RAPID CHANGES OF SANDY SOILS CAUSED BY VEGETATION CHANGES

Development of Typic Haplorthods in a heathland area in Denmark respon ded over a short period of time (decades) to changes of vegetation. Pa rt of the heath, Hjelm Hede, was left undisturbed and was invaded by t rees, mainly oak and a few aspen and conifers. Another part of the hea th was planted with Norway and Sitka spruce 60-70 yr ago. The soils un der heath, oak and spruce were studied. Major differences were found, some visible in the field and others detectable in the laboratory. Und er oak, relative to heath, horizon boundaries were less distinct, pH i ncreased in the top horizons, organic carbon and C/N ratio decreased, and iron and aluminum contents in the upper B horizons decreased. Comp ared with the original heath podzol, the soil under spruce had a lower pH in the O, E and upper B horizons, higher organic carbon content an d C/N ratio in the top horizons, increased cementation, and a placic h orizon. However the pyrophosphate-extractable iron and aluminum conten t was significantly lower than in any of the other soils. The soil und er oak showed ''depodzolization'' features, whereas the soil under spr uce was increasingly podzolized, though the podzolization mechanism mi ght be different from that under heath. Analyses of phenolic compounds in the soil water were consistent with these conclusions. The three m ain components of substituted benzoic acids were gallic acid, protocat echuic acid and coumaric acid, which are all strongly complexing agent s believed to take part in the podzolization process. Generally, the h ighest concentrations were found under spruce and the lowest under oak .