Abstract. As atmospheric emissions of S have declined in the Northern Hemisphere, there has been an expectation of increased pH and alkalinity in streams believed to have been acidified by excess S and N. Many streams and lakes have not recovered. Evidence from East Bear Brook in Maine, USA and modelling with the groundwater acid-base model MAGIC (Cosby et al. 1985a,b) indicate that seasonal and yearly variations in soil PCO₂ are adequate to enhance or even reverse acid-base (alkalinity) changes anticipated from modest decreases of SO₄ in surface waters. Alkalinity is generated in the soil by exchange of H⁺ from dissociation of H₂CO₃, which in turn is derived from the dissolving of soil CO₂. The variation in soil PCO₂ produces an alkalinity variation of up to 15 meq L^-1 in stream water. Detecting and relating increases in alkalinity to decreases in stream SO₄ are significantly more difficult in the short term because of this effect. For example, modelled alkalinity recovery at Bear Brook due to a decline of 20 meq SO₄ L^-1 in soil solution is compensated by a decline from 0.4 to 0.2% for soil air PCO₂. This compensation ability decays over time as base saturation declines. Variable PCO₂ has less effect in more acidic soils. Short-term decreases of PCO₂ below the long-term average value produce short-term decreases in alkalinity, whereas short-term increases in PCO₂ produce short-term alkalization. Trend analysis for detecting recovery of streams and lakes from acidification after reduced atmospheric emissions will require a longer monitoring period for statistical significance than previously appreciated.

Keywords: CO₂ , alkalinity, acidification, recovery, soils, climate change