SCOPE 48 - Sulphur Cycling on the Continents

The Scientific Committee on Problems of the Environment (SCOPE) of the International Council of Scientific Unions (ICSU) launched a major series of projects on biogeochemical cycles in 1974. Initially these projects focused on the study of the cycles of carbon, nitrogen, sulphur, and phosphorus separately. It was however understood from the beginning that none of these cycles operates independently from the others and that attempts would be made to include studies of elemental interactions in each unit's activities. Much of the work on individual cycles has been coordinated by the SCOPE/ UNEP international units working separately: carbon, under the direction of the late Professor Egon Degens in Hamburg, Germany; nitrogen, under the direction of Professor Thomas Rosswall in Uppsala, Sweden; sulphur, under the direction of Academician Mikhail Ivanov in Moscow, USSR; and phosphorus under the direction of Dr C. Vernon Cole in Fort Collins, USA. SCOPE also has sponsored efforts to have groups of scientists unravel many of the mechanisms of mutual interactions (SCOPE 17, edited by G .E. Likens, 1981, and SCOPE 21, edited by B. Bolin and R.B. Cook, 1983). Also various international units have met jointly to explore specific interactions between two element cycles, as between carbon and sulphur (SCOPE 42, edited by Degens et al. 1991, Biogeochemistry of Major World Rivers). 

The SCOPE/UNEP International Sulphur Unit's first investigation, The Global Biogeochemical Sulphur Cycle (SCOPE 19, edited by Ivanov and Freney, 1983), brought together information on sulphur in all aspects of the globe and attempted to construct a simulation model of the modern sulphur cycle. Since many of the processes of global sulphur fluxes were not adequately related to their controls, the focus of the book was on areas where knowledge of processes and effects (actual or potential) of anthropogenic inputs of sulphur on the environment were poorly understood. The second phase of the SCOPE/UNEP International Sulphur Unit, started in 1982 and continuing until 1986, had two important tasks. The first was to examine the information on the global cycle and its perturbations that could be obtained from the sulphur isotopic composition of various reservoirs, fluxes and ecosystems. This material has been published as SCOPE 43, 1991, edited by H. R. Krouse and V. A. Grinenko, entitled Stable Isotopes: Natural and Anthropogenic Sulphur in the Environment. The second activity was a meeting in Tallinn, Estonia, which drew together information on the evolution of the sulphur cycle, allowing the current anthropogenic perturbations to be placed in a context related to past changes over geological time. This material was published in 1989 as SCOPE 39, edited by P. Brimblecombe and A. Yu. Lein, entitled, Evolution of the Global Biogeochemical Sulphur Cycle. 

The third and final phase of the activities of the SCOPE/UNEP International Sulphur Unit was started in 1986 and focused on the cycling of sulphur in continental ecosystems. Continental was interpreted as including soils, wetlands, inland lakes, rivers, and estuaries. Sulphur cycling in oceans was dealt with in part in SCOPE 39. 

Three meetings were held as part of this third phase. Two of these were in conjunction with the SCOPE/UNEP International Carbon Unit. The first, a joint workshop to deal with interactions in estuaries, was held at Texel in the Netherlands in September, 1987. The second, also a joint meeting between the International Carbon and Sulphur Units, entitled, The Biogeochemical Carbon and Sulphur Cycles in Lakes and Water Bodies, was held at Irk tusk, Lake Baikal, in September 1988. The main objective of this workshop was a discussion and synthesis of the existing information on biogeochemical processes and fluxes of elements in lakes and water bodies of different types. Attempts were made to identify knowledge gaps in data on biogeochemical processes, and to discuss the evaluation of existing methods for investigation and modelling of freshwater aquatic ecosystems. The information accumulated from the first two workshops was synthesized and published as Interactions of Biogeochemical Cycles and Ecosystems, edited by E. T. Degens, S. Dempe, A. Lein, and Y. Sorokin, University of Hamburg Press. 

The third UNEP/SCOPE workshop in this series was held at Trent University, Peterborough, Canada in May 1989. This workshop was designed so that in addition to an overview of atmospheric sulphur and biogenic fluxes of sulphur to the atmosphere, that major focus would be on sulphur cycling processes and fluxes in natural and cultivated wetlands, forest and agroecosystems. The outcome of this third meeting is the current book. It includes some material from the earlier meetings at Texel and Lake Baikal. 

In organizing this workshop, it was realized that there was a need for scientists working in different disciplines and in different types of ecosystems to share information with one another. For instance, the community of scientists working in agricultural systems and that working in natural ecosystems do not always have the opportunity to interact, and therefore processes that are important in one system are not seen to have the same relevance in another. Most of the earlier work on the sulphur biogeochemistry of cultivated soils was carried out in regions where sulphur was a limiting nutrient, and attempts focused on understanding the natural cycling of the element before adding extra sulphur as a fertilizer. In areas of Australia and western Canada, an initial emphasis was on understanding the requirements of important agricultural crops such as legumes and/or oilseed crops, with some study of microbial processes in cycling sulphur in plant residues through the soil making it available as a nutrient. Similarly, studies on the application of elemental sulphur in soils concentrated on the rate at which it would be oxidized to the form more available to plants, and the microbial populations that aided this process. This was in complete contrast to another body of knowledge which had focused on the effect of anthropogenic additions of sulphur to natural forests and lakes from acid rain. Here the emphasis was not on the lack of sulphur in the ecosystem, but with excess sulphur. In some cases, the amounts of sulphur cycling through decomposer organisms was insignificant in comparison to the capacity of the soil to maintain its viability under heavy sulphur excess loading. Unless brought together to discuss the complete sulphur cycle, these two groups of scientists would rarely meet.

Similarly, the questions that were asked of scientists working in wetlands were completely different. In cultivated wetlands which are mostly used in rice production, the emphasis has again been on the efficient supply of sulphate and the efficiency of use of applied sulphur fertilizer. On the other hand, research on marshes and other wetlands has tended to be much more comprehensive and much more ambitious. A driving force behind research in natural wetlands has been an understanding of the overwhelming importance of sulphate reduction to the metabolism of these systems. In general, the emphasis of research in natural wetlands has been on microbial and biogeochemical processes and their controls rather than on the net effect of a sulphur input over the 90-day period of growth of a rice crop. Also, much more attention has been paid to diurnal changes in wetland conditions and properties, and the resulting effect this has on both emissions of sulphur to the atmosphere and in change of sulphur forms in the soil. For freshwater wetlands, it has been extremely important to understand sulphur cycling and fluxes in the context of how they have been affected by inputs of acid rain. These two groups of scientists, both working in wetlands, have rarely been in close contact. This workshop brought them together for such interaction.

A common problem faced by all scientists studying the sulphur cycle is the difficulty of the analytical chemistry of sulphur. This element is difficult to analyse accurately in pure solutions, and extremely difficult to understand in a heterogeneous system with solids and/or a changing oxidation-reduction potential. Our understanding of sulphur biogeochemistry has often been limited by these difficulties, and the literature is full of methodological disagreements. Scientists working in different types of ecosystems have frequently used different analytical methods for sulphur, often with different and contradictory underlying assumptions. This workshop allowed a cross- fertilization among these disciplines and furthered communication towards better approaches. There is always a need to use new developments in analytical chemistry, and/or isotope technology, to comprehend fully the processes associated with sulphur transformations under oxic or anoxic conditions, and to develop and use methods that give unequivocal results.

The workshop consisted of a mixture of lectures, discussions and small meetings of working groups. The insights developed by the working groups have been included in chapters of this book, and the chapters have benefited from this collective wisdom. The working groups were at least partly successful in breaking down some of our traditional disciplinary boundaries. We hope that this volume will further break down these boundaries and lead to progress in understanding the interaction of sulphur with other element cycles in a variety of ecosystems. Different disciplines use different units for presenting data on the sulphur cycle. To further communication, in this volume information in each chapter is provided in both mass and molar units.

The continuous support provided over the years by the United Nations Environment Programme has ensured the successful development of this study. We are indebted to the participants in the meeting for their interest and free discussion. Each of the chapters received critical comments from outside reviewers, and these reviewers were also major contributors to the overall book. This book would not have been possible without the capable editorial assistance of Ms Diane Sherman.