Sulphur isotope composition in tree rings
a possibility for a retrospective evaluation of atmospheric S input into ecosystems

Frieder Hofmann, Anette Giesemann, Ulrich Schlechtriemen


Introduction

Research on the influence of anthropogenic sulphur (S) pollution on ecosystems has been carried out for many years. Gaseous S species like SO2 and H2S can lead to severe damage if S is not readily incorporated into the plant’s metabolism or detoxified.
In order to distinguish between different S sources, stable S isotope analysis has shown to be a useful tool.
For example fossil fuels, a major source of anthropogenic S pollution in the atmosphere, often are enriched in d34S compared to S naturally present in ecosystems. As trees grow by building up one tree ring per year, they can be expected to reflect the respective S input situation over time. The S isotopic composition of tree rings therefore could provide information on changes in the S impact on forest ecosystems within the past years.


Methods

Tree ring samples from an oak tree (Quercus robur) - 120 years old - at a rural forest site in North Germany were analysed for their S isotope composition. Sample site and tree were chosen according to results of a previously carried out monitoring programme. Sequences covering periods of five years were cut out of a freeze-dried stem disc, homogenised, divided into 3 sub samples and S concentration was analysed using an automated S analyser (LECO SC132). S isotope composition was determined using the coupling of an elemental analyser (Carlo Erba NA 1500) to a stable isotope mass spectrometer (FINNIGAN MAT Delta S). The S isotopic composition is expressed as d34S value. The Mn/Ba-ratio resultet from a multi-element-analysis done by ICP-MS. Tree ring width was measured using the semi-automatic LINTAB/TSAP device.


Results and discussion


Please click to enlarge!

S concentration in the tree rings ( fig. 1 ) was more or less constant between 1880 and mid 1940, reflecting the “normal” level of S as essential nutrient. A steady increase was observed between 1950 and 1980, giving evidence of excessive amounts of S available for the tree. This increase was paralleled by a sharp upraise of the Mn/Ba ratio in the tree rings over a critical level (fig. 3), which indicates specifically that the soil pH dropped below 4,2 through acidification due to S stress already at that time. From 1985 on, the S concentration increased to a very high level. This increase is explained by accumulation of S through decomposition processes by infested fungii. The analysis of tree ring width shows in figure 4, that the sample tree as well as most of the trees in the stand got suppressed in growth at that time and finally died because of severe changes in site conditions (wetness).

The S isotopic composition ( fig. 2 ) in the tree rings shows clearly three trends over the years. The d34S value decreased first steadily from 1880 until about 1950. After 1950 the trend changes characteristically with d34S values raising slightly. This change is closely related to the general increase of S pollution due to the expanding industrialisation after World War II. The third trend is seen after 1970 with a strong increase in d34S values. It does not reflect the general trend of atmospheric S pollution. This trend is specifically due to S impacts from a local emitter, who has started working at that time. The change of the d34S value after 1987 belongs to the fungal decay processes.

Conclusions

The results proof, that the S concentration and the stable isotope ratio can be analysed successfully in tree rings. Both, S and d34S value show clearly trends over time, that reflect the history of atmospheric S impacts onto the ecosystem in respect to the local conditions. The results fitted well to data gained from an environmental monitoring programme. S concentration together with tree ring width and the Mn/Ba-ratio indicate on a biological effect level the natural, endogenic background. Both reveal, when limits in the forest ecosystem are going to be exceeded. The analysis of the S isotope composition allows to distinguish the different pollution sources of the S impacts.


The results are of importance for:
» Ecosystem research on sources and sinks of nutrients and pollutants in forest ecosystems.
» Case studies to distinguish the specific environmental impacts of pollution sources.
» Studies on Global Change of pollution effects during the past centuries.


Contact:
TIEM - Team Integrated Environmental Monitoring c/o
Dipl.-Biol. Frieder Hofmann, cert. EuroBiol ECBA
Ökologie Büro - Ecology Office

Rennstieg 25, D-28205 Bremen, Germany
phone: +49-421-706474 fax: +49-421-74106
e-mail: f.hofmann@oekologiebuero.de


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Erstellt am 30.01.03, Frank Müller