## 4 Discussion
### Variations in OC Content and DHA
Therefore, no identical trends in changes in the organic carbon content in the horizons of O and A were observed for the research plots in the subsequent study years. Despite the fact that many authors reported a seasonal variation in soil enzyme activity [49; 60], the obtained results do not coincide with their observations. These authors linked the observed seasonal changes of enzyme activity with the combined effects of temperature, moisture, substrate availability and other environmental factors. As dehydrogenases are only active in living cells, these enzymes display strong fluctuations in their activities during the year. The lowest value of DHA were noted in winter and the highest in autumn [49]. However other authors found the highest DHA in the spring period [60]. In this study only a small increase in DHA was observed during autumn months. The difference in these results might be connected to different weather patterns and availability of easily decomposable organic substance. In the case of this study lack of significant seasonality of DHA might be caused both by not taking into account the winter period and a different plant community (a different type of fresh organic matter inflow with diverse susceptibility to microbial attack). Additionally, this might be related to the slow process of microorganism's population modification after the land use change from agricultural to forestry. According to the research of [PERSON] et al. [40], [PERSON] [61], after the afforestation of farm land, the population of bacteria and actinomycetes slowly decreases with a simultaneous increases in the share of fungi. The different nature of incoming organic matter and changes in nutrient availability in young forest stand result in lower microbiological and thus enzymatic activity [62]. In order to recognize sharp changes in the parameters studied, change-point analysis was applied. An abrupt decrease in DHA was observed in spring 2014, which continued to the end of research period. A decrease in the activity of this enzyme's group could be a result of lower amounts of precipitation observed in 2014 and 2015 and also a change in the distribution of rainfall. Moreover, the higher temperatures noted in these years increase evapotranspiration leading to lower soil moisture affecting microbial communities through starvation (Figure S3). In the case of OC content a marked increase in the O horizon was observed in 2013. This change coincides with a period of lower precipitation (538 mm, 525 mm and 509 mm in 2013, 2014 and 2015, respectively) compared to previous years and the multi-year average (553 mm) [25]. Lower sums of precipitation together with a decrease in environmental humidity contributed to a decrease in microbial activity, measured by DHA. This was probably one of the reasons for the increase in OC content in the O horizon. At the same time, studies by [PERSON] et al. [63], showed that during research years, the meteorological conditions had a greater impact on the photosynthesis process (gross ecosystem photosynthesis (GPP)) than on the respiration of the ecosystem. The GPP decrease from 58.21 in 2013 to 53.65 and 48.87(MgCO\({}_{2}\cdot\)ha\({}^{-1}\)) in 2014 and 2015, respectively. At the same time, ecosystem respiration in 2013 amounted to 15.82 and in the following years 13.79 and 10.51 (MgCO\({}_{2}\cdot\)ha\({}^{-1}\)), thus suggesting that an increased inflow of plant residues to the litter horizon contributed to the higher content of OC.