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| Titel |
Geochemical and isotopic evolution of soil solutions over the last 25 years in a forested granitic catchment (the experimental Strengbach watershed case, France). |
| VerfasserIn |
M.-C. Pierret, J. Prunier, P. Stille, F. Chabaux |
| Konferenz |
EGU General Assembly 2009
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 11 (2009) |
| Datensatznummer |
250028235
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| Zusammenfassung |
| The upper most meter of the soil represents the most sensitive and the most reactive zone of
surface facing anthropological or global changes.
Indeed, the processes and the exchanges which take place in this zone, in particular
between soils, plants, waters, and atmosphere can be strongly modified by the disturbances of
the environment.
It is the reason why it is essential to study the various compartments of the soil and in
particular theirs time evolutions.
The soil solutions represent an essential vector of migration of elements within the soil
profile and until the deeper levels feeding springs and rivers. Besides, they also reflect the
atmospheric contributions and the processes such as the chemical weathering, the root uptake
or the biological recycling.
Our study presents the variations over more than 20 years of the elemental and isotopic
compositions of soil solutions along two profiles in a granitic environment undergoing acid
rains in the 60s-80s, as well as an extensive forestry development.
Two experimental plots were monitored, which are part of the Environmental
Hydro-Geochemical Observatory (OHGE; http://ohge.u-strasbg.fr) located on the small
granitic watershed of the Strengbach creek, in the Vosges Mountain (North-East of France).
Meteorological, hydrological and geochemical data are recorded since 1986. The
soil solutions and the soils were analysed (elemental and isotopic compositions) at
different depths, under beeches and spruces, and in both podzolic and acid brown
soils.
The data covering more than 2 decades show important evolutions with time in the
geochemical composition of the soil solutions from the both plots, both with respect to the
elementary data and the isotopic ratios.
For instance, the annual chemical flux in soil solutions of certain nutrients such as the Ca
strongly decreased in depth for more than 20 years, while they remained constant or
increased for lithogenic elements as Na or Si.
The rain and throughfall records show that the annual dry and wet atmospheric inputs in
Ca, Na, K, Mg, Si remain constant since 1986. Then the decrease of the Ca concentration
with time, in the two profiles, cannot be related to diminution of dissolution processes nor to
declining of atmospheric inputs.
At the same time, the evolution of the Sr isotopic ratios in soil solutions in depth
below 30 cm, which become more radiogenic, shows that the source of elements
also changed. Apatite, which is the main source of Ca in this system almost has
disappeared from the upper most levels of soils. The contribution to the flux of Ca from
secondary minerals such as clays or of exchangeable fraction increases. At the
present time, these phases represent a new main source of Ca, more radiogenic in
Sr.
All our results lead us to propose that the source of Ca in soils decreased for the past 20
years which raises the problems of the present-day nutrient availability in forested soils, such
as those developed in acid granitic bedrock. |
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