Boreal Mixed‐Wood Watershed Riparian Zone Cation Cycling during Two Contrasting Climatic Years

Abstract

Riparian zones are key transfer points for water, alkalinity, and base cations from terrestrial to aquatic ecosystems. Base cation cycling in, and fluxes from, forested and alder swamp riparian zones were investigated for one climatically “normal” and one dry season. The soil extractable base cation pools and pH were higher in the alder riparian type. Combined measurements of litter, forest floor, mineral soil, and soil solution indicate that base cation uptake in both riparian zones occurs from the mineral soil. It is not known, however, whether uptake is from the upper (<20 cm) or deeper (>20 cm) mineral soil. Litter N, P, Ca, and Mg concentrations indicate that the forested riparian zone is more sensitive to base cation depletion than the alder riparian zone. Soil solution base cation concentrations in the forested riparian zone were attributed to Si mineral weathering and organic matter mineralization. In contrast, carbonate, probably calcite, weathering was more common in the alder riparian zone. Base cation stoichiometry was similar between years and riparian types, but the proportion of anions shifted to dominance by SO42− and organic acids during fall of the drier year, which resulted in higher fluxes of those anions. The anion shifts, however, were not reflected in base cation fluxes. Uncertainties in shifts in litter quality, mineralization, and hydrologic flux in response to temperature and moisture change require further documentation to adequately deduce the role of such changes, along with rainfall amounts, on base cation cycling in riparian zones.