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Research activity associated with various observations at the Strengbach catchment in the Vosges Massif (880-1150 m) addresses many questions in the domains of hydrology and geochemistry. The catchment is the observation and experimental site of the Observatoire Hydro-Géochimique de l'Environnement appointed by the Centre National de la Recherche Scientifique. It also belongs to the research facilities that form the French Network of Critical Zone Observatories (OZCAR), which supports a network of critical zone observatories. The catchment is small (0.8 km 2 ) with steep slopes (20-30%) on granitic bedrock that mainly allow for forestry (spruce and beech stands) as the main land cover. Meteorological, hydrological, and geochemical data have been monitored since 1986. The first studies conducted were dedicated to the elucidation of acid rain effects on forest ecosystems and particularly on forest decline. Multidisciplinary research studies conducted on the Strengbach catchment enable exploration of the following issues: (i) hydrological functioning at the scale of a small catchment and questions regarding the evolution and preservation of the water resources in mountainous environments (stock, recharge, infiltration, and water pathways), (ii) exchange processes observed at the soil-plant-atmosphere continuum and in particular weathering processes and the evolution of soil mineral fertility (Ca, Mg, K, P), (iii) processes responsible for the export of water and for associated fluxes (dissolved chemicals, suspended materials, bed loads) and their dynamic at the outlet, and (iv) responses of the ecosystems to environmental disturbances (acid rain, forest management, and climate change) and their current and future modeling. University of Strasbourg), is well suited for addressing such issues owing to (i) a long-term dataset available for the area (from 1985;Probst et al., 1987), (ii) the availability of field equipment and monitoring devices, and (iii) numerous scientific studies that have been performed on the area. Research performed at this catchment, which is located in the Vosges Mountains in northeastern France, has focused on the evaluation of water and soil resources in relation to climatic changes (i.e., rainfall regimes and atmospheric chemistry trends) and anthropogenic patterns (e.g., forest management and acid precipitation) occurring in this moderate-altitude mountainous region.Usually, in this type of ecosystem, water is scarce and not evenly distributed, and stocks may rapidly vary over time. In addition, although wood production is an economically important activity in these regions, soil degradation (e.g., from acidification and nutrient depletion) modifies soil fertility features and thus affects the development of forests.The scientific objective of the OHGE is to develop a detailed understanding of water transport processes and of related chemical fluxes (including nutrients and contaminants) through the critical zone extending from the near atmosphere to the near subsurface. The main purposes of...

Modeling of Water-rock interaction in the Mackenzie Basin: competition between sulfuric and carbonic acids.

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[1] Anthropogenic sources are widely accepted as the dominant cause for the increase in atmospheric CO 2 concentrations since the beginning of the industrial revolution. Here we use the B-WITCH model to quantify the impact of increased CO 2 concentrations on CO 2 consumption by weathering of continental surfaces. B-WITCH couples a dynamic biogeochemistry model (LPJ) and a process-based numerical model of continental weathering (WITCH). It allows simultaneous calculations of the different components of continental weathering fluxes, terrestrial vegetation dynamics, and carbon and water fluxes. The CO 2 consumption rates are estimated at four different atmospheric CO 2 concentrations, from 280 up to 1120 ppmv, for 22 sites characterized by silicate lithologies (basalt, granite, or sandstones). The sensitivity to atmospheric CO 2 variations is explored, while temperature and rainfall are held constant. First, we show that under 355 ppmv of atmospheric CO 2 , B-WITCH is able to reproduce the global pattern of weathering rates as a function of annual runoff, mean annual temperature, or latitude for silicate lithologies. When atmospheric CO 2 increases, evapotranspiration generally decreases due to progressive stomatal closure, and the soil CO 2 pressure increases due to enhanced biospheric productivity. As a result, vertical drainage and soil acidity increase, promoting CO 2 consumption by mineral weathering. We calculate an increase of about 3% of the CO 2 consumption through silicate weathering (mol ha −1 yr −1 ) for 100 ppmv rise in CO 2 . Importantly, the sensitivity of the weathering system to the CO 2 rise is not uniform and heavily depends on the climatic, lithologic, pedologic, and biospheric settings.

Weathering of silicate minerals in the Critical Zone (CZ) is fundamental for numerous environmental and societal issues. Despite decades of efforts to accurately record biogeochemical variables controlling mineral reactivity in the field and to reproduce them in the laboratory, weathering rates estimates still differ from those observed in natural settings.Here we examine the biogeochemical environment of mineral surfaces exposed to contrasted weathering conditions in various compartments of a temperate CZ (Strengbach observatory, France). A novel approach was developed to probe both in-situ mineral dissolution rates and bacterial diversity associated to mineral surfaces. Labradorite and olivine minerals were either buried in the A and C horizons of a soil profile, directly exposed to meteoric fluids or immersed in stream water. Dissolution rates recorded in the soil profile were up to 2 orders of magnitude slower than those predicted using a numerical weathering model. Samples directly exposed to meteoric fluids exhibited contrasted dissolution rates that could not be explained by simple abiotic weathering, while dissolution rates of samples incubated in stream water were particularly low. In soil profiles, the field-laboratory discrepancy by up to 2 orders of magnitude was attributed to heterogeneity of fluid circulation and local variation of reaction conditions.Mineral substrates changed bacterial communities of the mineralosphere after 9 and 20 months of incubation in the CZ. However, we observed that this effect could be delayed or driven by extrinsic factors. Although mineral probes in soil horizons were enriched in bacterial phylotypes potentially involved in mineral weathering (e.g., Pseudomonas sp., Collimonas sp., Burkholderia sp., Janthinobacterium sp., Leifsonia sp., and Arthrobacter sp.), the relative contribution of biotic weathering could not be quantified in-situ. Altogether, the heterogeneity of in-situ mineral dissolution rates in key compartments of the CZ reveals the crucial need to improve spatial characterization of hydrogeochemical properties at the soil profile scale, and to evaluate the quantitative role of microbial communities to mineral weathering.