Future Ecological Outlook

The use of nitrogen and phosphorus compounds, particularly as fertilisers, is likely to increase throughout the present century (Chapter 12.2). This will be necessary in order to feed the rapidly expanding world population, and the ecology of these two elements has become a subject of major importance. 

Nitrogen supplies are obtained from the atmosphere. The latter forms an inexhaustible reservoir because of a relatively short natural cycle of the element involving continuous bacterial processes of fixation, nitrification and denitrification and so forth (Chapter 11.5). The eventual depletion of readily available phosphate rock supplies, on the other hand, seems not unlikely. 

The build-up of phosphates in the oceans or on the ocean beds may, in a few centuries, make the ocean the most economic if not the sole convenient source of supply. As an alternative to the direct mining of sea bed apatite, however, it may become possible to develop species of edible marine plants which could obtain their phosphate directly from (shallow) ocean waters, thus ranoving the necessity for fertiliser manufacture as we know it today [74]. 

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In this chapter, we outline the issues and principles that are relevant to toxicity assessments of combined exposures. The scope of this overview is limited to combinations of chemicals, but excludes the topic of nonchemical stressors acting in concert with chemicals. Because the issues are of a generic nature, we draw on examples from human, environmental, and ecological toxicology. Section 3.2 briefly outlines approaches to mixture effects assessment (Chapter 4 elaborates these approaches in more detail), Section 3.3 discusses mixture effects in relation to modes and mechanisms of action, and Section 3.4 addresses the problems and possibilities of predicting mixture effects. In Sections 3.5 and 3.6, emphasis is on the predictability of synergism and on effects at low concentration or dose levels of chemicals in mixtures. Section 3.7 provides an overview of scarcely available data on mixture effects in real-world exposure scenarios. This chapter ends with an outlook to the future. 

All this leads to the following outlook Ethanol as a major fuel is presently not under consideration and will not be in the near future. For the Gasohol project in the USA today, ethanol is seen as an anti-knock-ing additive, which might be of interest as a substitute for methyl-tertiary butyl ether (MBTE), which is already expected to eventually cause ecological problems. In Brazil, there is some tendency to reduce alcohol production for economical and ecological reasons, but social considerations and the investment in 100% alcohol-fueled cars (once subsidized by government) do not allow a fast termination. 

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