Predicting reversibility of soil acidification in beech (Fagus sylvatica) stands

Comparison between chemical parameters of soil from the infiltration zone of stemflow near the base of the stem and from the between trees areas in 152 old beech stands by Lindebner (1990) in the Vienna Woods (sample collection in 1984/85) proved a significant impact of deposition of atmospheric pollutants: soil acidification, increased sulfur (S) contents and loss of the base cations calcium and magnesium. Meanwhile, SO2 emissions, the most important precursor of Acid Rain, declined sharply since the 1980s. However, revisiting the Acid Rain topic is worthy, since in many regions mass balance estimates of S are negative due to release of previously stored S, delaying the recovery of pH of soils and surface waters, depending on soil properties. Preliminary results after re- sampling of 19 of these 152 sites showed a clear trend of top soil pH increase, with higher trends of recovery for the stem area than for the between trees area. Changes of soil chemistry with increasing distance from the stem in the Flysch Vienna Woods were measured by Sonderegger (1982) and Kazda (1983) in the 1980s and are well documented. The working hypotheses are: a) soils recovered from acid deposition due to a strong decline of SO2 emissions since the 1980s; b) reversibility of soil acidification is a function of acid loads, received in the past, and time; c) supply of historically lost base cations helps to define and to evaluate recovery over time; d) the net S- balance is crucial for predicting recovery and is related to S input and soil parameters. According to the four stated hypotheses the methodological part will focus on these four lines as well: i) a very straight forward approach by comparing historic soil chemical parameters of the 1980s with a recent survey of the same, approximately 90 beech stands within the Vienna Woods. ii) Using the spatial heterogeneity of soil columns at five distances between 27 and 300 cm (at 3 soil depths) downhill of individual beech stems at the same 3 nutrient-rich sites on Flysch (Sonderegger, 1982; Kazda, 1983) and at 3 nutrient-poor sites on Molasse enables the study of reversibility of soil acidification as a function of historic acid loads (stem area received much higher deposition loads in the past than the between trees area) and time (a "false chronosequence" is expected, since increasing soil solution fluxes due to additional stemflow with decreasing distance from the stem cause a quicker steady state of soil sulfate pools in response to decreasing inputs). These predictions will be compared with dynamic acidification modelling using PnET-BGC with special emphasis on the fact, how iii) resupply of lost base cations (experimental additions in the mid 1990s, Ca-pump of beech, weathering) effects recovery parameters over time. iv) Since solute sulfate fluxes through these columns and the corresponding net S balances are crucial for predicting soil recovery, various S sources will be evaluated, including the use of stable S isotopes.

From the beginning of the twentieth century, sulfur (S) emissions and deposition increased steadily and S deposition in forested ecosystems of Europe peaked in the early 1980s. As a transboundary pollution issue, legislation to reduce acidifying emissions has taken place at an international level. E.g., in Austria, SO2 emissions declined from 1980 (385.000 t) to 2013 (17.000 t) by 95%. Billions have been invested in cleaning up the emissions that cause Acid Rain, so it is politically worthy to know how much improvement has been achieved. Revisiting the Acid Rain topic is scientifically worthy, since in many regions mass balance estimates of S are negative due to release of previously stored S, delaying the recovery of pH of soils and surface waters.We resampled 97 old-growth beech stands in the Vienna Woods. This study exploits an extensive data set of soil (infiltration zone of stemflow and between trees area at different soil depths) and foliar chemistry from three decades ago. Stemflow of beech (Fagus sylvatica) represents a high input of water and elements, which is why deposition of acidifying substances may be significantly higher close to the stem compared to areas affected by throughfall only. It is concluded that the top soil will continue to recover from acidic deposition. However, in the between trees areas and especially in deeper soil horizons recovery may be highly delayed. The beech trees of the Vienna Woods showed no sign of recovery from acidification although S deposition levels decreased. Release of historic S even increased foliar S contents. Base cation levels in the foliage declined. Further nutritional imbalances will predispose trees to vitality loss.In addition, soil was sampled uphill (-55 cm) and downhill (five distances at 27, 55, 100, 150 and 300 cm) of individual beech stems at three nutrient rich sites on Flysch and three nutrient poor sites on Molasse and chemically analyzed (forest floor and mineral soil down to 50 cm depth). Throughfall and soil solution was collected bi-weekly. We concluded that net mineralization of organic S compounds on Flysch (top soil) and desorption of historically deposited S on Molasse (deep soil) are the main S sources, controlling sulfate leaching. These soil samples were characterized for S fractions (e.g., adsorbed sulfate-S and organic S) as well, indicating clear S distribution patterns with respect to the distance from the tree stem base. After setting up a new method, stable isotopes of S were measured in the sulfate of the solute samples. Lower ?34SVCDT values in the soil solution than in throughfall indicates that a considerable portion of the atmospherically deposited sulfate is cycled through the organic S pool before being released to the soil solution.Finally, we set up a new sulfur biogeochemistry model, adjusted to the above three beech stands on the nutrient rich soils on Flysch. The model succeeded to reproduce the current soil S pool, as well as an observed time series of sulfate concentration in soil solution, allowing an assignation of fluctuations of solute sulfate to actual biogeochemical processes. The model identified the microbial interplay of S immobilization and mineralization as a major driver of the short time dynamics of solute SO4 in the soil. A net S release, responsible for the observed output surplus, seems to be caused by the mineralization of more stable organic sulfur, originating from plant biomass. It will take a very long time till the system will approach a new steady state (Input = Output).