Evolution of the mineral fertility of an acidic soil during a period of ten years in the Vosges mountains (France). Impact of humus mineralisation

A very acidic soil under an adult spruce (Picea abies) stand, in the Basses-Vosges (North-eastern France), was analysed in 1986, and ten years later in 1996. The chemical composition of the mineral horizons did not show much variation, but the sum of exchangeable Ca and Mg in the mineral horizons and their total contents in the OL, OF and OH horizons decreased significantly. In ten years, the Mg in particular had decreased by about 30 kg center dot ha(-1), i.e. about 1.7% per year of the exchangeable Mg present in 1986. This decrease, which is worrying for the future, may be explained by peaks of humus mineralisation during warm summers combined with the inability of the mineral horizons, highly saturated in aluminium, to retain Ca and Mg coming from this mineralisation in the organic horizons. As a result, these elements were lost in drainage water instead of being taken up by the roots. As this soil was poor in weatherable minerals, the loss could not be compensated for by mineral decomposition. It is probable that a similar situation occurs at the end of a forest rotation when organic horizons are exposed to light and heat. Thus, the degradation of the humus from mull to dysmoder, followed by mineralisation, together with a high exchangeable Al content in mineral horizons and acid rain, all lead to losses of Mg and secondarily Ca at different stages of development of forest stands on acidic soils. Thus humus mineralisation, combined with a high exchangeable Al content, is one of the key mechanisms of soil acidification and impoverishment.