyme Activities and Microbial Community Sructure. Ecological Engineering, 37, pp. 1889-1894

Stępniewski, W.; Stępniewska, Z.; Gliński, J.; Brzezińska, M., Włodarczyk, T.; Przywara, G.; Varallyay, G. &Rajkai, J. (2000). Dehydrogenase Activity of Some Hungarian Soils as     Related To Their Water and Aeration Status. International Agrophysics, 14, pp. 341-354

Wolińska, A. & Bennicelli, R. (2010). Dehydrogenase Activity Response to Soil Reoxidation Process Described as Varied Condition of Water Potential, Air Porosity and Oxygen         Availability. Polish Journal of Environmental Studies, 19, pp. 651-657

Yuan, B. & Yue, D. (2012). Soil Microbial and Enzymatic Activities across a       Chronosequence of Chinese Pine Plantation Development On The Loess Plateau of       China.Pedosphere, 22, pp. 1-12.

 

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Factors stimulating and hindering Dehydrogenase activity in Soil.pdf