Chapter 12 - The Global Cycles of Nitrogen, Phosphorus and Potassium

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Nitrogen often limits the rate of net primary production on land and in the sea. The most abundant form of nitrogen at the surface of the Earth, N2, is the least reactive species. Nitrogen fixation converts atmospheric N2 to one of the forms of reactive nitrogen that can be used by biota. Denitrifying bacteria return N2 to the atmosphere, lowering the overall stock of nitrogen available for life on Earth. Rocks of the continental crust hold the reservoir of phosphorus that becomes available to the biosphere through rock weathering. In most habitats, the availability of P is controlled by the degradation of organic forms of P. The global P cycle is complete only when sedimentary rocks are lifted above sea level and the weathering begins again. Because supplies of nitrogen and phosphorus often define soil fertility, humans have added enormous quantities of these elements to soils to enhance crop production, leading to inadvertent enrichments of ecosystems downwind or downstream of the point of application. In this chapter, we attempt to balance N and P budgets for the world's land area and the sea. The balance between N fixation and denitrification through geologic time determines the nitrogen available to biota and the global nitrogen cycle. One of the byproducts of nitrification and denitrification is N2O (nitrous oxide), which is both a greenhouse gas and a cause of ozone destruction in the stratosphere.