Recent changes in soil chemistry in a forested ecosystem of Southern Quebec, 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 (Acersaccharum 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 mu mol L-1 yr(-1); alpha = 0.05) and B (-0.78 mu mol L-1 yr(-1); alpha = 0.01) horizons. This SO4 decline is associated with a reduction in dissolved Ca and Mg in the B (-1.83 and -0.38 mu mol L-1 yr(-1) for Ca and Mg, respectively; alpha = 0.001) and, to a lesser extent, in the LFH horizons (-2.09 and -0.69 mu mol L-1 yr(-1) for Ca and Mg, respectively; alpha = 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 mu mol kg(-1) yr(-1); alpha = 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 mu mol H+ L-1 yr(-1), at = 0.01) and in the solid phase (mean rate of +36.4 mu mol H+ kg(-1) yr(-1), a = 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); a = 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.