**3.1. Soil moisture**

184 Dehydrogenases

in the soil.

activities in soil.

variability of DHA at diverse soil environment.

results of laboratory experiments were conducted on different soil types, representing dominant types of arable soils in Poland, in order to demonstrate changeability and

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

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

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

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

Among different environmental factors with special emphasis on enzymatic activities in the soil environment it is possible to screen some, which have positive impact on DHA. The

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

process of material and energy conversion (Gu et al., 2009).

compounds are highly diverse (Salazar et al., 2011).

**3. Soil factors stimulating dehydrogenase activity** 

most important soil factors stimulating soil DHA are described below.

Life in the soil environment, as well as land use is related to alternate cycles of humidification and drainage (Wolińska & Bennicelli, 2010). Water availability strongly affects on soil microbial activity, community composition (Geisseler et al., 2011), and consequently on soil enzymatic activities. As soils dry, the water potential increases, and as well microbial activity as intracellular enzyme activity slows down (Geisseler et al., 2011). In the case of wet soils, increased moisture could bring into soil solution soluble OM, what might be responsible for increase of bacterial population number (Subhani et al., 2001). What is important, we should have consciousness that any compound, which alters the number or activity of microorganisms, could on the other hand affect on soil biochemical properties, and ultimately also on soil fertility and plant growth (Subhani et al., 2001).

A basic hydrophysical characteristic of soil is water retention, that can be described as a dependence between soil water content and soil water potential. Soil water content in the function of the soil water tension is described by pF curve, which provides information about the ability for water retaining by the soil pores at any given water tension, or conversely, how tightly a water is held between soil aggregates (Wolińska & Bennicelli, 2010).

The Figure 1 demonstrates diminishing trend for DHA behaviour at different soil moisture, described as water potential values. During this experiment gig set of soils (n=315), including all representatives among the most typical Polish mineral soils (*Eutric Cambisol*, *Eutric Histosol*, *Eutric Fluvisol*, *Mollic Gleysol*, *Orthic Podzol*, *Rendzina Leptosol*, *Haplic Phaeozem*) were investigated. However, each of soil unites displayed DHA reducing trend with increase of soil pF value, what means that maximum values of DHA in the soil profiles are indirectly connected with maximum soil moisture (pF 0).

**Figure 1.** DHA (µgTPFg-1min-1) dependence from water potential (pF) at different mineral Polish soil types, during reoxidation (n=315), according to Wolińska (2010)

Statistical relationships between DHA and soil water content, described as pF value in the range of pF0 – pF3.2, determined by Wolińska & Bennicelli (2010) are presented in Table 1.

Founded significant negative relationships between DHA and pF are confirmed by our above mentioned observations, that DHA is strongly affected by soil moisture. These strong

correlations are undoubtedly connected with the fact that the metabolism and the survival of soil microorganisms are also strongly impacted by the availability of water (Uhlirova et al., 2005), what is essential for microbial survival and activity. Consequently, low water availability can inhibit microbial activity by lowering intracellular water potential, and thus by reducing of hydration and enzymes activity (Wall & Heiskanen, 2003). Periods of moisture limitation may affect microbial communities through starvation. Thus, the most common environmental stress for soil microorganisms is perhaps drought (Wolińska & Stępniewska, 2011).

Dehydrogenase Activity in the Soil Environment 187

*Eutric Fluvisol depth* **0-30 cm**

AD ODR

**ODR(µgm-2s-1)**

3.2, were the reason of DHA inhibition and stimulation of ODR level in the *Rendzina Leptosols* and *Eutric Fluvisol* soil samples (Fig. 2). We also stated that soil DHA at pF 3.2 was

**y = 1,02x2**

**DHA**

**(µg TPFg-1 min-1)10-6**

**R2 = 0,98**

 **- 9,54x + 32,40**

0 1,5 2,2 2,7 3,2

**soil water potential, pF**

**Figure 2.** The response of soil DHA to varied aeration factors (pF and ODR), at surface layers of *Rendzina Leptosols* and *Eutric Fluvisol* , during reoxidation process (according to Wolińska & Bennicelli,

AD ODR

**ODR (µgm-2s-1)**

The Figure 3. demonstrates that low oxygen diffusion rate (2.8-25 µg O2 m-2 s-1) was optimal for DHA, what was also confirmed by correlation coefficient (Wolińska & Bennicelli, 2010; Wolińska, 2010). Our results and founding's are compatible with work of Stępniewski et al.

Statistical relationships between DHA and ODR, determined for two soil types (*Rendzina Leptosols* and *Eutric Fluvisol*) by Wolińska & Bennicelli (2010) are presented in Table 2. At

> **(0-30cm)**  j

y=8.55E-5-3.5E-5\*log(x)

**R= -0,31\*** 


**ODR (µg O2 m-2 s-1)** 

**Figure 3.** Relationship between DHA (µgTPFg-1min-1 \* 10-6) and ODR (µg m-2 s-1), in surface layer of

95% p.ufności

2010). Averaged values of three replicates with standard deviations are presented

(2000), Brzezińska et al. (2001), and Yang et al. (2005).

**DHA (µgTPFg-1min-1\*10-6)** 


different mineral Polish soil types (n=315, *P*<0.05), according to Wolińska (2010)

every case negative correlations DHA-ODR were determined.

lower by about 60.86%, in comparison to the activity estimated at pF 0.

*Rendzina Leptosols depth 0-20 cm*

0 1,5 2,2 2,7 3,2 **soil water potential, pF**

**y = 0,66x2**

**DHA**

 **(µgTPFg-1 min-1)10-6**

**R2 = 0,98**

 **- 18,98x + 121,16**

It was shown in many studies that DHA is significantly influenced by water content and dropped with the decrease of soil humidity. For example, Gu et al. (2009) observed higher DHA level (even by 90%) in flooded soil, rather than in non-flooded conditions. The higher DHA values in flooded conditions agreed also with results presented by Zhao et al. (2010) and Weaver et al (2012).


\*, \*\*, \*\*\* - indicate significance at the 5, 1 and 0.1% level, respectively, n.s. – not significant differences

**Table 1.** Statistical significance of differences between DHA and pF described by correlation coefficient (R) (95% LSD method, n=15), according to Wolińska & Bennicelli, 2010

The decline of DHA with an increase of pF value, could be also explained by the fact, that flooding of soil with water significantly increased the electron transport system (Wolińska & Stępniewska, 2011). Dehydrogenases however, are responsible for electron transport in the soil environment. It was also reported that DHA is higher in flooded, anaerobically soils, than aerobically incubated soils (Trevors, 1984; Subhani et al., 2001).
