Transitioning from anthropogenic to natural acidification in a humic catchment in Norway: projections of deposition and climate change effects

Abstract

Five decades of monitoring data (1974-2022) at the acidified, acid-sensitive forested catchment of Langtjern in southern Norway document strong chemical recovery and browning of surface water, related to changes in sulfur (S) deposition. We used the process-oriented model MAGIC to simulate water chemistry from 1860 to 2100 using historical and projected deposition and climate. New in MAGIC is i) a solubility control of dissolved organic carbon (DOC) from S deposition, which allows inclusion of the changing role of organic acids in chemical recovery, and ii) climate-dependency of weathering rates. MAGIC successfully described measured chemical recovery and browning, and the change towards organic acid dominated acidification status. Hindcasts of pH suggested lower preindustrial pH than previously modelled with MAGIC, simulated without sulfate-dependency of DOC solubility. Climate scenarios indicated substantially wetter climate, leading to increased base cation losses and slight reacidification of the surface waters. A sensitivity analysis of weathering rates revealed that a doubling of weathering rates is needed to reach pre-industrial ANC in 2100, given that S deposition is expected to be reduced to a minimum. We conclude that impacts of climate change are most likely to lead to slight reacidification of surface waters, and that enhanced weathering rates could partly compensate this trend.