Soil water leachate from two forested catchments on the Precambrian Shield, Ontario

Abstract

A predominantly coniferous catchment on the Precambrian Shield had significantly more acid, base-poor, and Al-rich soil solutions than a predominantly deciduous catchment. Eight-year median depth profiles of the forest floor solutions revealed that ceramic tension lysimeters collected significantly higher levels of Al and Si, lower amounts of NO3−, and equal amounts of dissolved organic carbon than plastic zero-tension lysimeters. There was no significant difference between lysimeter types in the deeper mineral soils. NO3−, NH4+, K+, organic C, organic Al, Fe, and to a lesser extent Ca2+ and Mg2+ were more abundant in forest floor solutions than in the mineral soils. Inorganic Al, F−, Na+, and SO42− were more abundant in the mineral horizons, and Cl− was uniform throughout the profile. Calculations based on the Na profile and the weathering of plagioclase feldspars suggested that secondary Al and Si minerals were accumulating in the mineral soil. Long-term median inorganic Al concentrations followed the microcrystalline gibbsite solubility curve (−log(Al3+) = −9.2 + 3.0(pH); R2 = 0.97), and F− was closely correlated (R2 = 0.7). NO3−, NH4+, and K+ in the forest floor exhibited the strongest seasonal patterns, with peaks during the winter–spring snowmelt and late summer. Their levels increased severalfold in response to summer drought, but there was little response in the lower horizons. Apparently because of a decline in SO42− and possibly Ca2+ deposition, there was a long-term decline in Ca2+ and SO42− in the stream draining the PCl mineral horizons (approximately −2.1 ± 0.4 μequiv.•L−1•year−1), and a similar Ca2+ decline in the mineral soil solutions themselves. However, there was no change in pH or inorganic Al levels.