Quantification of the Effect of Plants on Weathering: Influence of a Residual Soil Layer on Chemical Fluxes

Abstract

The weathering of Ca and Mg silicate minerals on the continents has exerted a major control on atmospheric CO2 over geologic time, and vascular plants may have played an important role in this process (e.g. Berner, 1997 and references therein). Before the Devonian, the land may have been populated by algae or lichens (Retallack, 1990) which can affect weathering (Barker et al., 1997), but the very slow metabolism of these plants and the lack of a fine root mass with high surface area argues against their having as strong an effect as vascular plants in accelerating weathering (Cochran and Bemer, 1996). We have examined the role of plants in weathering by measuring the chemistry of waters draining adjacent portions of some basaltic rocks of western Iceland which are either 1) completely bare rock, 2) essentially unvegetated with soil cover, or 3) populated by trees. The study area was chosen to maximize vegetational differences and to minimize differences in microclimate, slope, aspect, and lithology, while avoiding hydrothermal waters and anthropogenic acid rain. In addition, the barren area without soil is compared to one with soil to assess the effect of residual soil cover on chemical fluxes. Results, which are corrected for evapo-transpiration and include data on cation uptake by growing trees, indicate that the rate of weathering release of Ca and Mg is 3 to 8 times higher in vegetated areas than in 'bare' ones. Field Area