3.2. SOC content in closed depressions
In the soils and sediments from the CDs bottoms one can observe higher SOC contents than in the thin soil cover on the slopes of the studied CDs (Figure 5). Both in the bottoms and on the slopes of the CDs, SOC content decreases to 0.2 % with increasing depth (Figure 5). On the slopes of the CDs this value is reached at 0.5 m depth. After a slight increase in the B horizons, the average SOC content decreases again to 0.2% already at a depth of 1.3 m (Figure 5). In the bottom of the CDs, down to a depth of ca. 1 m, a colluvial layer is observed where the average SOC content is the highest, ranging from 0.5% to 1.1%. With increasing depth, the average SOC content drops to about 0.2% at a depth of 2 m (Figure 5). A slight increase of SOC content to 0.6% at a depth of 1.2-1.4m accompanies the humus horizons of fossil soils (Figure 5).
Mean SOC content in colluvial sediments C1 and C2 infilling the studied CDs ranges from 0.47 to 0.82 % (Table 2). However in younger colluvial sediments C2 usually is up to twice the value of SOC content from the older colluvial sediments C1. Fossil soils S1 have a smaller SOC content than that of colluvial sediments i.e. 0.2-0.55 % (Table 2).
Volume of colluvial sediments in particular CDs varies between 509.6 and 1715.59 m3 and depends on the CD size. The mass of colluvial sediments infilling the studied CDs ranges from 787.52 to 2651.5 t. The SOC content in these colluvial sediments ranges from 5.28 to 20.39 t (Table 2). SOC content in fossil soils S1 covering the original bottoms of the CDs varies from 4.12 to 15.98 t and is smaller than SOC content in colluvial sediments (Table 2). There may be two reasons for this: longer time for decomposition of organic matter in these Late Vistulian-Early Holocene soils than in colluvial sediments as well as the areal extent of these soils and of the colluvial sediments in the CDs. In total, between 11.66 and 31.79 t SOC are stored in the studied CDs, the average being about 20 t SOC (20.62) per CD (Table 2).
Between 206.73 and 178.96 Mg SOC·ha-1 (20 673.27 and 17 896.01 Mg SOC·km-2) average 192.85 Mg SOC·ha-1 (19 285.58 Mg SOC·km-2) are stored in the bottoms of all CDs (Table 3). Between 1.87 and 7,69 Mg SOC is stored in the topsoil of each CD (Table 4). In regional scale between 42.34 and 34.18 Mg SOC·ha-1 (4234.14 and 3418.73 Mg SOC·km-2) and average slightly more than 38.26 Mg SOC·ha-1 (3826.44 Mg SOC·km-2)(about 1/4) are stored in the topsoil of all CDs whose total area is slightly over 7 km2 (1.46% of the region’s area) (Table 5). This shows that in the bottoms of the CDs, the highest SOC reserves are below the topsoil. This is opposite to the situation on the loess plateaus and slopes (Table 6).