Vegetation effects on pedogenetic forms of Fe, Al and Si and on clay minerals in soils in southern Switzerland and northern Italy

The older forest type Quercetum– Betuletum (oak/birch; Q-type vegetation) in southern Switzerland and northern Italy was to a large extent replaced by chestnut forests ( Castanea sativa; C-type vegetation) in roman times. When laurophylloid vegetation (L-type vegetation) invaded some of these chestnut systems during the last few decades, it caused detectable changes in organic chemistry. The invasion of the L-type vegetation was predominately due to increased winter temperatures. We tested whether these vegetation changes led to measurable long-term and short-term responses of the mineral matrix by comparing soils under Q-type with C-type vegetation (probing for long-term effects; > 100–2000 years) and soils under C-type with L-type vegetation (short-term effects; a few decades). To do so, we examined soil characteristics including the dithionite-, oxalate- and pyrophosphate-extractable forms of Fe, Al and Si as well as the phyllosilicate mineralogy of the clay fraction with a pair-wise comparison procedure. On L-type patches, short-term changes resulted in higher contents of secondary, poorly crystalline Fe. The accumulation of pedogenetic Fe probably results from a lower availability of organic complexing moieties under L-type vegetation and thus leading to a reduced removal. As soil acidity did not change with the vegetation types, a strong effect of organic compounds on Fe and Al chemistry must be assumed. A correlation analysis confirmed that metal binding to organic matter was different between L-type stands (correlation of Fe and Al with organic matter was less significant) and C-type stands. The differences in clay phyllosilicate assemblage between the different vegetation sites were rather small. There was, however, a trend towards higher contents of hydroxy-interlayered vermiculite (HIV) under L-type vegetation when compared to C-type sites. It seems that Al-polymers fixation in interlayers of 2:1 clay minerals was increased or their removal hindered at sites having L-type vegetation. Changes in the long-term (response to C-type vegetation) were only measurable for the pyrophosphate-extractable Si content (formation of phytolithe?). The colonisation of laurophyllous species led in the short-term to significant alterations of the soil system that were even more pronounced than the long-term effect of chestnut on soil quality.