Nitrogen in future biosphere studies

This article suggests a theme for future nitrogen studies, involving the fu nctioning and evolution of the biosphere, together with certain aspects of human biology. It is hoped that this theme could be developed into a multi- authored book at some point in the future, although numerous new case studi es may be required. The biosphere can be considered to be a single interactive system, comprise d of numerous component parts, including the atmosphere, rain, rivers. lake s, oceans, continental ecosystems, marine ecosystems, soils and sediments. In order to study the internal complexity of each of the components, and th e relationships that exist between them, one can choose a common theme. The author believes that nitrogen is a good prospect because of its ubiquitous nature and complex chemistry. The burial of sedimentary volatiles in the continental crust has contribute d to the long-term chemical evolution of the exterior environments of the E arth. The most marked effects, notably the accumulation of atmospheric oxyg en, have been due to the burial of reduced carbon. An understanding of the passage of sedimentary volatiles through the crust will provide important c onstraints on the evolution of the biosphere, and may lead to a more meanin gful comparative planetology. Carbon is difficult to trace through crustal processes because its high temperature form (graphite) is incompatible with the crystal structures of silicate minerals. However, the high temperature form of sedimentary nitrogen (ammonium) readily enters into potassic miner als, and it can be easily traced through crustal processes. Herein, the fate of sedimentary nitrogen is traced from wet sediments, thro ugh metamorphic rocks, into granites and other crustal melts. Without the t rapping of nitrogen in the crust, and the liberation of oxygen that is a co nsequence of biological nitrogen fixation, the N-2/O-2 of the atmosphere ma y have been about 82.9% to 16.3%, and the total atmospheric pressure about 1.2 atm. Most of the changes in the oxygen content is due to dilution by ni trogen, with only about 5% of the present atmospheric oxygen being a conseq uence of nitrogen fixation. The ancient continental surfaces would have been volcanic deserts containin g little (<1 ppm) or no nitrogen. Today, nitrogen-rich soils support contin ental ecosystems. One can use nitrogen as a 'window' to speculate on the co lonisation of the continents by land plants. Herein, nitrogen fixation by l egumes is considered in detail, and by drawing analogies with the present d ay, the author speculates on the colonisation of the continents by land pla nts. Having speculated on the co-evolution of the atmosphere, soils and continen tal ecosystems, one can place a human being in the centre of a tropical for est, and begin to examine how they relate to the modern biosphere. The chos en example is the metabolism of plant protein. and the roles of glutamate d ehydrogenase (GDH), vitamin B-2, and flavin mononucleotide (FMN). The realisation of this theme would imply numerous new case studies, notabl y those concerning the continental crust, where nitrogen studies were linke d to mineralogy. petrology and other geochemical tracers, including Sr, O, Al, and K, and where the short-range order of delta N-15 were taken into co nsideration. (C) 2001 Elsevier Science B.V. All rights reserved.