Vaclav Smil

The importance of global biogeochemical cycles is easily stated: all economic systems are just subsystems of the biosphere, dependent on its resources and services. The biosphere cannot function without incessant cycling of scarce elements needed for prokaryotic and eukaryotic metabolism.

The water cycle is the biosphere's most rapid and the most massive circulation. It is driven overwhelmingly by evaporation and condensation. Compared to the ocean, the living organisms have only a negligible role in storing water, and they are of secondary importance in affecting its flows. (Evapotranspiration supplies only about 10 per cent of all water entering the atmosphere.) Human activities have drastically changed some local and even regional water balances and pronounced anthropogenic global warming would accelerate the global water cycle. But, with the exception of globally negligible withdrawals from ancient aquifers, and water vapor from combustion, we do not add to the compound's circulating mass.

In contrast, human activities - above all the combustion of fossil fuels - have been introducing large amounts of carbon (C), nitrogen (N) and sulfur (S) into the biosphere. These elements are doubly mobile, being transported in water as ionic solutions or in suspended matter, and through the atmosphere as trace gases. Theirs are the three true biospheric cycles as they are dominated by microbial and plant metabolism. They involve numerous nested subcycles, and operate on time scales ranging from minutes to millions of years, as the elements may move rapidly among reservoirs or be sequestered (assimilated, mineralized, immobilized) for extended periods of time.

High mobility of C, N and S makes the three elements readily available in spite of their relative biospheric scarcity. But it also means that human interference in these cycles has become evident on the global level (for example, rising atmospheric concentrations of CO2, CH4 or N2O) and/or that it has major impacts on large regional or continental scales (atmospheric deposition of sulfates and nitrates). Environmental problems arising from these changes include potentially rapid global warming, widespread acidification of soils and waters, and growing eutrophication of aquatic and terrestrial ecosystems. These topics have been receiving a great deal of research attention in recent years (Turner et al. 1990; Schlesinger 1991; Butcher et al. 1992; Wollast et al. 1993; Mackenzie and Mackenzie 1995; Smil 1997; Agren and Bosatta 1998).

Finally, those mineral elements that form no, or no stable, atmospheric gases are moved through the biosphere solely by the sedimentary-tectonic cycle. Weathering liberates these elements from parental materials and they travel in ionic solutions or as suspended matter to be eventually deposited in the ocean. They return to the biosphere only when the reprocessed sediments re-emerge from ancient seabeds or from the mantle in ocean ridges or hot spots. We thus see mineral cycles only as one-way oceanward flows, with human activities (mineral extraction, fuel combustion) enhancing some of these fluxes, particularly by mobilization of heavy metals.

This chapter reviews the basics of C, N and S cycles. Because of the element's indispensable role in food production and the intensity of its anthropogenic mobilization, it also looks briefly at phosphorus (P) flows in the biosphere. In every case it stresses the extent of recent human interference - the essence of industrial ecology.

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