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Two forest soil inventories have been conducted on the Flemish transnational forest vitality grid: one in 1993 and one in 2004.
A detailed profile description and classification following the World Reference Base for Soil Resources on the 10 Level I plots was made by J.H. Mikkelsen in 2006 during the BioSoil Inventory (Mikkelsen et al. 2008).
In 1993, the organic layer and 3 depth layers of the mineral soil (0-5 cm, 5-10 cm and 10-20 cm) at each observation plot were sampled according to a fixed procedure, resulting in 4 mixed samples per plot. Each mixed sample consisted of 36 subsamples, collected in the 4 quadrants of the plot. L, F and H layers were sampled together. Chemical analyses were carried out according to the reference methods of the ICP Forests soil Submanual (PCC – UNEP – UN/ECE, 1993), except for total nitrogen, for which Kjeldahl digestion was used.
In 2004, samples of the organic layers (OL, OF and OH) and at fixed depth intervals (0 – 5 cm, 5 – 10 cm, 10 – 20 cm, 20 – 40 cm and 40 – 80 cm) were taken. Analytical methods follow the reference methods described in the ICP Forests manual (FSCC, 2006).
In 1993, the pH (CaCl2) values in the plots ranged from 2.9 to 5.6 in the mineral layers and from 3.1 to 6.8 in the organic layer. Nine plots out of ten had acidic soils, with pH below 4 in organic and mineral layers. Cation exchange capacity (CEC) and base saturation in the mineral layers were in general low to moderate. Median values for effective CEC were 6.3 cmol kg-1 in the 0-5 cm layer, 3.7 cmol kg-1 in the 5-10 cm layer and 3.5 cmol kg-1 in the 10-20 cm layer. Base saturation values lower than 20 % in at least 2 mineral depth layers were found in 7 plots. Very low base saturation values (< 5 %) were found in 3 plots (Roskams, 1997).
These results can be partly explained by the nutrient poor parent material and the coarse soil texture in many plots; they indicate a high sensitivity to soil acidification. Other studies (e.g. Ronse et al., 1988), however indicated that a significant acidification occurred in the upper layers of Podzols and Regosols under forests before the start of the soil monitoring on these plots.
In 2004, the pH(CaCl2) values in the plots ranged from 2.8 to 5.8 in the mineral layers and from 3.1 to 3.4 in the organic OFH layer. In all nine plots currently still reported to the international ICP Forests database the pH(CaCl2) of the surface mineral layer (0-5 cm) remained below 3.5. Both cation exchange capacity and base saturation decreased compared to 1993. Median values for effective CEC were 4.0 cmol kg-1 in the 0-5 cm layer, 3.5 cmol kg-1 in the 5-10 cm layer and 3.5 cmol kg-1 in the 10-20 cm layer. Base saturation below 20% was found on all nine plots involved in the international tree vitality survey. Very low base saturation values (< 5 %) were found in the surface mineral layer (0 -5 cm) of 3 plots but on 7 plots for the 5-10 and 10- 20 cm layer.
In 1993, 7 out of 10 plots the C/N ratio of the organic layer (average = 27) was smaller than the C/N ratio of the mineral layer (average = 32). However, in healthy forests the C/N ratio of organic layers is distinctly higher than the ratio in underlying mineral layers. Lower C/N ratios in the organic layer may be the result of atmospheric nitrogen deposition. In 2004, the C/N of the organic layer (average on 10 plots = 27) was smaller than the C/N ratio of the topsoil (0 – 10 cm) of the mineral layer on only 3 out of 10 plots. Though, changes in the chemical method for total N determination might explain the observed difference.