Mathematics – Logic
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003trgeo...8..557g&link_type=abstract
Treatise on Geochemistry, Volume 8. Editor: William H. Schlesinger. Executive Editors: Heinrich D. Holland and Karl K. Turekian.
Mathematics
Logic
2
Scientific paper
Once upon a time nitrogen did not exist. Today it does. In the intervening time the universe was formed, nitrogen was created, the Earth came into existence, and its atmosphere and oceans were formed! In this analysis of the Earth's nitrogen cycle, I start with an overview of these important events relative to nitrogen and then move on to the more traditional analysis of the nitrogen cycle itself and the role of humans in its alteration.The universe is ˜15 Gyr old. Even after its formation, there was still a period when nitrogen did not exist. It took ˜300 thousand years after the big bang for the Universe to cool enough to create atoms; hydrogen and helium formed first. Nitrogen was formed in the stars through the process of nucleosynthesis. When a star's helium mass becomes great enough to reach the necessary pressure and temperature, helium begins to fuse into still heavier elements, including nitrogen.Approximately 10 Gyr elapsed before Earth was formed (˜4.5 Ga (billion years ago)) by the accumulation of pre-assembled materials in a multistage process. Assuming that N2 was the predominate nitrogen species in these materials and given that the temperature of space is -270 °C, N2 was probably a solid when the Earth was formed since its boiling point (b.p.) and melting point (m.p.) are -196 °C and -210 °C, respectively. Towards the end of the accumulation period, temperatures were probably high enough for significant melting of some of the accumulated material. The volcanic gases emitted by the resulting volcanism strongly influenced the surface environment. Nitrogen was converted from a solid to a gas and emitted as N2. Carbon and sulfur were probably emitted as CO and H2S (Holland, 1984). N2 is still the most common nitrogen volcanic gas emitted today at a rate of ˜2 TgN yr-1 (Jaffee, 1992).Once emitted, the gases either remained in the atmosphere or were deposited to the Earth's surface, thus continuing the process of biogeochemical cycling. The rate of transfer depended on the reactivity of the emitted material. At the lower extreme of reactivity are the noble gases, neon and argon. Most neon and argon emitted during the degassing of the newly formed Earth is still in the atmosphere, and essentially none has been transferred to the hydrosphere or crust. At the other extreme are carbon and sulfur. Over 99% of the carbon and sulfur emitted during degassing are no longer in the atmosphere, but reside in the hydrosphere or the crust. Nitrogen is intermediate. Of the ˜6×106 TgN in the atmosphere, hydrosphere, and crust, ˜2/3 is in the atmosphere as N2 with most of the remainder in the crust. The atmosphere is a large nitrogen reservoir primarily, because the triple bond of the N2 molecule requires a significant amount of energy to break. In the early atmosphere, the only sources of such energy were solar radiation and electrical discharges.At this point we had an earth with mostly N2 and devoid of life. How did we get to an earth with mostly N2 and teeming with life? First, N2 had to be converted into reactive N (Nr). (The term reactive nitrogen (Nr) includes all biologically active, photochemically reactive, and radiatively active nitrogen compounds in the atmosphere and biosphere of the Earth. Thus, Nr includes inorganic reduced forms of nitrogen (e.g., NH3 and NH4+), inorganic oxidized forms (e.g., NOx, HNO3, N2O, and NO3-), and organic compounds (e.g., urea, amines, and proteins).) The early atmosphere was reducing and had limited NH3. However, NH3 was a necessary ingredient in forming early organic matter. One possibility for NH3 generation was the cycling of seawater through volcanics (Holland, 1984). Under such a process, NH3 could then be released to the atmosphere where, when combined with CH4, H2, H2O, and electrical energy, organic molecules including amino acids could be formed (Miller, 1953). In essence, electrical discharges and UV radiation can convert mixtures of reduced gases into mixtures of organic molecules that can then be deposited to land surfaces and oceans ( Holland, 1984).To recap, Earth was formed at 4.5 Ga, water condensed at 4 Ga, and organic molecules were formed thereafter. By 3.5 Ga simple organisms (prokaryotes) were able to survive without O2 and produced NH3. At about the same time, the first organisms that could create O2 in photosynthesis (e.g., cyanobacteria) evolved. It was not until 1.5-2.0 Ga that O2 began to build up in the atmosphere. Up to this time, the O2 had been consumed by chemical reactions (e.g., iron oxidation). By 0.5 Ga the O2 concentration of the atmosphere reached the same value found today. As the concentration of O2 built up, so did the possibility that NO could be formed in the atmosphere during electrical discharges from the reaction of N2 and O2.Today we have an atmosphere with N2 and there is energy to produce some NO (reaction of N2 and O2). Precipitation can transfer Nr to the Earth's surface. Electrical discharges can create nitrogen-containing organic molecules. Simple cells evolved ˜3.5 Ga and, over the succeeding years, more complicated forms of life have evolved, including humans. Nature formed nitrogen and created life. By what route did that "life" discover nitrogen?To address this question, we now jump from 3.5 Ga to ˜2.3×10-7 Ga. In the 1770s, three scientists - Carl Wilhelm Scheele (Sweden), Daniel Rutherford (Scotland), and Antoine Lavosier (France) - independently discovered the existence of nitrogen. They performed experiments in which an unreactive gas was produced. In 1790, Jean Antoine Claude Chaptal formally named the gas nitrogène. This discovery marked the beginning of our understanding of nitrogen and its role in Earth systems.By the beginning of the second half of the nineteenth century, it was known that nitrogen is a common element in plant and animal tissues, that it is indispensable for plant growth, that there is constant cycling between organic and inorganic compounds, and that it is an effective fertilizer. However, the source of nitrogen was still uncertain. Lightning and atmospheric deposition were thought to be the most important sources. Although the existence of biological nitrogen fixation (BNF) was unknown at that time, in 1838 Boussingault demonstrated that legumes restore Nr to the soil and that somehow they create Nr directly. It took almost 50 more years to solve the puzzle. In 1888, Herman Hellriegel (1831-1895) and Hermann Wilfarth (1853-1904) published their work on microbial communities. They noted that microorganisms associated with legumes have the ability to assimilate atmospheric N2 (Smil, 2001). They also said that it was necessary for a symbiotic relationship to exist between legumes and microorganisms.Other important processes that drive the cycle were elucidated in the nineteenth century. In the late 1870s, Theophile Scholesing proved the bacterial origins of nitrification. About a decade later, Serfei Nikolaevich Winogradsky isolated the two nitrifers - Nitrosomonas and Nitrobacter - and showed that the species of the former genus oxidize ammonia to nitrite and that the species of the latter genus convert nitrite to nitrate. Then in 1885, Ulysse Gayon isolated cultures of two bacteria that convert nitrate to N2. Although there are only two bacterial genera that can convert N2 to Nr, several can convert Nr back to N2, most notably Pseudomonas, Bacillus, and Alcaligenes (Smil, 2001).By the end of the nineteenth century, humans had discovered nitrogen and the essential components of the nitrogen cycle. In other words, they then knew that some microorganisms convert N2 to NH4+, other microorganisms convert NH4+ to NO3-, and yet a third class of microorganisms convert NO3- back to N2, thus completing the cycle.The following sections of this chapter examine the biogeochemical reactions of Nr, the distribution of Nr in Earth's reservoirs, and the exchanges between the reservoirs. This chapter then discusses Nr creation by natural and anthropogenic processes and nitrogen budgets for the global land mass and for continents and oceans using Galloway and Cowling (2002) and material f
No associations
LandOfFree
The Global Nitrogen Cycle does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with The Global Nitrogen Cycle, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and The Global Nitrogen Cycle will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1732397