ABSTRACT: The response of soils to environmental changes in a forested watershed of Canadian Shield.

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Abstract

We analyzed the temporal trends in the solid and liquid phases of the soil in response to environmental changes and fluctuations. The study was conducted in the Hermine, a 5.1 ha watershed of the Lower Laurentians, Québec (45o 59’ N, 74o 01’ W, altitude ~ 400 m). The forest canopy is dominated by sugar maple (Acer saccharum), American beech (Fagus grandifolia) and Yellow birch (Betula alleghaniensis) growing on Podzols formed in a shallow (< 2 m) anorthositic till. Soil solutions (LFH and B horizons every two weeks) and soils (FH and B horizons every two months) were collected from May to November in nine 300-m2 plots (three zones) from 1993 to 2005. The results show a significant long-term decrease of dissolved SO4 concentrations in both the LFH and B horizons. Sulfate release from soils presently occurs even under constant S deposition levels and, thus, seems to continue to respond to changes in atmospheric deposition that occurred in the 1980s. This SO4 decline is associated with a significant reduction in dissolved Ca and Mg concentrations in the B horizon but not in the LFH horizons. The combined change in dissolved Ca and Mg in the B horizon not only follows that of SO4 but it proceeds at a rate three times faster than that of SO4.For SO4, the concentration changes in the soil solutions are accompanied by a moderate but significant depletion of the H2O-soluble adsorbed SO4 pool in the podzolic B horizon of two of the three zones of the watershed. As for Ca and Mg, decreasing trends in exchangeable forms are present but they are not systematically significant. A tendency towards soil and solution acidification is noted in the organic horizons but not in the B horizon. The long-term increase in exchangeable Al in LFH horizons could reflect these downward pH trends. A strong decrease in exchangeable Mn that reaches up to 50% of this pool is also documented in the FH and B horizons. The decline in soil Mn is partly balanced by the higher Mn uptake and accumulation in sugar maple trees since 1994. On a seasonal basis, a recurrent pattern in exchangeable cations is observed where exchangeable Al increases systematically at the expense of Ca, Mg, Mn and K during the growing season. The trend apparently reverts as plant uptake decreases at the onset of the dormant season and as nutrient accumulate back at the soil-root interface. At the event scale, dissolved cation and anion concentrations in soils quickly respond to precipitation events following prolonged dry spells as accumulated decomposition and weathering products are flushed by infiltrating waters. This study shows that long-term, seasonal and episodic trends in soil properties create a complex temporal signal that needs to be recognized and partitioned when assessing the response of soil materials to changes in environmental conditions.

Bibliography of Canadian Geomorphology