Recent Changes in Soil Chemistry in a Forested Ecosystem of Southern Québec, Canada

We analyzed the temporal trends of elemental changes in the soil of the Hermine, a 5.1-ha watershed of the Lower Laurentians, Quebec, Canada, from 1993 to 2002. The forest canopy of the Hermine is dominated by sugar maple (Acer saccharum Marshall) growing on Podzols formed in a shallow (<2 m) anorthositic till. The results show a significant long-term decrease of SO₄ concentration in the solution of both the LFH (−1.33 μmol L⁻¹ yr⁻¹; α = 0.05) and B (−0.78 μmol L⁻¹ yr⁻¹; α = 0.01) horizons. This SO₄ decline is associated with a reduction in dissolved Ca and Mg in the B (−1.83 and −0.38 μmol L⁻¹ yr⁻¹ for Ca and Mg, respectively; α = 0.001) and, to a lesser extent, in the LFH horizons (−2.09 and −0.69 μmol L⁻¹ yr⁻¹ for Ca and Mg, respectively; α = 0.01). Thus, the combined change in dissolved Ca and Mg in the B horizon not only follows that of SO₄ but it proceeds, on an equivalent basis, at a rate almost three times faster than that of SO₄ For SO₄, the concentration changes in solution are accompanied from 1994 to 2002 by a moderate depletion of the H₂O-soluble SO₄ pool in the podzolic B horizon of Zones A and C (mean rate for Zones A and C of −3.5 μmol kg⁻¹ yr⁻¹; α = 0.05). Indeed, SO₄ desorption from the B horizon occurs even under constant S deposition levels and seems to respond to changes in atmospheric deposition that occurred decades ago. In the case of exchangeable calcium and magnesium (Caₑₓ and Mgₑₓ), decreasing trends are present in the FH horizons but they are statistically significant only for Mgₑₓ in the B horizons of Zones A and C. A decrease of up to 50% of the Mnₑₓ pool is observed in the FH and B horizons, a decline that is partly balanced by an increase in Mn uptake by sugar maple since 1994. A tendency toward acidification is also noted in the solution (mean rate of +3.48 μmol H⁺ L⁻¹ yr⁻¹, α = 0.01) and in the solid phase (mean rate of +36.4 μmol H⁺ kg⁻¹ yr⁻¹, α = 0.001) of the organic horizons. The increase in exchangeable aluminum (Alₑₓ) in the FH horizons of zones B and C (mean rate for zones B and C of +0.16 cmol₍₊₎ kg⁻¹ yr⁻¹; α = 0.01) could reflect these decreasing pH trends. Finally, a recurrent seasonal pattern in exchangeable cations is observed where Alₑₓ increases at the expense of Caₑₓ, Mgₑₓ, and exchangeable manganese and potassium (Mnₑₓ and Kₑₓ) during the growing season. This study shows that long-term, seasonal, and episodic trends in soil properties create a complex temporal pattern that needs to be recognized and partitioned when assessing the response of soil materials to changes in environmental conditions.