3.4. The spatial distribution of SOC stocks in the agriculturally used eroded loess landscape of the Nałęczów Plateau
At regional scale, a strong variation of SOC distribution within non-valley areas can be observed (Figure 6), which results from the presence of a microtopography (closed depressions) on the loess plateau. In most non-valley areas of the Nałęczów Plateau, SOC storage is 100-105 Mg·ha-1 (Figure 6). The CDs within the loess plateau tops constitute local stores of sediments and SOC originating from soil erosion in their catchment. The CDs are not evenly distributed (see Figure 4), and their presence leads to a local increase in SOC storage within the plateau tops and slopes to more than 105 Mg·ha-1 and even more than 120 Mg·ha-1 (max. 144.1 Mg·ha-1) and to a considerable variation of its spatial distribution (Figure 4 and 6). A high density of the CDs (above 1 CDs·ha-1) is usually accompanied by a considerable mass of stored SOC i.e. 105.1-144.1 Mg·ha-1. In these areas about 10-50 Mg·ha-1 and sometimes above 50 Mg·ha-1 SOC is present in the CDs (Fig. 7).A relationship is observed between CDs density on the plateau tops and slopes and SOC enrichment (Figure 8). SOC enrichment is 4-25 Mg·ha-1 in areas with a high density of CDs ( i.e. > 1 CDs·ha-1). The Pearson’s correlation coefficient is 0.57. Some areas with a smaller density of CDs (i.e. < 1 CDs·ha-1) show a similar or higher (max. 42 Mg·ha-1) value of SOC enrichment, which is explained by the larger sizes of the CDs observed there (Figure 8). The size of the CDs is of secondary importance, however, because most CDs are of similar size. More than 76% of the CDs have diameters less than 65 m, and the area of more than 65% of the CDs does not exceed 1400 m2 (Kołodyńska-Gawrysiak & Poesen, 2017).
At the regional scale, most of the analyzed plateau and sloping areas (95.96%) have a number of CDs that represent a SOC enrichment in the eroded loess landscape by a few percent (up to about 10%) (Figure 9).
For 3.83% of the analyzed area of plateau and slopes, SOC stored in the CDs leads to a local SOC enrichment by 10-20% i.e. 10.1-30 Mg·ha-1 at most, and for 0.32% of these areas, up to 20-30% i.e. 30.1-41.8 Mg·ha-1 respectively (Figure 9, 10). An average increasing of SOC storage in the studied eroded landscape is 3.08%. A relationship can be observed between CDs density and SOC enrichment (%) of loess plateau (Figure 11). The Pearson’s correlation coefficient is 0.60. A high density of the CDs (above 1 CDs·ha-1) is usually accompanied by a considerable enrichment of SOC (i.e. 4-20 %). The presence of a small number of large CDs results in a higher SOC enrichment (by up to 20-28%) for a CDs density less than 1 CD·ha-1