Geochemical and isotopic (Sr, U) monitoring of soil solutions from the Strengbach catchment (Vosges mountains, France): Evidence for recent weathering evolution

- Geochemical tracing of lithogenic fluxes in surface soil solutions
- Lithogenic origin of the colloidal-U in soil solutions
- Different geochemical time-series for soil solutions under spruce and beech
- Evidence of recent weathering changes
- Testing the transient vs. steady state of a forest ecosystem by U-Sr isotope ratios

This study presents the results of a geochemical and isotope (U and Sr) study of soil solutions from two experimental sites in the Strengbach catchment, one under spruce trees and the other under beech trees. The soil solution data, combined with the data from soil and vegetation samples, indicate that the “dissolved” U (filtered to 0.45 μm) in the soil solutions is primarily of lithogenic origin, implying that the colloidal U is secondarily associated with organic matter or organo-metallic complexes present in the soil solutions. The uranium activity ratios and Sr isotope ratios of soil solutions are used to decipher the nature of the minerals that control the lithogenic contribution to the soil solutions. A comparison of soil solution data collected over the period 2004-2006 with those collected over the period 1993-2010 indicates a decrease in the Ca and K exported by the soil solutions between 1998 and 2004 at the spruce site and a significantly smaller decrease at the beech site. In addition, the Sr isotope ratios of the soil solutions vary significantly between these two periods at the spruce site but not or much less at the beech site. Such observations suggest that the forest ecosystem at the spruce site is in a transient state associated with recent changes in weathering reactions. The data also suggest that the comparison of U and Sr isotopes in soil solutions and plant samples could be a relevant method for determining whether a forest ecosystem is in a steady or transient state.