L’altération des minéraux dans les sols forestiers du Bouclier Canadien : quels facteurs environnementaux affectent la variabilité spatiale et temporelle de la mise en solution des cations basiques?

Mineral weathering is an important nutrient provider for plants, and these processes play a key role in the neutralization of acid inputs in terrestrial and aquatic ecosystems. Silicate weathering and its coupling with the vegetation also constitutes a carbon sink and controls the long-term evolution of the climate. Despite the central role of mineral weathering processes in the overall biogeochemical cycle of elements, there is no standard method for quantifying the rates at which minerals weather. Moreover, the impact of environmental changes on these processes remains difficult to anticipate in natural envi-ronments. Mineral weathering results from the complex interaction of hydro-climatic (precipitation, temperature, soil moisture), biochemical (pH, organic matter, biological activities) and physical factors with soils or parental material. However, the multidimensional nature of mineral weathering has rarely been considered in studies aiming at analyzing the impacts of environmental variables on these processes in the natural environments. Furthermore, approaches that allow to analyze the integrated effects of several simultaneously acting factors have been little explored. Through this thesis, we have: 1) analyzed the spatial variability of historical and contemporary weathering rates of base cations (Ca, Mg, Na and K) in a wide geographical area in Quebec; 2) evaluated the agreement between three methods for estimating mineral weathering rates; 3) developed a new approach for estimating base cation weathering rates on an annual time scale in order to analyze temporal variability in mineral weathering rates; and 4) examined the impact of the watershed characteristics and environmental factors on the variability of weathering rates.   The results show that climate-related variables accounted for up to 51.6% of the spatial variation in contemporary weathering rates of base cations. Variables related to ion sources (lithology, sulphate deposition and soil properties) also explained a significant proportion of the variation. As for historical base cation weathering rates, soil properties, such as surface area and mineralogical composition, were the main determinants of their spatial variability in the study area. Regarding the temporal variability of base cation weathering rates, the results of this research show that they depend essentially on the evolution of hydro-climatic conditions. Overall, base cation weathering rates are positively affected by temperature, percent deciduous canopy cover, precipitation as rain, sulphate deposition, and soil surface area, while they are negatively affected by precipitation as snow and the number of frost days. These findings provide an original contribution to the understanding of mineral weathering processes in natural environments, and they are likely to improve knowledge about the potential impact of climate change on mineral weathering and hence on the ability of soils to supply essential plant nutrients to sustain long-term forest growth.

Keyphrases: podzols, climat, PROFILE, Mineral weathering, forest ecosystems, spatial variability, soil profile mass balance, watershed input-output budget, sequential extraction, environmental factors, climate, multivariate statistics