**3.4. pH**

190 Dehydrogenases

and 0.98\*.

lowest values of DHA.

DHA was its level above 25%.

DHA (µg TPF g-1 min-1)

to Wolińska & Stępniewska (unpublished data)

forest stands (two at low and two at higher attitudes).

number was reported (Fontaine et al., 2003).

determined significant (*P*<0.0001) correlation between TOC-DHA (Fig. 5). Mentioned strong relationship was also confirmed by high value of correlation coefficient (r=0.99\*\*\*). In our laboratory conditions the optimal value of TOC content for reaching maximal values of soil

**Figure 5.** Relationship between DHA and TOC content in the *Mollic Gleysol* (n=9, r=0.99\*\*\*), according

TOC (%)

Analogically to our investigations also Koper et al. (2008) found and reported strong significant relationships between DHA and organic carbon content in *Haplic Podzol* soil samples, and they described mentioned correlations by r coefficient ranged between 0.56\*

The study of Kumar et al. (1992) indicated that DHA displayed the close, positive correlations not only with OM content but also with fungal population abundance in four

High correlation coefficient reported for enzymatic activities and TOC level suggested an important role of these enzymes in transformations of basic components of soil OM (Wolińska & Stępniewska, 2011). There is in general agreement with previous results indicated by Pascual et al. (2000), who found that soils characterized with low microbial and biological activity (e.g. low microbial carbon and low respiration rate), also display the

Summarizing, the higher content of OM, the more active the soil microorganisms. Microorganisms accelerate the degradation of OM, which is reflected in soil respiration and release of carbon dioxide from the rizosphere (Zhang et al., 2010), thus DHA is positively correlated with OM content. Similarly, increase of DHA with higher microorganisms The literature data, currently available, referring to the connections between DHA and soil pH are still ambiguous.

Generally, enzyme activities tend to increase with soil pH (Błońska, 2010; Moeskops et al., 2010) – please put a space before Moeskops. Błońska (2010) determined significant positive correlation (r=0.50\*\*\*) DHA-pH(water) in the pH range 3.67-5.88.

Fernandez-Calviño et al. (2010) noted significantly positive correlations among soil DHA and pH in the range of 4.1 (pHKCl) and 4.9 (pHwater), suggesting that acidity suppressed potential enzyme activity.

Adequately, a study by Levyk et al. (2007) demonstrated that acidic conditions in the pH range between 1.5–4.5 resulted with strong DHA inhibition in relation to alkaline soils, whereas Ghaly & Mahmoud (2006) noted that under acidic conditions with pH less than 6.5, the rate of TTC - specific substrate for DHA, did not decrease.

According to Frankenberger & Johanson (1982), the weakening of enzymatic activity in soil with the increase of soil acidity is the effect of destroying ion and hydrogen bonds in enzyme active centre.

On the other hand, study performed by Włodarczyk et al. (2002) indicated maximum DHA at pH 7.1, similarly to the work of Ros et al. (2003), where optimum for DHA was noted for pH 7.6-7.8. Also Brzezińska et al. (2001) reported that the best pH conditions for DHA ranged between 6.6-7.2.

Natywa & Selwet (2011) noted positive correlation between DHA and pH in soils under maize growth at pH range from 5.17 to 7.27.

Trevors (1984) concluded that very little DHA is observed below pH 6.6. and above pH 9.5. According to Nagatsuka & Furosaka (1980) the optimum range for DHA is contained between 7.4–8.5. However we should realize that many heterogeneous soil types might not be included in mentioned above range.

Our investigations, performed on *Mollic Gleysol* sample (from Kosiorów village) indicated however, that DHA also reached high level at lower pH values–between 5.5-5.73 (Fig. 6). Significant inhibition of DHA (even by 95%) we scarcely noted when soil pH was above 5.75.

It is often assumed that pH may affects soil enzymes level in three different ways (Shuler & Kargi, 2010):


Thus, the pH factor is considered to be the best predictor of DHA in the soil environment (Quilchano & Marañon, 2002; Moeskops et al., 2010).

Dehydrogenase Activity in the Soil Environment 193

**Figure 7.** The dependence between DHA and temperature incubation in the *Mollic Gleysol*, according to Wolińska & Stępniewska (unpublished data). Averaged values of three replicates with standard

Casida et al. (1964) indicated that incubation of soil samples at 37ºC increased of soil DHA

Trevors (1984) described positive significant correlation among DHA and temperature in the range from 5 to 70ºC and determined r coefficient on the level of 0.99\*. Moreover, study by Trasar-Cepeda et al. (2007) reported that increased temperatures up to 57-70ºC enhanced the product formation in the reaction catalyzed by soil dehydrogenases increased with, explained by the fact that specific substrate (TTC), used for DHA determination, is

Analogically, Subhani et al. (2001) noted positive correlation in soil samples incubated at 10, 25 and 40ºC (under constant moisture – flooded conditions), what confirmed by r=0.82\*.

As suggested by Cirilli et al. (2012) optimum temperature for soil DHA is 30ºC, what is in agreement with our findings. Similarly, Brzezińska et al. (1998) indicated that under

Seasonal variations in both microbial biomass and soil enzymatic activities reflect the combine effects of temperature, moisture, substrate availability and other environmental factors. Dehydrogenases belong to the enzymes displaying strong fluctuations in their activities caused by season of the year, as they are in close relationships with dynamic of

laboratory conditions DHA demonstrated the highest activity at 28-30ºC.

above the value normally observed at lower temperatures.

chemically reduced at high temperatures.

deviations are presented

**3.6. Season of the year** 

microbial activity.

**Figure 6.** Relationship between DHA and pH values in the *Mollic Gleysol* (n=18, r=-0.70\*\*), according to Wolińska & Stępniewska (unpublished data)
