## 5 Conclusions  
The results of a five-year study on the seasonal variability of organic carbon content in topsoil horizons showed that during the spring and autumn period, the organic carbon content was higher than the average values recorded in the summer. However, there was no such regularity regarding the content of organic carbon in the O horizon. Moreover, no statistically significant differences were observed between the content of organic carbon in individual years but only for certain years. During the research period, an increase in OC content was observed only in the O horizon whereas in the A horizon it either remained at the same level or was slightly lower. The result of PCA analysis showed that OC content in the study area was a function of the interaction of four groups of factors: soil reaction, rate of microbial processes, human disturbance and meteorological. The correlation analysis indicated that OC content in soils were a function of forest stand age and progressive acidification of soil. Furthermore, OC content in the litter horizon was negatively correlated with soil moisture and dehydrogenases activity, suggesting that periods with higher microbial activity lead to lower accumulation of carbon in the O horizon.  
Dehydrogenases activity was related to the presence of OC in the O horizon and soil moisture and the age of forest stand. Despite the fact that dehydrogenase shows a strong seasonality, such a phenomenon has not been recorded in this pine forest. However, a difference in DHA was observed in the years studied. An abrupt drop of DHA was noted in spring 2014 and an increase in OC content in the O horizon was recorded.  
In order to confirm these seasonal variations in OC content and dehydrogenases activity, further studies with time-series data are needed. A much longer period of study is required, especially to recognize the potential of carbon sequestration by terrestrial ecosystems in the temperate forest zone.  
The following are available online at [[https://www.mdpi.com/article/10.3390/f12040459/s1](https://www.mdpi.com/article/10.3390/f12040459/s1)]([https://www.mdpi.com/article/10.3390/f12040459/s1](https://www.mdpi.com/article/10.3390/f12040459/s1)), Figure S1: Scheme of the research plot soil sampling, Figure S2: Sum of precipitation and average annual temperature of the study period in Tuczno research field, Figure S3: Soil moisture (average) fluctuations on particular research plots during study years, Table S1: Person correlation analysis between organic carbon content in A and O horizons and physicochemical and biochemical soil properties, Table S2: The principal component analysis of soil properties.  
Conceptualization, K.W., J.K. and J.O.; methodology, J.K.; software, K.W.; validation, K.W. and J.K.; formal analysis, K.W.; investigation, K.W., J.K.; resources, K.W.; data curation, K.W.; writing--original draft preparation, K.W.; writing--review and editing, J.K.; visualization, K.W.; supervision, J.K.; project administration, J.O.; funding acquisition, J.O. All authors have read and agreed to the published version of the manuscript.  
This research was supported by funding from States Forest National Forest Holding in Poland (project LAS, No OR-2717/27/11).The publication was financed within the framework of Ministry of Science and Higher Education programme as,,Regional Initiative Excellence\" in years 2019-2022, Project No. 005/RID/2018/19.  
Not applicable.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.