The Nitrogen Cycle

12 Key excerpts on "The Nitrogen Cycle"

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  Global biogeochemical cycles

  • Butcher(Author)
  • 1992(Publication Date)
  • Academic Press

    (Publisher)

  12.5 The Global Nitrogen Cycle The global nitrogen cycle is often referred to as The Nitrogen Cycles, since we can view the overall pro-cess as the result of the interactions of various bio-logical and abiotic processes. Each of these pro-cesses, to a first approximation, can be considered as a self-contained cycle. We have already con-sidered the biological cycle from this perspective (Fig. 12-1), and now we will look at the other pro-cesses, the ammonia cycle, the NO x cycle, and the fixation/denitrification cycle. The Nitrogen Cycle 273 12.5.1 Nitrogen inventories We will consider the inventories of nitrogen in the following compartments: crustal, terrestrial, oceanic, and atmospheric. In general, there is more agreement among researchers over the values for the nitrogen burdens than for the fluxes. In considering these inventories, it is significant to recall that 99.96% of the non-crustal nitrogen exists as un-combined atmospheric N 2 and it is this fact that causes nitrogen to often be a limiting nutrient in the condensed phase. The principal form of nitrogen in terrestrial sys-tems is as dead soil organic matter, with biomass accounting for only about 4%, and inorganic nitrogen about 6.5%, on a global average (Soderlund and Svensson, 1976). There is, however, a large difference in the distribution of nitrogen in the tropics and the polar regions, with tropical regions having a larger proportion of nitrogen contained as biomass. Most of the reservoirs for organic nitrogen and biomass have been estimated by knowledge of the carbon content of soils and biomass and an appropriate ratio of carbon to nitrogen. The inventories of inorganic forms of nitrogen have a higher relative uncertainty. Dissolved N 2 is the principal form of nitrogen in the oceans, accounting for 95% of the total oceanic nitrogen. The remainder of the oceanic nitrogen is principally NO; and dead organic matter. The oceans hold about 0.5% of the total non-crustal nitrogen (as N 2 ) .

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  Structure and Functioning of Seminatural Meadows

  • M. Rychnovská(Author)
  • 2012(Publication Date)
  • Elsevier Science

    (Publisher)

  16 The Nitrogen Cycle Nitrogen is indispensable for all forms of life. The greatest reservoir of nitrogen is the atmosphere. While the C 0 2 concentration in the air is 0 . 0 3 % that of nitrogen is about 7 9 % . However, only a small part of this large nitrogen pool enters into ecosystems. Only a few groups of micro-organisms are able to trans-form gaseous atmospheric nitrogen at first into organic and then into mineral nitrogenous substances. The micro-organisms also mediate most of the decom-position and transformation processes taking place in the soil which involve nitrogenous substances; thus they have a key position in nitrogen cycling. During recent decades, The Nitrogen Cycle has become distinctly affected by industrial and agricultural activities of m a n and by his extensive use of road vehicles on the local, regional and global levels. The industrialized type of agriculture increases nitrogen inputs into the biosphere through the application of mineral fertilizers. Concurrently, the amounts of nitrates percolating into the groundwater and watercourses increase on both local and regional scales, and so do the losses of nitrogen to the atmosphere; the turnover of nitrogen in the biosphere is becoming more intensive. The growth of industry is associated with a high output of acidic emissions to the atmosphere. The emitted gasses of the N O x and S O x type are stripped from the air back to the earth by rain. Acid rain adversely affects both biotic and abiotic components of ecosystems. It retards the production of trees on a global scale, and often they eventually die off over large areas. Some of the biotic components of ecosystems can be similarly eliminated from trophic chains on a local scale. Some of the nitrogen oxides escape to the stratosphere, contributing to the destruction of the ozone layer which protects the Earth from excessive ultraviolet radiation.

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  Mineral Nitrogen In The Plant-Soil System

  • R Haynes(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  Chapter 1 Origin, Distribution, and Cycling of Nitrogen in Terrestrial Ecosystems R. J. HAYNES I. INTRODUCTION During ecosystem development, from unproductive rocks devoid of soil and vegetation to an ecosystem with a deep soil profile and abundant vegetation, both total biomass Ν and soil Ν increase (Stevens and Walker, 1970; Jenny, 1980). Such an increase is achieved by wet and dry deposition of atmospheric Ν and through the actions of a specialized group of microorganisms that fix atmospheric N 2 . An equilibrium level of Ν is obtained within the mature ecosystem. Indeed, when a natural ecosystem is in a steady state, the rates of both Ν input and loss are characteristically very small and equal. In contrast, large quantities of Ν are cycled within the system. On a global scale, 90 to 97% of the Ν content of the net primary production of plant biomass is derived from recycling of Ν within the biosphere (Rosswall, 1976), leav-ing approximately 3 to 10% as annually fixed. Mineral Ν in the soil represents a very small and usually transitory pool of Ν in terms of the total Ν stock of any ecosystem. Indeed, the major forms of mineral Ν (NH^- and NO^-N) usually account for less than 2% of the total Ν content of soils (Melillo, 1981; Woodmansee et al. y 1981). It is, nevertheless, this Ν that is available for direct uptake by plants. Nitrate Ν is easily lost from soils through leaching to groundwater and through denitrification. Maintenance of a low rate of nitrification (which ι 2 1 . Distribution and Cycling of Ν itself results in gaseous losses of N) is therefore essential in Ν conserva-tion in most natural ecosystems (Verstraete, 1981); in general, the rate of nitrification appears to be regulated by the supply of NH4 -N within the soil (Robertson and Vitousek, 1981; Adams and Attiwill, 1982).

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  Fundamentals of Geobiology

  • Andrew H. Knoll, Don E. Canfield, Kurt O. Konhauser, Andrew H. Knoll, Don E. Canfield, Kurt O. Konhauser(Authors)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  4 THE GLOBAL NITROGEN CYCLE

  Bess Ward

  Department of Geosciences, Princeton University, Princeton, NJ 08540 USA

  4.1 Introduction

  The geobiology of nitrogen is dominated by large inert reservoirs and small biological fluxes. The largest reservoirs are nitrogen gas (in the atmosphere and dissolved in the ocean) and sedimentary nitrogen (sequestered in continental crust). Because most organisms cannot utilize gaseous nitrogen, we distinguish between fixed nitrogen compounds (which contain no N–N bonds) that are biologically available, and the dinitrogen gases (N2 and N2 O) that are largely inaccessible to organisms. Fluxes into and out of the fixed nitrogen pools are biologically controlled, and microbes control the rates of transformations and the distribution of nitrogen among inorganic and organic pools.

  Possibly more than any other biologically important element, the global nitrogen cycle has been perturbed by anthropogenic activities. The rate of industrial nitrogen fixation now approximately equals the natural rate, resulting in a two- to threefold increase in the total inventory of fixed N on the surface of the Earth through agricultural fertilizer applications (Galloway et al ., 2004). Nitrogen oxides enter the atmosphere via fossil fuel combustion and catalyse the atmospheric chemistry of ozone through pathways that were either nonexistent or insignificant prior to humans. Because of the relative biological inaccessibility of nitrogen, ecosystems have responded to the increased flux of fixed N with changes in the rates and fates of production, and in some cases, large changes in ecosystem chemistry and health.

  The microbiology of nitrogen transformations has been intensely studied for over a century, but important new discoveries have been made in only the last decade. These involve the discovery of previously suspected, but unknown processes, as well as the discovery of new and diverse microbes involved in many of The Nitrogen Cycle fluxes.

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  Biogeochemistry

  An Analysis of Global Change

  • William H. Schlesinger(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  We will attempt to balance an Ν and Ρ budget for the world's land area and the sea. For N, the rate of fixation through geologic time determines the nitrogen available to biota and the global biogeo-chemical cycle. We will review ideas about the rate of nitrogen fixation and denitrification in the geologic past. One of the products of nitrifica-tion and denitrification is N 2 0 (nitrous oxide), which is both a greenhouse gas and a cause of ozone destruction in the stratosphere (Chapter 3). We will formulate a tentative budget for N 2 0 in the atmosphere, based on our current, limited understanding of the sources of this gas. The Global Nitrogen Cycle Land Figure 12.2 presents the global nitrogen cycle, showing the linkage be-tween the atmosphere, land, and sea. The atmosphere is the largest pool The Global Nitrogen Cycle t Permanent burial 10 Figure 12.2 The global nitrogen cycle. Pools and annual flux in 1 0 12 g Ν. Modified from Söderlund and Rosswall (1982) based on values derived in the text. 12. The Global Cycles of Nitrogen and Phosphorus 325 (3.8 x 10 2 1 g Ν; Table 3.1). Relatively small amounts of Ν are found in terrestrial biomass (3.5 x 10 1 5 g) and soil organic matter (95 x 10 1 5 g; Post et al. 1985). The mean C/N ratios for terrestrial biomass and soil organic matter are about 160 and 15, respectively. The pool of inorganic nitrogen, N H 4 + and N 0 3 ~ , on land is very small. The transformations of nitrogen in the soil and the uptake of Ν by organisms are so rapid that little nitrogen remains in inorganic form, despite a large annual flux through this pool (Chapter 6). The nitrogen that bathes the terrestrial biosphere is not available to most organisms; the great strength of the triple bond in Ν 2 makes this molecule practically inert. All nitrogen that is available to the terrestrial biota was originally derived from nitrogen fixation—either by lightning or by free-living and symbiotic microbes (Chapter 6).

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  Nitrogen, the Confer-N-s

  • K. van der Hoek, J. Willem Erisman, S. Smeulders, John Robert Wisniewski(Authors)
  • 2012(Publication Date)
  • Elsevier Science

    (Publisher)

  Part 2 The Global Nitrogen Cycle Passage contains an image

  The global nitrogen cycle: changes and consequences

  James N. Galloway,     Department of Environmental Sciences, University of Virginia, Charlottesville, VA 22903, USA

  Abstract

  In the absence of human activities, biological N fixation is the primary source of reactive N, providing about 90–130 Tg N year−1 (Tg = 1012 g) on the continents. Human activities have resulted in the fixation of an additional ~150 Tg N year−1 by energy production, fertilizer production, and cultivation of crops (e.g., legumes, rice). Some sinks of anthropogenic N have been estimated (e.g., N2 O accumulation in the atmosphere; loss to coastal oceans), however due to the uncertainty around the magnitude of other sinks (e.g., retention in groundwater, soils, or vegetation or denitrification to N2 ) a possibly large portion of the N fixed by humans is missing. While we know that N is accumulating in the environment, we do not know the rate of accumulation. Due to the myriad of effects of excess N on humans, ecosystems, and the atmosphere, and their cascading nature (i.e., one atom of N can have a large number of different effects as it is transformed to different N species), this lack of knowledge is unfortunate. There are limited options available to society to reduce the amount of N mobilized by human action because there is, in effect, a N imperative — it is required for food production. As population and per capita consumption of food (especially animal products) increase, more and more N will be converted from unreactive to reactive forms in the future. This is especially true in less developed regions.

  Keywords Nitrogen fertilizer Asia global change food production

  Introduction

  Imagine that you were to double the amount of nitrogen that you ate. Further imagine that some of the processes occurring in your body were limited by nitrogen (e.g., ability to synthesize fat), and that other processes were adversely affected by nitrogen (e.g., organ function). If this was the case, then a doubling of your N intake would make you larger, and you would suffer damage to critical components of your body. Not the best of all worlds.

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  Structure and Function in Agroecosystem Design and Management

  • Masae Shiyomi, Hiroshi Koizumi, Masae Shiyomi, Hiroshi Koizumi(Authors)
  • 2001(Publication Date)
  • CRC Press

    (Publisher)

  Today, mankind produces nitro-gen fertilizer by fixing atmospheric nitrogen and supplies this to croplands in order to increase production, disturbing the natural nitrogen cycle. GLOBAL BALANCE The amount of nitrogen in Earth’s nitrogen cycle is shown in Table 17.1. Such data are continuously being refined, and the numbers will inevitably change as better estimates become available. A great deal of nitrogen exists in the atmosphere. N 2 gas, which exists in the ocean in a dissolved state, abounds. Dissolved or suspended nitrogen in the ocean and organic nitrogen Table 17.1 Estimates of Active Pools in the Global Nitrogen Cycle. Million tonnes N 2 Air N 2 3,900,000,000 N 2 O 1,400 Land Plants 15,000 Animals 200 of which is people 10 Soil organic matter 150,000 of which is microbe microbial biomass 6,000 Sea Plants 300 Animals 200 In solution or suspension 1,200,000 of which is NO 3 —N 570,000 of which is NH 4 —N 7,000 Dissolved N 2 22,000,000 Jenkinson, 1990. NITROGEN CYCLE AND AGRICULTURE 355 in the soil also abound. The nitrogen included in the biomass is very small compared with these quantities. The nitrogen moves between these pools. This circulation is illustrated in Figure 17.2. The nitrogen fixation rate by microorganisms in the terrestrial ecosystem is estimated as 139 Tg/year, an estimation based on Burns and Hardy (1975). This quantity is equivalent to approximately 10 kg/ha/year for each hectare of the Earth’s land surface. The details of the estimation are shown in Table 17.2. Hardy has estimated the fixation in the ocean at 36 Tg/year. Jenkinson (1990) has summarized the rel-evant previous research. It has been estimated that nitrogen fixation quantity in nature is 60 to 260 Tg/year (Jenkinson, 1990). Some nitrogen fixation occurs when there is lightning, but this quantity is much smaller than that fixed by bacteria. Jenkinson has estimated this as 2 to 8 Tg/year. Nitrogen fix-ation also occurs with the high-temperature combustion of fossil fuel.

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  Ecological Aspects of Nitrogen Metabolism in Plants

  • Joe C. Polacco, Christopher D. Todd(Authors)
  • 2011(Publication Date)
  • Wiley

    (Publisher)

  * Nitrogen in our environment has both benefits as well as negative consequences. The primary benefit of nitrogen is the stimulation of plant growth in agriculture for food, feed, and fuel, whereas the negative aspects include almost all environmental impacts. Compared to elements such as carbon, sulfur, or phosphorus, nitrogen contributes to a variety of negative impacts, most interrelated, such as climate change, eutrophication, soil acidification, degraded human health, loss of biodiversity, etc. In 2008 we celebrated the one-hundredth anniversary of the invention of the production of ammonia by Fritz Haber in 1908. Ammonia is the basis for fertilizer production, and Carl Bosch was able to turn that ammonia production into an industrial process (Smil, 2001; Erisman et al., 2008). Bosch and Haber were awarded the Nobel Prize for their achievements. Ammonia is not only the basis for fertilizer, but also for many industrial chemicals, including explosives. So, overall, the Haber-Bosch process was beyond doubt one of the most important inventions of the twentieth century (Erisman et al., 2008). Over the past decades the production of fertilizer has become very energy and economically efficient (Kongshaug, 1998) and on a large scale has increased agricultural productivity. Without fertilizer input, the biosphere would produce 48% less food (Erisman et al., 2008). Therefore, at present, we cannot live without fertilizer. At the same time, industrialization increased the use of fossil fuels. To use the energy from fossil fuels, they are burned, resulting in a large release of oxidized nitrogen (NOx) into the atmosphere. Vehicular traffic, energy use, and industry are the principal sources of oxidized forms of Nr. The dispersion of NOx has affected human health and increased nitrogen deposition in remote areas leading to eutrophication (Erisman and Fowler, 2003).

  This book is dedicated to plant “strategies” to acquire, assimilate, and conserve N. Much emphasis is given to plant interactions with microorganisms (Frankia , rhizobia, mycorrhizae). Biological N-fixation is prominently covered and is an important part of The Nitrogen Cycle. However, in order to establish the need for knowledge in the area of biological N fixation (and the N “conduits” that mycorrhizae provide for delivering soil N to plant), we have to address the “big picture” first, providing background on the major components of The Nitrogen Cycle on different scales, emphasizing the human contribution to the increased cycling of nitrogen in the biosphere and the resulting impacts on ecosystems and humans. This introductory chapter addresses these issues as a global overview of the human influence on The Nitrogen Cycle.

  The Preindustrial Nitrogen Cycle

  Nitrogen is an important element—the most abundant constituent of the atmosphere, hydrosphere, as well as the biosphere. It is also one of the essential elements for the growth of plants and animals and has a crucial role in ecology and in the environment. It is useful to look at the reactions of elements in the form of a closed cycle. Such a cycle is often termed a biogeochemical cycle because chemistry, biology, and geology all provide important inputs. Cycling of elements is often governed by kinetics and may involve the input of energy, so that chemical equilibrium states are not attained. The ultimate source of energy for driving energetically uphill reactions is the sun. The earth’s surface receives an average radiation input of 100–300 W/m2 % day, depending on latitude. Some of this is captured by photosynthesis, and is used to produce high-energy content molecules, such as oxygen. Because of the inherently low efficiency of the photosynthetic process and the production of phytomass, energy supply from this source has low power densities and hence high land demands. Recent estimates of the global terrestrial net primary productivity (NPP) average approximately 120 gigaton (Gton) of dry biomass produced annually, and that contains some 1,800 × 1018

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  Biogeochemistry

  An Analysis of Global Change

  • William H Schlesinger(Author)
  • 2012(Publication Date)
  • Academic Press

    (Publisher)

  We will review ideas about the rate of nitrogen fixation and denitrification in the geologic past. One of the products of nitrifica-tion and denitrification is N 2 0 (nitrous oxide), which is both a greenhouse gas and a cause of ozone destruction in the stratosphere (Chapter 3). We will formulate a tentative budget for N 2 0 in the atmosphere, based on our current, limited understanding of the sources of this gas. The Global Nitrogen Cycle Land Figure 12.2 presents the global nitrogen cycle, showing the linkage be-tween the atmosphere, land, and sea. T h e atmosphere is the largest pool The Global Nitrogen Cycle P e r m a n e n t b u r i a l 10 Figure 1 2 . 2 The global nitrogen cycle. Pools and annual flux in 10 12 g N. Modified from Soderlund and Rosswall (1982) based on values derived in the text. 1 2 . The Global Cycles of Nitrogen and Phosphorus 325 (3.8 x 1 0 21 g N; Table 3.1). Relatively small amounts of Ν are found in terrestrial biomass (3.5 x 10 1 5 g) and soil organic matter (95 x 1 0 15 g; Post et al. 1985). T h e mean C/N ratios for terrestrial biomass and soil organic matter are about 160 and 15, respectively. T h e pool of inorganic nitrogen, N H 4 + and N C >3~, on land is very small. T h e transformations of nitrogen in the soil and the uptake of Ν by organisms are so rapid that little nitrogen remains in inorganic form, despite a large annual flux through this pool (Chapter 6). T h e nitrogen that bathes the terrestrial biosphere is not available to most organisms; the great strength of the triple bond in N 2 makes this molecule practically inert. All nitrogen that is available to the terrestrial biota was originally derived from nitrogen fixation—either by lightning or by free-living and symbiotic microbes (Chapter 6). T h e rate of nitrogen fixation by lightning, which produces momentary conditions of high pressure and temperature allowing N 2 and 0 2 to combine, is poorly known.

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  Ecology

  Principles and Applications

  • J. L. Chapman, M. J. Reiss(Authors)
  • 1998(Publication Date)
  • Cambridge University Press

    (Publisher)

  In terrestrial systems, the great majority of the nitrogen (97%) is found in soil organic matter, litter and soil inorganic nitrogen; only 3% is found in plants and animals (Rosswall, 1983; Jefferies & Maron, 1997). From an ecological perspective it is most con- venient to break The Nitrogen Cycle down into a number of stages. For the sake of brevity, we will concentrate on the terrestrial environment. Ammonification When organisms excrete nitrogenous waste or die, their nitrogen is converted to ammonium ions by the action of saprotrophic fungi and bacteria. This process is known as ammonification. The saprotroph microbes use the ammonia to synthesise their own proteins and other nitrogen-containing organic com- pounds. Inevitably, though, some of the ammonia leaks into the surrounding soil and so becomes available to other bacteria and to plants. Nitrification In warm, moist soils with a pH near to 7, ammoni- um ions (NH4) are oxidised within a few days of their formation or their addition as a fertiliser (Salisbury & Ross, 1985). This oxidation benefits the bacteria 152 13.4 The Nitrogen Cycle N, in atmosphere nitrogen fixation by bacteria 110 N 3 O in atmosphere nitrogen fixation by industry for fertilisers 140 NH 3 in atmosphere volcanic emissions animalsf £1 terrestrial vegetation ammonia from "animals 90-190 denitrification from land 100-160 denitrification from sea 30-80 various oxides of nitrogen in atmosphere factories < nitrogen turnover \ in plants 2000 f runoff from land to sea 10-25 nitrogen fixation 30-130 i nitrogen turnover in » \ plankton 4000 f to sediments 401 Figure 13.7 The global nitrogen cycle. Estimates are given of the annual rates of flow (in units of 10 u g). (Based on Rosswall, 1983; Jefferies & Maron, 1997; Pearce, 1997a.) performing the reactions by releasing energy which the bacteria can use for the synthesis of ATP.

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  Geochemistry of Earth Surface Systems

  A derivative of the Treatise on Geochemistry

  • Heinrich D Holland, Karl K. Turekian(Authors)
  • 2010(Publication Date)
  • Academic Press

    (Publisher)

  al . (2003b). This section summarizes that discussion. The nitrogen cascade is defined as the sequential transfer of Nr through environmental systems, which results in environmental changes as Nr moves through or is temporarily stored within each system.

  Two scenarios in Figure 5 illustrate the nitrogen cascade. The first example shows the fate of NOx produced during fossil fuel combustion. In the sequence, an atom of nitrogen mobilized as NOx in the atmosphere first increases ozone concentrations, then decreases atmospheric visibility and increases concentrations of small particles, and then increases precipitation acidity. After that same nitrogen atom is deposited into the terrestrial ecosystem; it may increase soil acidity (if a base cation is lost from the system), decrease biodiversity, and either increase or decrease ecosystem productivity. If discharged to the aquatic ecosystem, the nitrogen atom can increase surface-water acidity in mountain streams and lakes. Following transport to the coast, nitrogen atom can countribute to coastal eutrophication. If the nitrogen atom is converted to N2 O and emitted back to the atmosphere, it can first increase greenhouse warming potential and then decrease stratospheric ozone.

  Figure 5 The nitrogen cascade illustrates the sequential effects a single atom of N can have in various reservoirs after it has been converted from a nonreactive to a reactive form.

  source

  Galloway et al. , 2003b

  Example 2 illustrates a similar cascade of effects of Nr from food production. In this case, atmospheric N2 is converted to NH3 in the HaberBosch process. The NH3 is primarily used to produce fertilizer. About half the Nr fertilizer applied to global agro-ecosystems is incorporated into crops harvested from fields and used for human food and livestock feed (Smil, 1999 , 2001 ). The other half is transferred to the atmosphere as NH3 , NO, N2 O, or N2 , lost to the aquatic ecosystems, primarily as nitrate or accumulate in the soil nitrogen pool. Once transferred downstream or downwind, the nitrogen atom becomes part of the cascade. As Figure 5 illustrates, Nr can enter the cascade at different places depending on its chemical form. An important characteristic of the cascade is that, once it starts, the source of the Nr (e.g., fossil-fuel combustion and fertilizer production) becomes irrelevant. Nr species can be rapidly interconverted from one Nr to another. Thus, the critical step is the formation

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  The Atmospheric Environment

  Effects of Human Activity

  • Michael B. Mcelroy(Author)
  • 2021(Publication Date)
  • Princeton University Press

    (Publisher)

  They are therefore intended to represent conditions for the pre-industrial environment. For example, the terrestrial system (biosphere plus soil) is postulated to annually re- ceive a quantity of nitrogen equivalent to 240 Mt N, of which 200 Mt N is supplied by the fixation of atmospheric N 2 , 30 Mt N by precipitation, and 10 Mt N by the recycling of N from sediments. Nitrogen is assumed to be removed from the terrestrial compartment by runoff in rivers (20 Mt N yr –1 ), by volatilization of fixed N (30 Mt N yr –1 ), and by denitrification (190 Mt N yr –1 ), with an integrated loss constrained to equal the postulated source of 240 Mt N yr –1 . 2 The corresponding balance for the ocean assumes an input of 20 Mt N yr –1 from rivers, 20 Mt N yr –1 from fixation, and 10 Mt N yr –1 from pre- 1  10 4 M tons fixed N Soil inorganic 2  10 9 M tons fixed N Sediments 3  10 0 M tons fixed N Atmospheric fixed N 4  10 9 M tons N Atmospheric N 2 and N 2 O 1  10 5 M tons fixed N Soil organic 3  10 4 M tons fixed N Land biosphere 2  10 7 M tons N, N 2 8  10 5 M tons fixed N Ocean (9 ) (1) (6 ) (3) (5 ) 30 30 20 (4 ) (11) (8 ) (7 ) 10 (2 ) 20 (10 ) 10 190 40 200 10 10 Figure 12.1 Overview of the global nitrogen cycle. Reservoir con- tents are expressed in units of MT N (10 12 g N). Transfer rates are in units of MT N yr –1 (10 12 g N yr –1 ). Processes involved in spe- cific transfer processes are numbered in parentheses (1)–(11). Specific processes are as follows: (1) terrestrial fixation, (2) marine fixation, (3) lightning, (4) transfer from land to ocean in rivers, (5) volatilization of fixed N from terrestrial sources, (6) precipita- tion on land, (7) precipitation on ocean, (8) incorporation in sedi- ments, (9) denitrification on land, (10) denitrification in ocean, (11) return of fixed N mainly to terrestrial systems by uplift of sediments.

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