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Forest, Range & Wildland Soils

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

^a Dép. de Géographie, Univ. de Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, QC, Canada, H3C 3J7
^b Dep. of Natural Resource Sciences, Macdonald Campus, McGill Univ., 21 111 Lakeshore Road, Ste.-Anne de Bellevue, QC, Canada, H9X 3V9
^c Dep. of Geography, Concordia Univ., 1455 de Maisonneuve W., Montréal, QC, Canada, H3G 1M8

^* Corresponding author (francois.courchesne{at}umontreal.ca)

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; = 0.05) and B (–0.78 µmol L^–1 yr^–1; = 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; = 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; = 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; = 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 (Ca_ex and Mg_ex), decreasing trends are present in the FH horizons but they are statistically significant only for Mg_ex in the B horizons of Zones A and C. A decrease of up to 50% of the Mn_ex 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, = 0.01) and in the solid phase (mean rate of +36.4 µmol H⁺ kg^–1 yr^–1, = 0.001) of the organic horizons. The increase in exchangeable aluminum (Al_ex) in the FH horizons of zones B and C (mean rate for zones B and C of +0.16 cmol₍₊₎ kg^–1 yr^–1; = 0.01) could reflect these decreasing pH trends. Finally, a recurrent seasonal pattern in exchangeable cations is observed where Al_ex increases at the expense of Ca_ex, Mg_ex, and exchangeable manganese and potassium (Mn_ex and K_ex) 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.

Abbreviations: Al_a, amorphous inorganic aluminum • Al_ex, exchangeable aluminum • BS, base saturation • Ca_ex, exchangeable calcium • CEC, cation exchange capacity • DOC, dissolved organic carbon • Fe_a, amorphous inorganic iron • K_ex, exchangeable potassium • Mg_ex, exchangeable magnesium • Mn_ex, exchangeable manganese