The Biogeochemical Cycling of Manganese in a Forested Ecosystem of the Canadian Shield.
Nathalie Gingras, Dépt de geographie, Univ de Montréal, 520, Côte Sainte-Catherine, C.P. 6128, Succ.Centre-ville, Montréal, QC H3C 3J7, Canada and François Courchesne, Dépt de Géographie, Univ de Montreal, 520 Côte Sainte-Catherine, C.P. 6128 Succ. Centre-ville, Montréal, QC H3C 3J7, Canada.
Recently, a decline reaching 50% of the exchangeable manganese (Mn) pools in the FH and B horizons was measured in the Hermine watershed (45°49' N, 74°01'W) for the period 1993 to 2002 (Courchesne et al., 2005, SSSAJ, 1298-1313). Considering the fact that Mn is an essential nutrient for the photosynthesis of the plants, this severe change in the stock of available Mn could have major consequences on the health of this forested ecosystem. In order to increase our understanding of Mn distribution and movement within the ecosystem, the objective of this project is to quantify the cycle of Mn, for the years 2003 to 2005, by 1) measuring the fluxes of dissolved Mn between the biogeochemical compartments of the Hermine and 2) establishing the impact of topographic position on the annual cycle of Mn. The 5 ha Hermine watershed is located 80 km north of Montreal (Canada) and receives 1100mm of precipitation per year. The vegetation is composed of Sugar maple (Acer saccharum), American beech (Fagus grandifolia) and Yellow birch (Betula alleghaniensis). Soils are classified as sandy orthic or gleyed humo-ferric and ferro-humic Podzols.The watershed is divided into six 300m2 sampling sites; three at an upslope position and three others downslope close to the stream channel. The sampling of precipitation, throughfall, soil and stream solutions is performed, at all sites, every two weeks. Within 24 hours, the solutions are filtered at 0,45 μm and acidified with 2mM EDTA. The concentrations of Mn dissolved are measured using ICP-AES or ICP-MS. Hydrological fluxes between each biogeochemical compartment are also determined. The following preliminary results are for the year 2003. They indicate that Mn fluxes are generally highest in throughfall and under the organic soil horizons. Furthermore, a temporal relation appears to link the concentrations of Mn in the stream, stream flow and the pH in the stream (r2=0.65 à 0.86, α=0.01). Significant seasonal differences, between the growing and the senescence periods, are also noted. These annual changes are mostly induced by the biological activity such as the decomposition by the microorganisms and the bioaccumulation. Only the organic horizons are significantly influenced by the topography. It is caused by the variability of the tree species and the accumulation of water at the downslope position. In general, the Mn budget of the watershed is positive, consequently there is an accumulation of Mn within the ecosystem on an annual basis. So, the decline in soil exchangeable Mn seems to be more strongly associated to increased absorption by the vegetation rather than by the leaching of soil horizons. This study will contribute to our understanding of the response of forested ecosystem to environmental changes such as variations in climate conditions and trends in the chemistry of atmospheric deposition.