Chapter 21: Global biogeochemical cycles
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...
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