Forest decline inventories in Germany have documented continuously high percentages of moderate to highly damaged trees over the last decade (Waldschadensbericht, 1995). Some of the observed symptoms (e.g. needle yellowing) are believed to be caused by potassium and magnesium deficiencies in the acid soils and consequently also in the foliage (Hüttl, 1991). Magnesium shortages are often counteracted by application of dolomite-rich carbonate. This amelioration procedure, however, is known to provoke increased humus degradation and nitrification which elevates nitrate concentrations in seepage and groundwater, thereby endangering adjacent drinking water supplies. As a consequence, the application of sulfate-based Mg and K fertilizers has gained increasing interest over the last few years since comparable negative side effects have not been documented.
In order to assess the environmental consequences of K2SO4 applications in Norway spruce forests, an isotope tracer experiment was designed at the Höglwald site near Munich (Germany) where an equivalent of 70 kg S ha-1 was applied as K2SO4 solution to 960 m2 forest floor in July 1990. The two major objectives of the experiment were
(1) to test if potassium is effectively retained in the soil and consequently available for forest nutrition, and (2) to monitor the sulfur flux through the biosphere and pedosphere in order to quantifiy its acidification potential for soils and adjacent aquatic ecosystems. The SO42- was derived from Silurian gypsum with a d34S-value of +26, whereas S in soil and seepage water before application had d34S-values near 0. This large difference between d34S-values of fertilizer and ecosystem S was ideal for tracing sulfur fluxes and transformations in the ecosystem, since sulfur isotope discrimination appears to be small under aerated conditions (Krouse et al., 1991).
Seepage water was sampled monthly underneath the forest floor, and at 20 and 100 cm depths. Sulfur isotope data for seepage water SO42- collected beneath the forest floor indicated that the tracer passed through the litter within four months after application. At 20 cm depth, the d34S-values increased during the first half year of the experiment to +14 representing approximately 50% tracer SO42-. Thereafter, they decreased slightly, indicating that the tracer was effectively retained in the upper mineral soil. At the same depth interval, potassium was effectively exchanged predominantly for Al and to a lesser extent for Ca, Mg, and Mn. Neither elevated Al concentration nor labelled SO42- was detected in the seepage water at 100 cm depth throughout 2.5 years after application. Isotope measurements on total S in soil before and 11 months after the tracer application support the findings from seepage water analyses. Whereas minor increases in the d34S values of total soil S indicated little S immobilization in the humus layer and Ah horizons, shifts of up to 5 showed that most of the tracer was retained in the A horizons. No added S was detected isotopically below 40 cm depth. Isotope measurements on various soil S compounds indicated that the applied sulfur was preferentially bound inorganically either by sulfate adsorption or precipitation of sulfate minerals.
Because of the effective retention of potassium and sulfate in the upper mineral soil, the application of sulfate-based potassium fertilizers seems to be an effective and - in comparison to liming - a less risky amelioration procedure in forest stands with high potential for nitrification.
Bundesministerium für Ernährung, Landwirtschaft und Forsten, Waldschadensbericht 1995 (Bonn, 1995).
Hüttl, R. F., Freiburger Bodenkundl. Abh. 28, 1-440 (1991).
Krouse, H. R., Stewart, J. W. B. & Grinenko, V. A., In Stable Isotopes Natural and Anthropogenic Sulphur in the Environment (Krouse, H. R. & Grinenko, V. A., eds.) 267-306 (J. Wiley & Sons, Chichester, 1991).