**5. Conclusions**

Our results confirm the beneficial e ffect of birch stand regeneration on the soil properties on post-agricultural land. We observed a clear trend of increasing carbon accumulation in the soil under the influence of birch trees. With age, greater amounts of carbon were accumulated in the surface soil

layers. Dehydrogenase activity is a suitable indicator of the condition of post-agricultural soils with birch stands and, in combination with soil chemical properties, reflects historical soil management. In this sense, the determination of dehydrogenase activity allows for an assessment of the processes occurring in post-agricultural soils, which are associated with the soil organic carbon accumulation. A high nutrient content and high pH are characteristic of post-agricultural soils, facilitating a greater biochemical activity in the initial stages of stand formation.

**Author Contributions:** Conceptualization, T.G. and S.M.; Methodology, T.G. and E.B.; Project administration, T.G. and S.M.; Supervision, S.M.; Writing—original draft, T.G. and E.B.; Writing—review & editing, T.G.

**Funding:** This research was funded by Ministry of Science and Higher Education of Poland, Grant/Award Number: DS 3420/ZELiR/2018 and the Polish National Science Centre within gran<sup>t</sup> N N305 400238, entitled "Ecological consequences of the silver birch (Betula pendula Roth.) secondary succession on abandoned farmlands in central Poland".

**Conflicts of Interest:** 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.
