Type of article: Special features to Ecological Research, “The Future Environment and Role of Multiple Air Pollutants”.

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

Heleen A. de Wit^1,2*, François Clayer¹, Øyvind Kaste¹ and Magnus Norling¹

1 Norwegian Institute for Water Research, Oslo, Norway

2 Centre for Biogeochemistry in the Anthropocene, Department of Biosciences, University of Oslo, Norway

*corresponding author

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.

Keywords: organic acids, weathering, climate change, chemical recovery, forest

Reductions in anthropogenic emissions of sulfur (S) and nitrogen (N) to the atmosphere has resulted in wide-scale chemical recovery of surface waters in acid-sensitive regions in Europe and North America (Garmo et al. 2014; Skjelkvale et al. 2005). Reductions in sulfate (SO₄), rather than in N deposition have been the dominant driver of water chemical recovery since N deposition change has been less distinct than for S, and N-retention in catchments diminish its impact on elemental runoff (Watmough et al. 2005).

Chemical recovery is impacted by other factors than reduced SO₄ deposition and associated changes in inorganic cations and anions. Reduced air pollution has also led to widespread browning of surface waters (de Wit et al. 2021; Monteith et al. 2007), where other factors such as precipitation also impact dissolved organic matter (DOM) concentrations (de Wit et al. 2016). Increases in DOM are relevant for assessing chemical recovery of surface waters since increases in organic acidity partly compensate for declines in mineral acidity (Evans et al. 2008a), also illustrated by the organic-acid adjusted acid neutralizing capacity (ANCoaa) which better describes acidification in humic lakes than ANC only based on major ions (Lydersen et al. 2004). Factors that affect surface water acidification on a more local or regional basis are seasalt spray, through mobilization of protons and aluminium (Hindar et al. 2004), catchment disturbances such as tree die-back from insect attacks, leading to mobilization of S and N (Oulehle et al. 2021), and hydrology through post-drought mobilization of SO₄ (Clark et al. 2006) and variation in flow-paths (Evans et al. 2008b). Possibly, chemical recovery may be enhanced by climate-induced increased weathering rates (Augustin et al. 2015) resulting in higher rates of base cation replenishment in soils, which have lost base cations through decades of mobilization and leaching due to acid deposition (Watmough et al. 2005).

Because of the high success of historical emission reductions of S, the potential for further reductions in Europe will be limited (Grennfelt et al. 2020; Schopp et al. 2003). That implies that other factors such as climate and land use will become relatively more important for future chemical recovery of surface waters (Kopacek et al. 2016; Vuorenmaa et al. 2017) and for the time required for reaching pre-industrial water quality, if at all possible (Helliwell & Simpson 2010).

For assessment of future surface water acidification, the process-oriented model MAGIC (Cosby et al. 1985; Norling et al. this issue) is commonly used (Larssen 2005; Posch et al. 2019). MAGIC has in its core descriptions of acid-base soil chemistry and elemental mass balances and has been further developed to include forest N cycling and C storage which potentially enable simulation of catchment disturbances and climate change effects on element cycling and surface water chemistry (Valinia et al. 2021). A new version of MAGIC called MAGIC-Forest is implemented in the model development framework Mobius (Norling et al. 2021) enabling flexible sensitivity analysis and addition of model features (Norling et al., this issue) , Simulation of effects of ‘confounding factors’ on expected recovery of surface water is necessary for credible predictions in the current era where factors other than deposition will influence surface water acidification status (de Wit et al. 2023). Until recently, browning of surface waters was not implemented in MAGIC while weathering rates were calibrated to a constant value. In the current era of low S deposition, changes in organic acids and weathering are likely to become more important drivers of surface water acidity status.

Here, we aim to test the MAGIC model on its ability to describe five decades of acidification and recovery in the acidified, forested Langtjern catchment using new features in MAGIC, e.g. browning (sulfate-control) and climate-dependency of weathering, and predict acidification under further reductions of atmospheric deposition and climate change. Our research questions are the following: i. can we reproduce empirical charge balances at Langtjern, including estimates of organic charge; ii. Given observed increases in DOM at Langtjern, can MAGIC reproduce acidification and recovery, and produce credible hindcasts and forecasts of pre-industrial and future ANC under changes in acid deposition? iii. How high must weathering rates be to reach preindustrial ANC in 2100, given projections of climate and deposition? The model calibration will be anchored in five decades of streamwater monitoring at Langtjern.