## 4 Discussion
### Variations in OC Content and DHA  
Our study indicates an existent of a seasonal pattern of OC content change. During spring and autumn months, higher amounts of OC were noted in comparison to summer months. The pattern of variations obtained is probably associated with lower soil temperature values and higher soil moisture in spring and autumn, which limits the intensity of abiotic mineralization. In the spring, the soil temperature still slows down the activity of microbes, thus the active forms of soil organic matter have not yet been broken down by heterotrophs. [PERSON] [52] and [PERSON] and [PERSON] [53] observed a similar seasonality of organic carbon content in the mineral epipedons of forest soils. These studies also took into account the differences in the species composition of forests. The seasonal character of soil organic carbon content was recorded only for coniferous stands: Caucasian fir, Cedar of Lebanon and Black pine. Moreover, [PERSON] et al. [41] observed the seasonality of the OC content in pine stands, but obtained significantly higher carbon values in the autumn period. Even though, in general, there were no statistically significant differences in OC content between the study years, some differences were observed between certain years. In the case of O horizon significant difference were observed between two first years of studies and years 2014, 2015 and 2016. While, in the A horizon such difference was found only between 2013 and 2014 as well as between 2014 and 2016. Additionally, [PERSON] et al. [54] analyzing OC changes in the topsoil of Swiss cropland over 25 years reported no statistically significant changes between study years. They observed inconsistent changes among site studied ranging from \(-\)12 to +11% over 10 years. Whereas our results showed variations in the A horizon from \(-\)22 to +11.2% within five years. Other authors [PERSON] et al. [55] and [PERSON] [56] reported variations of 15% within one year and 14-16% within 39 months, respectively. The decrease in OC content in mineral horizon noted for 2013 and later increase in the next years was probably caused by a combination of microorganism activity and meteorological conditions. Taking into consideration that dehydrogenases play a significant role in the biological oxidation of soil organic matter by transferring hydrogen from organic substrates to inorganic acceptors [51; 57] activity of this enzyme group may significantly change the content of active pool of organic matter. The activity of soil microorganisms, assessed on the basis of dehydrogenase activity, was the highest in 2012 (Figure 6). This could be due to the higher amount of rainfall recorded this year (657 mm- very wet year) (Figure S2). [PERSON] [58] and [PERSON] [51; 59] reported that DHA is strongly affected by soil moisture. They associated this relationship with the fact that the metabolism and the survival of soil microorganisms are also strongly impacted by the water availability. Consequently, low water availability can inhibit microbial activity and reduce enzymes activity. Periods with high soil moisture may support microbial communities and thus increases the rate of organic matter turnover. Additionally, the distribution of precipitation in 2013 changed, over 75% of annual rainfall was recorded before the end of June. During summer and autumn seasons combination of higher air temperature (average 16.22 \({}^{\circ}\)C) and small amounts of precipitation (127 mm) favor mineralization processes. Considering the fact that the content of organic carbon in the mineral horizon is the result of the dynamic equilibrium between the input of organic substance and the processes of its mineralization taking place over several years, the dry year effect was visible in the results of the next research years. According to [PERSON] et al. [12], active fractions of organic carbon undergo changes within 1-2 years.