## 3 Results  
### Organic Carbon Content in Topsoil Horizons  
Soil cover of the studied area was mainly represented by Brunic Eutric Arenosol (Ochric) that developed from fluvio-glacial outwash materials (sand and gravel of 1 level outwash plain). Only research plot 6 was represented by Brunic Eutric Arenosol (Cumullic, Ochric), which was related to the location of this surface in a local depression. In the case of the research plot located in the higher part of the relief (No. 7), soil was classified as Lamellic Luvisol (Arenic, Ochric) [26]. The analyzed soils are characterized by a well-developed A horizon, which abruptly and smoothly separates from the below endopedones, that is a typical characteristic of post-agricultural soils. Due to the nature of incoming debris and the nature of the parent materials, these soils, both in the O and A horizons, were characterized by very strongly acid to strongly acid pH (pH of O horizon 3.38-4.40 and A horizon 3.59-4.49). The organic carbon content in the analyzed profiles in the O horizon ranged from 197.8-446.4 g\(\cdot\)kg\({}^{-1}\), while the thickness did not exceed 4 cm, except for the research plot No. 6, where the thickness of this horizon was 6 cm. In the case of A horizon OC range from 6.87 to 52.28 g\(\cdot\)kg\({}^{-1}\). The thickness of A horizon did not exceed 20 cm, except research plot No. 6, where this horizon was noted to the 62 cm depth. For 2012-2016, the average content of organic carbon in the organic horizon (O) in the analyzed research plots was in the range of 221.3-412.6 g\(\cdot\)kg\({}^{-1}\). In the mineral accumulation horizon (A), the average organic carbon content in the studied period ranged from 9.40 (plot No. 1) to 15.68 g\(\cdot\)kg\({}^{-1}\) (plot No. 3) (Table 1). Despite the close location and similar age of the stand, there were statistically significant differences in the chemical properties of soils between the analysis plots. Comparing the OC content among the plots analyzed, it can be concluded that the OC amount in the O horizon of plot No. 1 was significantly different from others (Figure 2). Moreover, it was noted that plots No. 4 and No. 5 contained significantly lower amounts of OC than research plot No. 7. In terms of organic carbon content in the A horizon, plot No. 3, covered with 60-year-old pine, differed significantly from other plots, except plot No. 2. Probably it was related to the localization of the plots in a different microrelief and thus to a different water and matter cycle. Both plots No.1 and No. 2, covered by 60-year-old pine, were located in a shoulder section of the hill. Additionally, plots No. 1 and No. 5, located in a 52-year-old stand and a 60-year-old stand, respectively, were significantly different from research plots No. 2 (Figure 2). No significant difference was found among plots No. 4, No. 6 and No. 7. For both horizon O and A OC content was significantly lower in research plot No. 1 (52-year-old stand) in comparison to other plots. There was no effect of soil type on the observed differences in organic carbon content in the A horizon. The [PERSON]'s correlation analysis (\(p\) < 0.05) showed that the content of organic carbon in both A and O horizons were positively correlated with stand age (Table S1). In the organic horizon of the 50-year-old stand (plot No. 4) the average OC content was 312.2 g\(\cdot\)kg\({}^{-1}\), while in the O horizon of the 60-year-old plots No. 2 and No. 3, these values were higher: 386.1 and 357.0 g\(\cdot\)kg\({}^{-1}\), respectively. In the case of mineral epipedon OC content in the youngest stand (plot No. 4) was 11.06 g\(\cdot\)kg\({}^{-1}\) and in the older stands (No. 2 and 3) amounted to 12.71 and 15.79 g\(\cdot\)kg\({}^{-1}\), respectively. The carbon content was also negatively correlated with soil pH and positively with exchangeable aluminum ions (Figure 3). The lowest values of soil pH were observed in the older forest stands (plots No. 2 and No. 3). With the age of stand, the influence of previous agriculture land use on soil pH disappears.