PLANT-IMPACT ON THE BIOGEOCHEMICAL CYCLE OF SILICON AND RELATED WEATHERING PROCESSES

The contribution of plants to the biogeochemical cycle of Si and relat ed weathering processes was studied in an equatorial rainforest ecosys tem (Congo) where the biologic turnover of Si is high (58 to 76 kg/ha/ y). Litterfall leaves, a soil profile and groundwaters were analysed. Phytoliths and organic matter have a similar distribution with depth i n the soil profile. The model of a bicompartmental distribution of org anic matter is applied to phytolith distribution and shows that about 92% of the biogenic silica input is rapidly recycled while about 8% of the biogenic silica input supplies a stable pool of phytoliths, with a lower turnover. Reprecipitation of silica was observed at the base o f the soil profile, indicating a local geochemical environment that is oversaturated with respect to amorphous silica. A balance in biogeoch emical cycle of Si requires that the vegetation absorb dissolved silic on released from weathering of minerals, which otherwise would be avai lable for mineral neoformation or export from the profile towards regi onal drainages. Plant uptake of Si increases the chemical weathering r ate without increasing the denudation rate. This study shows that the uptake, storage, and release of Si by the vegetation have to be taken into account when using dissolved Si for tracing chemical weathering d ynamics. Copyright (C) 1997 Elsevier Science Ltd.