Evaluation of sulphur cycling in managed forest stands by means of stable S-isotope analysis

Sulphur cycling was evaluated in a 20 to 60 year old Norway spruce (Picea abies L. Karst) ecosystem in the Black Forest near Schluchsee, SW Germany, by means of stable sulphur isotope analysis.Soil and plant material were analysed for S-content and S-isotopic composition to gather information on the S-distribution in the ecosystem. Two out of three adjacent watershed areas, highly comparable to each other were fertilized with MgSO₄ and (NH₄)₂SO₄ respectively, where sulphate was enriched in the ³⁴S-isotope compared to the sulphur present in the ecosystem. As the fertilizer S served as a tracer, comparison of the S-isotopic composition of total and inorganic S in the soil and S in spruce needles from both the treated and the control sites led to new information of S-turnover processes.The S-isotopic composition of spruce needles changed markedly after the fertilizer application. Within half a year a shift towards the S-isotopic composition of the fertilizers sulphate indicated uptake of the sulphate by the trees, although this uptake did not become visible with the S content of the needles.Regarding the soil, a shift in the S-isotopic composition of the total sulphur was not that striking as with the needles, although the phosphate extractable sulphate showed a clear shift towards the S-isotopic composition of the fertilizer sulphate.