Temporal and Spatial Variations in Soil Water Chemistry at Three Acid Forest Sites

As acid deposition declines, recovery from acidification is delayed by the fact that the soil processes that earlier buffered against acidification are now being reversed. Monitoring of within catchment processes is thus desirable. However, soil sampling is destructive and not suitable for long-term monitoring at a single site, whereas sampling of soil water with suction lysimeters may be more suitable. In this paper we evaluate 8–11 years of soil water chemistry from E- and B-horizons in three acid forest soil plots within monitored catchments. Five years of sampling also included the C-horizon. To our knowledge, this is the first long-term lysimeter study including the E-horizon showing recovery from acidification, and one of few studies including the B-horizon. Soil water concentrations of SO₄ decreased significantly between –9.5 and –1.4 μeq L^-1 yr^-1, with much higher rates of change at two southern sites compared to a northern site, where levels and changes of deposition were lower. The average annual bulk deposition of S ranged between 3 kg S ha^-1 at the northernmost site to 11 kg S ha^-1 at the southernmost site. The SO₄ decline in E-horizons was smaller than the decline in deposition, which indicated leaching of SO₄ from the O-horizon. At the two southern sites, a weaker decline in SO₄ in the B-horizon compared to the E-horizon indicated desorption of SO₄. The negative trends in SO₄ were to a large extent balanced by decreases in base cations but there were also tendencies of recovery from acidification in soil solution at the southern sites by increasing pH and ANC. However, these were contradicted by increasing Al concentrations. A high influence of marine salts in the early 1990s may have delayed the recovery. Decreasing trends of the Ca/(H⁺)² ratio in the soil solution, most pronounced at one of the southern sites, suggested that the soils were becoming more acidic, although the soil solution tended to recover.