## 3 Results
### Variability in OC Content and DHA  
The analysis of the organic carbon content in the A horizon in 2012-2016 indicates the existence of a certain pattern of seasonal variations in this parameter. In the spring and autumn period, the organic carbon content was higher than the average values recorded in the summer seasons (Figure 4b). In the case of organic carbon content in the O horizon, such regularity was not observed (Figure 4a). In the research plots No. 4 and No.6, an increase in the content of organic carbon in the soil was observed from spring to autumn. A downward trend was observed on plot No. 5 from spring to autumn. In the case of research plots No. 1, No. 3 and No. 7, the same trend of variations was observed as in the A horizon. Research plot No. 2 was characterized by a different course of seasonal variations in organic carbon content; the highest values were recorded in the summer period. Despite the statistically significant seasonality of the organic carbon content in the A horizon, no significant differences were found for the O horizon in any of the research plots. Figure 3: Correlation network analysis between analyzed soil characteristic (\(n=196\)). The correlation network was built using Pearson correlation coefficients and visualized in Cytoscape 3.8.2. The thickness of the edges is proportional to absolute value of Pearson correlation coefficients (red—positive correlations; blue—negative correlations; **:**\(p<0.05\); **:**\(p<0.01\)). Analyzing the content of organic carbon for particular plots in individual years, no statistically significant differences were observed between them, both in the O and A horizons. However, in both O and A horizons, some variations in the content of organic carbon were observed in certain years of the study. These fluctuations were more visible in the mineral, A horizon. For the O horizon in research plots No. 1, No. 3, No. 4 and No. 7 higher mean values of OC content in 2016 (268.4-477.5 g\(\cdot\)kg\({}^{-1}\)) than in 2012 (181.1-342.9 g\(\cdot\)kg\({}^{-1}\)) were recorded (Figure 5a). In the rest of research plots (No. 2, No. 5, No. 6) the highest average OC content was noted in 2014. The Tukey test conducted for all data (\(n=196\)) obtained for the O horizon showed that the content of OC in 2012 and 2013 was statistically different from years 2014, 2015 and 2016. In the A horizon, in contradiction to the O horizon, there was no increase in the OC content in the subsequent research years. Average values of the OC content were either lower in 2016 or similar to those in 2012 and 2017, respectively. Figure 4: The average seasonal content of OC in the (**a**) O horizon and (**b**) A horizon. (Figure 5b). In the mineral horizon of all of the research plots a decrease in OC content was observed in 2013 (8.7-14.55 g\(\cdot\)kg\({}^{-1}\)) and maximum was noted in 2014 (9.81-18.15 g\(\cdot\)kg\({}^{-1}\)). The analysis of all data of OC content in A horizon (one-way ANOVA, without considering plots division, \(n=525\)) showed that a statistically significant difference was observed between 2013 and 2014 and additionally between 2014 and 2016. Although previous statistical analyses showed the dependence of dehydrogenase activity on the age of the stand, soil moisture and carbon content at the O horizon, the research plots analyzed did not differ from each other in terms of the activity of this  
Figure 5: Annual average organic carbon content in (**a**) O and (**b**) A horizons. enzyme (Figure 6). Only research plot No. 4 (the youngest one) exhibited significantly lower dehydrogenases activity than plots No. 2, 3 and 7, and plot No. 1 (52-year-old stand) in comparison to plot No. 3. On the youngest forest stands (No. 4 and No.1), lower values of this enzyme activity were recorded than on 60-year-old stands, on average 0.01, 0.014 and 0.025 umol TPF-g d.m.\({}^{-1}\).20 h\({}^{-1}\), respectively. Analyzing temporary changes in DHA, no seasonal pattern was found.