SCOPE 48 - Sulphur Cycling on the Continents

This book reviews and synthesizes information on the processes that govern the cycling of sulphur in the various ecosystems of the continents. Whenever possible, chapters have synthesized information both on rates of sulphur-cycle processes and on the controls of these processes. An understanding of these processes and their controls is essential to determine accurately the effect of anthropogenic sulphur deposition on ecosystem structure and function and to predict the role of altered sulphur cycling in global change. Current evidence indicates that biogenic sources of sulphur to the atmosphere, with the exception of marine sources of DMS, are much smaller than once thought, and so anthropogenic sources of atmospheric sulphur dominate (Chapter 2 by Whelpdale and Chapter 3 by Andreae and Jaeschke) .These anthropogenic sulphur emissions have started to decrease in North America and Europe and are predicted to continue to decrease, but large increases in Asia, Africa, and South America seem likely. A knowledge of sulphur cycling at the scale of the ecosystem is required to evaluate the recovery of areas already polluted by acid deposition (SCOPE 19) and the potential impact of further sulphur deposition and acidification in tropical countries (SCOPE 36).

Detailed and relatively accurate information on sulphur processes is generally now available for a variety of specific sites in a variety of types of ecosystems. However, measurements are usually made at a fine areal scale (a few square metres) , and a challenge exists in developing the ability to integrate these to regions, to continents, and to global basis (SCOPE 35). One approach may be to synthesize process information into credible mathematical simulation models that accurately mimic the behaviour of sulphur and natural ecosystems. Obviously we have much to do before we can achieve these aims, despite the large amount of study over the past two decades of sulphur and nutrient cycling, element interactions, acid deposition, and climate change. None the less, remarkable progress in understanding has been made, and we believe the synthesis in this book of the existing material from workers using different techniques and approaches will be extremely useful. We found that bringing this material together exposed many gaps in information, uneveness in progress, and sometimes a lack of communication between groups looking at the same problem from different angles or in different ecosystems. Thus much was gained from bringing groups together and discussing common problems at the Trent workshop. We hope that the readers of this book will profit from this synthesis.

Sulphur and sulphur compounds are not easy to determine analytically. Thus, from the onset of work on sulphur in soils and sediments, there has been much discussion and debate on analytical methodology. Despite progress in this area (Appendix by Tabatabai), further method development is needed for many questions, such as measuring the diurnal fluctuation of sulphur in the vicinity of a plant root under aerobic or anaerobic conditions, or accurately estimating fluxes of sulphur gases from point sources to the atmosphere. Similarly, the complexity of the interactions that occur in the decomposition of plant material in soils and sediments has not allowed us to determine the chemical nature of much of the sulphur that is held in organic material. Indeed, most methods of studying sulphur transformations rely heavily on isotopic methods to gain estimates of the fluxes of sulphur in such systems and do not attempt to measure the exact quantities of chemical compounds. These methods themselves are complicated, especially with regard to isotopic exchange among pools and the discrimination that occurs in reduction and oxidation processes between the heavier and lighter isotopes of sulphur. However, an understanding of the discrimination processes has allowed this particular technique to advance our knowledge of anthropogenic fluxes of sulphur in the environment (SCOPE 43; Appendix to Chapter 3 by Nriagu and Krause, this volume).

The analytical challenge of sulphur has been further complicated by the number of additional sulphides that have been discovered in the atmosphere during the last 15 years, such as dimethylsulphide, carbonyl sulphide and carbon disulphide. Thus, as our knowledge expands on the existence of natural sources of atmospheric sulphur compounds to the atmosphere, we require much more information on the processes of formation and decomposition that occur, both in the atmosphere and in terrestrial systems (Chapter 3). Andreae and Jaeschke (Chapter 3) summarize the most important naturally emitted sulphur compounds known at the time of writing and present best estimates of these fluxes. They emphasize three uncertainties regarding continental emissions of biogenic sulphur compounds. First, there is the difficulty of being able to determine these at low levels. Second, there are technical problems of either measuring point sources or estimating these across regions. Third, knowledge of the exact processes involved is incomplete, as are all the controls on these processes. Understanding these controls is complicated by the great diversity of interactions of the sulphur cycle with other element cycles (Chapter 4).

Despite problems and complications, substantial progress has been made during the past two decades, especially through intensive studies to understand the effects of anthropogenic deposition on lakes, wetland ecosystems, and upland forests. These studies have emphasized the critical role that sulphur plays in the biogeochemistry of these ecosystems.

Much progress has been made in our knowledge of sulphur cycling in marine and freshwater wetlands (Chapter 5 by Giblin and Wieder). In marine wetlands, the tidal fluxes of sulphur are very large in comparison to the rates of sulphur processing within the sediment, and rapid cycling occurs primarily through inorganic reduced forms of sulphur. Sulphate reduction is the dominant form of decomposition in these systems. Gaseous fluxes are high in comparison to any other ecosystems, but represent only a tiny fraction of a cycle within the wetlands. Also, because of the relatively limited area of these marine wetlands, their influence on the global atmospheric sulphur cycle is limited. The freshwater wetlands have a much lower sulphur input and the cycling of organic sulphur is much more important. One study, which examined the effect of increased sulphur deposition on a freshwater peatland, suggested that it may increase rates of anoxic decomposition and thereby decrease storage of organic carbon in the peat (See Appendix to Chapter 5 by Wieder et al. ). Increased inputs of sulphur to a wetland also clearly reduce the fluxes of methane from the wetland to the atmosphere. The chemistry of sulphur in wetland systems is complicated due to the wide range of oxidation states of sulphur, -2 to +6, the wide variety of both organic and inorganic sulphur compounds found, and the methodological problems already mentioned. Chapter 6 by Luther and Church integrates information on the chemistry of sulphur (emphasizing marine wetlands) and suggests major transfers among inorganic and organic species in natural ecosystems.

The research of the past two decades on sulphur cycling in lakes and other inland water bodies has been similarly encouraging. Freshwater lakes are among the ecosystems most obviously damaged by acid deposition. Research has clearly noted the resulting changes that occur in the sulphur cycle, the attendant changes in acid-base chemistry of lakes, the alteration of interactions with carbon, nitrogen, phosphorus, and other major elements, and the ability of the lakes to partially counteract acidification. Cook and Kelly (Chapter 7) point to the key features in freshwater lakes as being assimilatory reduction and uptake of sulphate in the water column by algae and bacteria, dissimilatory sulphate reduction by bacteria in anoxic zones, reaction of reduced S species with organic matter and reduced metals, and the production of sulphur gases likely to accompany decomposition of sulphur-bearing organic matter .

In Chapter 8, Lein and Ivanov explore another form of alteration of the sulphur cycle in aquatic ecosystems: the intensification of sulphate reduction which accompanies eutrophication in continental and coastal seas. The processes which occur are the same as in the freshwater lakes subject to acidification, but the magnitude and consequences of the processes differ. As with marine wetlands, sulphate reduction dominates the decomposition which occurs in the sediments of seas. Increased inputs of nutrients and organic carbon to these seas increase the rates of sulphate reduction, with a concomitant increase of hydrogen sulphide in the waters and rise in the depth of the anoxic-oxic boundary.

Chapter 9 by Mitchell et al. deals with sulphur dynamics in forest ecosystems and summarizes much of the information gained from studying the impacts of acid-sulphate deposition on forest vegetation and soils. The role that sulphur retention or release plays in affecting the flux of acidity to downstream ecosystems is now clear, as is the interaction of sulphur with other elemental cycles, especially nitrogen and basic cations. However , Chapter 9 notes that there are still inconsistencies in the measurement of dry and cloud-water deposition, and that there are still major unanswered questions with regard to sulphur retention and adsorption in soil. For example, the effect of sulphur storage in organic compounds has to be understood in terms of release rates over a longer period: is this organic sulphur held in a recalcitrant organic matter pool or can it be released quickly? The fluxes of sulphur gases from forests also remain poorly studied, particularly for tropical forests and for fluxes directly from forest vegetation.

Significant progress also has been made in understanding sulphur cycling in upland agricultural systems (Chapter 10 by Schoenau and Germida). We now have a basic understanding of the forms and amounts of sulphur in agricultural soils and of the processes controlling the supply of sulphur to plants. Again, the exchange of sulphur gases between the soil and plant system and the atmosphere is less well understood, as is the immediate and long-term effect of sulphur loading from the atmosphere. The authors of Chapter 10 suggest we still need a better understanding of the basic forms of sulphur in the soil, and of transformations through chemical and biological reactions. An emphasis is also needed in extrapolation from square metre to field and to region, and a requirement for more emphasis on the construction of models of mineralization and volatilization processes that include information on controls such as temperature, moisture, substrate and microbial composition response functions.

Managed lowland systems, specifically wetlands used in rice culture, offer many opportunities for expansion of knowledge. In many rice-paddy soils, high yield agriculture has stressed natural reserves of soil sulphur to an extent where sulphur deficiency is becoming a common occurrence (Chapter 11 by Lefroy et al. ) .Partially for this reason, research has concentrated on the supply of sulphur to plants and on the efficiency of fertilizer use, and much of the work reviewed in Chapter 11 concentrates on the efficiency of recovery by plants of applied fertilizer sulphur. In comparison with the other ecosystems discussed in this book, much less information is available on the processes of sulphur cycling within rice paddies. The focus of research on use of fertilizer sulphur provides practical short-term solutions to sulphur limitations, but it may not deal adequately with the long-term supply of this essential element. Also left relatively under-studied are the potential toxic effects of sulphur on rice and the effect of sulphur cycling on methane fluxes from rice paddies. We suggest that there should be a major effort towards research on sulphur processes and their controls in rice-paddy ecosystems, as there has been in other types of ecosystems, including managed upland agricultural systems. Such an emphasis on understanding sulphur cycling is especially needed as there has been a recent trend to re-examine organic amendments or incorporation of legumes in sustainable rice cultivation, a type of green manuring. This will require a better understanding of the dynamics of sulphur, nitrogen, and carbon in these systems. In our discussions at Trent, the synergism between those working on natural wetlands and cultivated wetlands gave rise to hopes that some of the methods recently developed in natural wetland studies could have application in cultivated wetland studies. Similarly, the managed nature of the rice wetlands provided the opportunity for understanding processes under more controlled conditions that could clarify the same processes occurring under natural conditions. This was a most encouraging aspect of the Trent workshop.

Finally, we conclude by noting that society has greatly altered sulphur cycles in a variety of ways, but the full magnitude of these is still unknown. Acid sulphur deposition has damaged forests and lakes, and the challenge has been accepted in industrialized temperate regions to begin to ameliorate the problem and restore damaged areas. Tropical areas are also known to be vulnerable to a greater or lesser extent, but questions arise as to our ability to control the magnitude of anthropogenic sulphur inputs to these systems and to limit their deleterious effects. Eutrophication of continental and coastal seas has increased the abundance of sulphides in the water column, to the detriment of life in these water bodies. And by developing high yield agriculture, we have stressed natural systems of sulphur supply and developed a need to add fertilizer sulphur in a manner that supplies plant needs without altering important soil quality characteristics and properties. Atmospheric sulphur comes principally from anthropogenic sources, and biogenic fluxes of sulphur from terrestrial ecosystems are thought to be relatively small. However, we do not have the capability to predict how they will change in response to added inputs of sulphur or other human influences. The major biogenic source of atmospheric sulphur appears to be oceanic DMS, but the exact magnitude of this flux and its control remain poorly known. We are unable at present to predict how this flux may change in response to climatic change or changing nutrient inputs to the oceans.