**4.1. Depth of the soil profile**

194 Dehydrogenases

microorganisms.

presented

January.

for September and October, respectively.

Yuan & Yue (2012) stated the highest DHA level in autumn season and the lowest value of DHA in winter time. The study performed by Piotrowska & Długosz (2012) indicated that DHA level in *Luvisols* revealed significantly higher values in April (by 96%) than in August, probably due to intensive winter wheat growth with an increased secretion of substrates such as polysaccharides, organic acids, which may have affected the growth and activity of

Similarly, our investigations demonstrated the highest level of DHA in *Eutric Fluvisol* sample taken in May (0.0087 µg TPF g-1 min-1), than in the same soil type taken in October, where DHA was reduced by 42.5 % (Fig. 8). Quite high level of DHA (lower by 14.9% from its maximum reaching in May) we also noted in July. Moreover, we did not found significant differences (*P*>0.05) in DHA values during autumn season, where DHA remained on similar level equaled 0.000598 (µg TPF g-1 min-1) and 0.0005 (µg TPF g-1 min-1),

**Figure 8.** Effect of the season of the year on soil DHA in the *Eutric Fluvisol*, according to Wolińska & Stępniewska (unpublished data). Averaged values of three replicates with standard deviations are

Analogical trend like our observations, related to high Oxidoreductases activity at the time form spring to autumn was noted by Januszek (1993). A study by Włodarczyk (2000) performed on *Orthic Luvisol* sample, showed that DHA demonstrated seasonal pattern and reached the highest values in September, whereas the lowest in winter time. Similarly effect noted Tripathi et al. (2007), who indicated maximum DHA in September and its reduction in

Spring season is strongly connected with increase in microbial activity, intensification of oxido-reduction reactions and temperature change, what is indirectly impacted with DHA, Depth of the soil profile is one of the most known and popular environmental factor reducing soil DHA level. It is well known that the highest microorganisms abundance is in the surface layer of the soil profile (till to the depth of 30 cm), at the deepest part of the soil the number of microbial cells is limited, and consequently also DHA level display diminishing trend.

**Figure 9.** DHA (µgTPFg-1min-1) at different depth of the *Mollic Gleysol* profile (n=18, *P*<0.001), according to Wolińska founding (unpublished data)

The confirmation of the above statement might be the Fig. 9, where effect of depth on DHA in *Mollic Gleysol* is presented. The highest level of DHA we noted in surface layer (0-20 cm), whereas at the deepest part of the soil profile (40-60 cm) DHA was reduced by 95%, in relation to the surface layer. This trend is undoubtedly connected with presented in literature data and mentioned above spatial distribution of soil microorganisms (Agnelli et al., 2004; Levyk et al., 2007; Wolińska, 2010), and its preference to inhabiting the surface layers, where optimum conditions for its growth and development are guaranteed.

Dehydrogenase Activity in the Soil Environment 197

Despite the fact that pesticides are important tools in agriculture that help to minimize economic losses caused by weeds, insects and pathogens, they also are recognized as a source of potential adverse environmental impacts (Tejada et al., 2010). It is often assumed that less than 0.3% of the pesticide reaches its target pest, the remaining 99.7% is released to the environment, representing a potential hazard for non-target organisms (Muñoz-Leoz et

Stepniewska et al. (2007) noted the relationship between soil DHA and Fonofos (Stauffer Chemical Co., Westport, USA) concentration in the *Mollic Gleysol*. In the investigated samples influence of pesticide on soil enzymatic activity started to be observed after one week of incubation, but since 14th day to the end of experiment this effect was significant and noticeable (Fig. 10). Generally, 1µg g-1 dose of Fonofos was responsible for about 26% inhibition of soil DHA, whereas ten times higher factor reduced activity for 46.6% at 21st day of incubation time, later fall of enzymatic activity ranged from 22.5% to 30% in relation to

**Figure 10.** Dynamic of DHA during incubation at 20ºC (0 - control, 1 - 1µg g-1 Fonofos supplement; 10 -

Our results suggest a negative effect of Fonofos on soil DHA in the first stage after application (1-7 day), later an initial, almost linear growth of DHA was observed and the final day of incubation resulted in significant extension of DHA, presumably because the process of Fonofos decomposition in the soil environment was almost completely finished.

Tejada et al. (2010) indicated that MCPA herbicide rate of 1.5 l ha-1 (manufactures rate recommended) was the reason of 39.3% soil DHA inhibition, what suggest that the MCPA

10 µg g-1 Fonofos supplement), according to Stępniewska et al., 2007. Averaged values of three

al., 2011).

the control samples was considered.

replicates with standard deviations are presented

**DHA (u**

**gTPF** 

**g-1 min-1**

**\*10-6)** 

Our results are in agreement and might be supported by the findings of Xiang et al. (2008), who observed that DHA was roughly 4-fold higher in surface (till to 5 cm depth), than in subsoil (90-100 cm). It was also suggested by study of Gajda (2008), that values of DHA noted in the anthropogenic soil, till depth to 4 cm were by c.a. 30% higher in relation to the deeper part of the soil profile.

Brzezińska (2006), reported even 9-fold increase of DHA in the surface layer of the soil, than in the subsurface parts and 25-fold higher enzymatic activity in surface than in subsoil.

Generally it is possible to state, that both diversity, abundance as distribution of microorganisms are more even under oxic (surface layers) conditions, relative to anoxic (deeper layers) conditions (Fierer et al., 2003; Wolińska & Stępniewska, 2011).
