Evidence of current soil acidification in spruce stands in the Vosges Mountains, north-eastern France

To demonstrate directly soil acidification under spruce stands in the Strengbach catchment, soils from two adjacent stands aged 40 and 90 years were sampled intensively in 1990 and resampled in 1996. Soils already were very acid in 1990. Between 1990 and 1996, soils had experienced a significant decrease in exchangeable Ca2+ and Mg2+ at all depths at the two sites except in the 0-10 cm layer, for which base saturation remained constant. Losses of Ca2+ calculated from a budget study at the same sites and from the comparison of exchangeable stores were similar. In contrast, the loss of Mg2+ from the exchange complex was higher than that computed from the field budget. Various reasons, including most probably uncertainties linked to the extremely low levels of Mg in the ecosystem, may explain this discrepancy. Since 1987, a general decrease of the (Ca2+ + Mg2+)/Al3+ ratio in soil solution was observed at both sites. These results are consistent with present day acidification of soils poor in weatherable minerals under spruce stands in the Vosges Mountains.