The relation between spruce monoculture and incipient podzolisation in ochreous brown earths of the Belgian Ardennes

Summary The effects of coniferous monoculture on the distribution of C, Fe and Al in the upper horizons of brown ochreous earths of the Belgian Ardennes were investigated by comparing 5 soil profiles developed under an 80 year old spruce stand with another 5 profiles developed under a climactic broad-leaved forest (beechwood).Organic carbon, Fe and Al were extracted with 0.1N NaOH/Na-tetraborate solution buffered at pH 9.7: recent studies have shown that this extraction is particularly appropriate for the detection of incipient podzolisation in brown earths-brown podzolic soils intergrades.Even if most of the classic podzolisation indexes fail to illustrate differences, nevertheless our results show that fulvic acids and organo-ferric complexes are present in significantly greater amounts in the upper part of the cambic (B)₁ horizon of the soils developed under conifers. Moreover, this podzolic tendancy is confirmed by the weathering patterns of the clay minerals in the A₁(B) horizons developed under spruces, i.e. a more pronouced weathering of chloritic layers than those observed in the beechwood soil, with a correlative genesis of more abundant smectite-like minerals. One may therefore conclude that the change in the humus type (moder to mor) after the planting of spruce trees, has been sufficient, within the local climatic and edaphic context, to promote incipient podzolisation.