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Upon oxygen depletion, a suite of alternate oxidants supports microbial respiration and inhibits the onset of sulfate (SO₄ ²) reduction. Waters in this intermediate state are referred to as ‘oxygen deficient’ or ‘suboxic’. This chapter discusses types of suboxic water column respiration, their occurrence, variability, and significance. Methodological problems are also discussed. Nitrate (NO₃ ^-) is the most abundant suboxic electron acceptor in oceanic water, but the suite of alternate oxidants includes nitrite (NO₂ ^-), nitric oxide (NO), nitrous oxide (N₂O), iodate (IO₃ ^-), manganese (Mn III and IV), iron (Fe III), and several other oxidants present at low concentrations. Although canonical denitrification, which involves reduction of nitrate to N₂O and N₂ is probably the single most important suboxic respiratory pathway in the water column, important additional respiratory pathways for dinitrogen (N₂) and N₂O production are also considered.

This chapter describes efforts to improve the fertility of California's soils. During the late nineteenth and early twentieth centuries, nitrogen depletion in California's soils was a major concern. Farmers relied on two imports from Chile—nitrogen-rich Chilean alfalfa (Medicago sativa) and Chilean sodium nitrate (NaNO₃)—to meet the nutrient demands of a continuously expanding agricultural system. Chilean alfalfa was indispensable to the emergence of Northern California's profitable dairy businesses, which made California into the nation's top milk butter, ice cream, and yogurt-producing state by the end of the twentieth century. Chilean sodium nitrate was essential to Southern California's prosperous citrus-fruit industry, which served as that region's primary engine of economic growth from the 1880s through World War II.

June, when the snows come hurrying from the hills and the bridges often go, in the words of Emily Dickinson. In the beaver meadow, the snows are indeed hurrying from the surrounding hills. Every one of the 32 square miles of terrain upslope from the beaver meadow received many inches of snow over the course of the winter. Some of the snow sublimated back into the atmosphere. Some melted and infiltrated into the soil and fractured bedrock, recharging the groundwater that moves slowly downslope and into the meadow. A lot of the snow sat on the slopes, compacted by the weight of overlying snow into a dense, water-rich mass that now melts rapidly and hurries down to the valley bottoms. North St. Vrain Creek overflows into the beaver meadow, the water spilling over the banks and into the willow thickets in a rush. I can hear the roar of water in the main channel well before I can see it through the partially emerged leaves of the willows. Overhead is the cloudless sky of a summer morning. A bit of snow lingers at the top of the moraines. Grass nearly to my knees hides the treacherous footing of this quivering world that is terra non-firma. I am surrounded by the new growth of early summer, yet the rich scents of decay rise every time I sink into the muck. I walk with care, staggering occasionally, in this patchy, complex world that the beavers have created. I abruptly sink to mid-thigh in a muck-bottomed hole, releasing the scent of rotten eggs, but less than a yard away a small pocket of upland plants is establishing a roothold in a drier patch. A seedling spruce rises above ground junipers shedding yellow pollen dust and the meticulously sorted, tiny pebbles of a harvester ant mound. I extract my leg with difficulty and continue walking. As I walk around the margin of another small pond, the water shakes. Sometimes the bottom is firm in these little ponds, sometimes it’s mucky—I can’t tell simply by looking through the water.

I shall now introduce you to one of the simplest kinds of chemical reaction: precipitation, the falling out from solution of newly formed solid, powdery matter when two solutions are mixed together. The process is really very simple and, I have to admit, not very interesting. However, I am treating it as your first encounter with creating a different form of matter from two starting materials, so please be patient as there are much more interesting processes to come. I would like you to regard it as a warming-up exercise for thinking about and visualizing chemical reactions at a molecular level. Not much is going on, so the steps of the reaction are reasonably easy to follow. There isn’t much to do to bring about a precipitation reaction. Two soluble substances are dissolved in water, one solution is poured into the other, and—providing the starting materials are well chosen—an insoluble powdery solid immediately forms and makes the solution cloudy. For instance, a white precipitate of insoluble silver chloride, looking a bit like curdled milk, is formed when a solution of sodium chloride (common salt) is poured into a solution of silver nitrate. Now, as we shall do many times in this book, let’s imagine shrinking to the size of a molecule and watch what happens when the sodium chloride solution is poured into the silver nitrate solution. As you saw in my Preliminary remark, when solid sodium chloride dissolves in water, Na+ ions and Cl– ions are seduced by water molecules into leaving the crystals of the original solid and spreading through the solution. Silver nitrate is AgNO3; Ag denotes a silver atom, which is present as the positive ion Ag+; NO3– is a negatively charged ‘nitrate ion’, 1. Silver nitrate is soluble because the negative charge of the nitrate ion is spread over its four atoms rather than concentrated on one, 2, as it is for the chloride ion, and as a result it has rather weak interactions with the neighbouring Ag+ ions in the solid.

Cheap meat, dairy, and eggs are an illusion—we pay for each with depleted forests, polluted freshwater, soil degradation, and climate change. Diet is the most critical decision we make with regard to our environmental footprint—and what we eat is a choice that most of us make every day, several times a day. Dietary choice contributes powerfully to greenhouse gas emissions (GHGE) and water pollution. Animal agriculture is responsible for an unnerving quantity of greenhouse gas emissions. Eating animal products—yogurt, ice cream, bacon, chicken salad, beef stroganoff, or cheese omelets—greatly increases an individual’s contribution to carbon dioxide, methane, and nitrous oxide emissions. Collectively, dietary choice contributes to a classic “tragedy of the commons.” Much of the atmosphere’s carbon dioxide (CO2) is absorbed by the earth’s oceans and plants, but a large proportion lingers in the atmosphere—unable to be absorbed by plants or oceans (“Effects”). Plants are not harmed by this process, but the current overabundance of carbon dioxide in the atmosphere causes acidification of the earth’s oceans. As a result of anthropogenic carbon dioxide emissions, the “acidity of the world’s ocean may increase by around 170% by the end of the century,” altering ocean ecosystems, and likely creating an ocean environment that is inhospitable for many life forms (“Expert Assessment”). Burning petroleum also leads to wars that devastate human communities and annihilate landscapes and wildlife—including endangered species and their vital habitats. Additionally, our consumption of petroleum is linked with oil spills that ravage landscapes, shorelines, and ocean habitat. Oil pipelines run through remote, fragile areas—every oil tanker represents not just the possibility but the probability of an oil spill. As reserves diminish, our quest for fossil fuels is increasingly environmentally devastating: Canada’s vast reserves of tar sands oil—though extracted, transported, and burned only with enormous costs to the environment—are next in line for extraction. Consuming animal products creates ten times more fossil fuel emission per calorie than does consuming plant foods directly (Oppenlander 18). (This is the most remarkable given that plant foods are not generally as calorically dense as animal foods.) Ranching is the greatest GHGE offender.

Stories of people doing their jobs well, treating each other with respect, and trying to make the world a better place are all examples of “good news.” Such stories don’t generate many website hits, nor do they bring people into the theaters. Instead, it seems readers and movie viewers would rather have the double pleasure of learning about bad behavior and its comeuppance. Five movies in this chapter overcome this problem; they are based on true stories. The advantage of such stories is the sympathy viewers feel as they appreciate the adversities the chemist has overcome to make their celebrated findings. For instance, in the documentary Me & Isaac Newton, which explores the motivations of seven scientists, pharmaceutical chemist Gertrude Elion is warm and charming as she describes why she decided to become a chemist. When she later describes her struggles to enter graduate school and then get a job as a chemist, the viewer is struck by her matter-of-fact, water-under-the-bridge tone. This all happened before she understood there was a climate of active discrimination against women that had nothing to do with their drive or abilities. Still later, she says the ultimate reward for all her work comes when someone thanks her for having developed the drug that cured a loved one. The disadvantage of using true stories is the need to create dramatic tension. The important moments in people’s lives rarely coincide with obvious indications that “this is the moment when everything fell into place,” whereas a movie’s linear narrative has to make that point clear to the audience. Another problem for moviemakers is that most people just aren’t very curious about the origins of everyday things. This is a challenge because very few chemicals cause the imagination to soar (unless you are a chemist), which may explain why all five movies based on true stories are about medicinal chemistry, which can be seen as the external evidence of the chemist’s desire to do good things for other people. Fictional movie chemists are less likely to develop medicines. Like the chemistry professors in chapter 8, they tend to develop chemical products for more selfish reasons.

At the time of publishing, it is exactly 50 years since Bob Dylan answered a number of enigmatic questions with the ambiguous line ‘the answer is blowin’ in the wind’ on the A-side of the record The Freewheelin’ Bob Dylan. But one of these, ‘How many years can a mountain exist before it’s washed to the sea?’ we can at least try to answer, as part of the solution lies in one of the more famous rules of thumb one learns as a novice chemist: positively charged metal ions combined with oxides (O2−), sulphides (S2−), phosphates (PO43− ), silicates (SiO42−), and carbonates (CO32−), are insoluble in water, whereas similar nitrates (NO3− ), chlorides (Cl−), and bromides (Br−), are soluble. In terms of stuff you’ve got in your kitchen, this means that when you put a spoon of table salt (NaCl) into water it will ‘disappear’, faster if you stir or heat, and the water will look exactly the same as before. For the insoluble stuff, we move to the more expensive regions of the cupboards and investigate the state of our silver and copperware. When things like these were to be on display, my mother used to have me clean them with silver or copper polish, as the oxides and sulphides tarnishing the metal surfaces did not go away in a normal wash with water—they are completely insoluble. A suitable but boring exercise, and as close to a chemistry set as I ever came as a child. What they normally don’t tell you in chemistry textbooks however, are the enormous consequences of these rules, visible all over the world. Why are mountains made of rocks from oxides, sulphides, phosphates, silicates, and carbonates? Because they are insoluble! Any mountains made from sodium chloride would indeed have been ‘washed to the sea’ thousands of years ago, and where NaCl can be mined it is also known as rock salt, and found either underground or in regions with a very dry climate. Which brings us to the hero and heroine if this chapter.

The problem of photochemically generated smog begins inside internal combustion engines, where at the high temperatures within the combustion cylinders and the hot exhaust manifold nitrogen molecules and oxygen molecule combine to form nitric oxide, NO. Almost as soon as it is formed, and when the exhaust gases mingle with the atmosphere, some NO is oxidized to the pungent and chemically pugnacious brown gas nitrogen dioxide, NO2, 1. We need to watch what happens when one of these NO2 molecules is exposed to the energetic ultraviolet photons in sunlight. We see a photon strike the molecule and cause a convulsive tremor of its electron cloud. In the brief instant that the electron cloud has swarmed away from one of the bonding regions, an O atom makes its escape, leaving behind an NO molecule. We now continue to watch the liberated O atom. We see it collide with an oxygen molecule, O2, and stick to it to form ozone, O3, 2. This ozone is formed near ground level and is an irritant; ozone at stratospheric levels is a benign ultraviolet shield. Now keep your eye on the ozone molecule. In one instance we see it collide with an NO molecule, which plucks off one of ozone’s O atoms, forming NO2 and letting O3 revert to O2. Another fate awaiting NO2 is for it to react with oxygen and any unburned hydrocarbon fuel and its fragments that have escaped into the atmosphere. We can watch that happening too where the air includes surviving fragments of hydrocarbon fuel molecules. A lot of little steps are involved, and they occur at a wide range of rates. Let’s suppose that some unburned fuel escapes as ethane molecules, CH3CH3, 3. Although ethane is not present in gasoline, a CH3CH2· radical (Reaction 12) would have been formed in its combustion and then combined with an H atom in the tumult of reactions going on there. You already know that vicious little O atoms are lurking in the sunlit NO2-ridden air. We catch sight of one of their venomous acts: in a collision with an H2O molecule they extract an H atom, so forming two ·OH radicals.

In 1905, Sir William Crookes published a book entitled The Wheat Problem in which he reiterated what he had said in his British Association address of 1898. The content and tone are familiar: “The fixation of nitrogen is vital to the progress of civilized humanity, and unless we can class it among the certainties to come, the great Caucasian race will cease to be foremost in the world, and will be squeezed out of existence by races to whom wheaten bread is not the staff of life.” A whole gamut of processes for fixing nitrogen was described in a book published in 1914, and in 1919 an eminent U.S. electrochemist, H. J. M. Creighton, published a series of three papers entitled “How the Nitrogen Fixation Problem Has Been Solved.” However, the broader story was only just beginning to unfold. In about 1925, J. W. Mellor, in a justly celebrated sixteen-volume compendium, simply took Creighton at his word and stated quite baldly: “The problem has since [Crookes’ lecture] been solved.” Mellor describes not one but six processes that he believed were of industrial significance. These were: (1) the direct oxidation of dinitrogen by dioxygen to yield, initially, nitrogen oxides, as was undertaken in the Norwegian arc process; (2) the absorption of dinitrogen by metal carbides, subsequently developed as the cyanamide process; (3) the reaction of dinitrogen and dihydrogen by what has become known as the Haber process, or, more justifiably, the Haber–Bosch process; (4) the reaction of dinitrogen with metals, followed by treatment of the resultant nitrides with water; (5) the reaction of dinitrogen with carbon to form cyanides; and (6) the oxidation of dinitrogen during the combustion of coal or natural gas. Of these, only the first three really reached the stage of industrial exploitation, and only the Haber–Bosch process has been applied to any degree of significance since about 1950. The history of these three major developments is traced below. One of the first industrially significant reactions to be developed at the beginning of the twentieth century had already been known for more than 100 years.

When a patient says ‘swallowing difficulty’, they could mean: • Dysphagia: difficulty swallowing. If they really mean dysphagia, try to understand when/where exactly it feels as though the food ‘gets stuck’. Those with high dysphagia (oropharyngeal and upper oesophageal) describe difficulty initiating a swallow or immediately upon swallowing. Those with low dysphagia (lower oesophageal) feel the food getting stuck a few seconds after swallowing. • Odynophagia: painful swallowing. Odynophagia may be due to malignancy, but is more commonly a feature of infection such as candidiasis. • Globus: the common sensation of having a lump in the throat without true dysphagia. Globus is very common and its aetiology is poorly understood—however, only a small proportion of affected patients will seek medical help and it is an entirely benign condition. Broadly speaking, high dysphagia is more likely to be due to generalized/systemic neuromuscular disease, whereas low dysphagia is more likely to be due to a local obstructing lesion. New-onset dysphagia in middle-aged to elderly patients is carcinoma until proven otherwise. • What is the duration of the symptoms? This is a key question: a food bolus stuck in the oesophagus will typically appear immediately during a meal; cancer typically presents with a short history of days to weeks (not because the cancer has appeared in such a short space of time, but because it has reached a size where symptoms rapidly become apparent), whereas chronic motility disorders such as achalasia present with symptoms lasting months to years. • Is the dysphagia progressive or intermittent? Progressive dysphagia is highly suggestive of a stricture (benign or malignant), whereas intermittent symptoms are more characteristic of motility disorders. • Is the dysphagia to solids, fluids, or both? If the patient is able to swallow fluid as normal but has difficulty with solid food items (which feel as if they are sticking) this points towards a mechanical obstruction, i.e. a stricture (benign or malignant). Of course, as the stricture becomes more severe, then the dysphagia may start to involve fluids as well.

The world today is a very different place from what it was in about 1900. It is a very different place from what it was even in the 1960s. This is not to say that the worries and preoccupations of 1900 and the 1960s have just disappeared. Rather, they still remain, but as a consequence of the activities of the Club of Rome and the many similar organisations that have arisen since then, people are much more conscious of them. The famous energy crisis of 1973, provoked by the rapid quadrupling of the price of oil, hardly a natural process, served to push such considerations to the fore. The simple questions that were once posed (such as “How shall we feed a growing population?”) have been joined to many others. Is there a limit to population growth beyond which the potential food supply will really be exceeded? Is there a limit beyond which the perturbation of the environment by human actions will produce changes that will irretrievably damage both people and the environment? Are there really limits to growth? What can we reasonably do that will not produce disaster? This is a far cry from the Victorian and even old-fashioned capitalistic and Soviet attitudes that seemed then and still seem to assume that humans, being at the pinnacle of evolution (or, alternatively, being placed at the pinnacle of animal life by God), were free to exploit Earth and its resources as much as seemed necessary. Even to attempt to answer such questions, it is necessary to understand what the current state of Earth and the environment really are, and this is not simply a matter of looking out of the window and making a snap judgement, or even looking out of several windows over a certain period. It is necessary to do serious research and then attempt to make sound judgements. This is no trivial matter because often there is little objective guidance as to what constitutes a sound judgement. The idea that human activities are upsetting the current equilibrium between people and the environment is based upon a misconception.

The Nitrogen Group of the Periodic Table contains the elements nitrogen N, phosphorus P, arsenic As, antimony Sb, and bismuth Bi. The outer electron structure ns2np3 characterizes all five of the elements, with n representing principal quantum numbers 2, 3, 4, 5, and 6, respectively. The ns2np3 indicates the possibility of oxidation states V, III, and -III. As one goes down the group, the metallic character increases, with N and P being distinctly non-metals, As a metalloid, and Sb and Bi metals. However, the major bonding in most of the compounds of the group is covalent, aqueous cationic species being formed only by Sb and Bi. A covalency of 5 is exhibited by all the elements except N, this being assignable to the considerable energy required to place 10 electrons around the atom. The pentavalent state is the most stable for P, with its stability falling off down the group, as the trivalent state stability increases. Covalent radii in pm are as follows: N (75), P (110), As(122), and Sb(143). Ionic radii (most hypothetical) in pm are these: Sb+3 (90), Sb+5 (74), Bi+3 (117), and Bi+5 (90). a. E–pH diagram. Figure 9.1 depicts the E–pH diagram for N with the soluble species (except H+) at 10−1.0 M. Equations for the lines that separate the species are displayed in the legend. The colorless strong acid nitric acid HNO3, its colorless anion nitrate NO3−, the colorless weak acid nitrous acid HNO2, its colorless anion NO2−, the colorless ammonium ion NH4+, and the colorless hypothetical compound ammonium hydroxide NH4OH are involved.

Photochemistry provides the important driving force that initiates chemistry in the atmosphere. We saw in Chapter II how light absorbed by ozone generates the important HO radical, and, in Chapter III, we reviewed how light absorption by NO2 leads to ozone formation. In this chapter, we discuss the photochemistry of the light-absorbing oxygenates: their photochemical lifetimes and the nature of the modes of photodecomposition they undergo. Of course, light of sufficient energy per quantum must be absorbed by a molecule if its photodecomposition is to occur. The hydrocarbons do not absorb tropospheric sunlight, as seen in Figure VIII-A-1. The light gray and dark gray lines, respectively, show the distribution of actinic flux present in the troposphere and upper stratosphere for overhead Sun. It can be seen that the larger alkanes, alkenes, and aromatic hydrocarbons absorb at somewhat longer wavelengths than the first member of the family, but none can be electronically excited by tropospheric radiation. Among the hydrocarbons, only the polycyclic aromatics absorb appreciable tropospheric sunlight, and their π → π* excitation does not result in decomposition but likely generates O2(1Δg) molecules by energy transfer; these molecules are usually quenched by collision to ground state O2(3Σg−) molecules (see Calvert et al., 2000). As atmospheric oxidation of the hydrocarbons occurs, initiated largely by HO radicals, a multitude of oxygenated organic species are generated. The absorption region for the oxygenates is generally shifted to longer wavelengths, although the alcohols, ethers, acids, and esters still show no overlap of the regions of tropospheric actinic flux. For the families of compounds shown, the only significant absorbers of tropospheric sunlight are the aldehydes (e.g., CH2O) and the ketones (e.g., CH3C(O)CH3). Formic acid and methyl formate, as well as the larger members of the acid and ester families, absorb sunlight available only at the higher altitudes of the stratosphere, where they are expected to photodecompose. However, these species are not expected to be present in the stratosphere because they are removed in the troposphere largely via HO reactions. In this chapter, we focus on the rates and pathways for photodecomposition of the aldehydes and ketones with less detailed considerations of the other less prevalent light-absorbing trace compounds.

A wide range of trace gases, including dimethyl sulphide (DMS) and organohalogens, are formed in the surface oceans via biological and/or photochemical processes. Consequently, these gases become supersaturated in seawater relative to the overlying marine air, leading to a net flux to the atmosphere. Upon entering the atmosphere, they are subject to rapid oxidation or radical attack to produce highly reactive radical species which are involved in a number of important atmospheric and climatic processes. Organohalogens can affect the oxidizing capacity of the atmosphere by interacting with ozone, with implications for air quality, stratospheric ozone levels, and global radiative forcing. DMS and iodine-containing organohalogens (iodocarbons) can both contribute to direct and indirect impacts of aerosols on climate through the production of new particles and cloud condensation nuclei (CCN) in the clean marine atmosphere. Therefore, marine trace gases are considered a vital component of the earth’s climate system, and changes in the net production rate and subsequent sea-to-air flux could have an impact on globally important processes. In recent years, attention has turned to the impact that future ocean acidification may have on the production of such gases, with the greatest focus on DMS and organohalogens. In this chapter, the current state-of-the-art in this growing area of research is outlined. The oceans are a major source of sulphur (S), an element essential to all life, and marine emissions of the gas DMS (chemical formula (CH3)2S) represent a key pathway in the global biogeochemical sulphur cycle. The surface oceans are supersaturated with DMS relative to the atmosphere, resulting in a oneway flux from sea to air (Lovelock et al. 1972; Watson and Liss 1998). DMS is a breakdown product of the biogenically produced dimethyl sulphoniopropionate (DMSP): . . . (CH3)2S+CH2CH2COO- → (CH3)2S + CH2CHCOOH (acrylic acid) (11.1) . . . Single-celled marine phytoplankton are the chief producers of DMSP, and this reaction is catalysed intra- and extracellularly by the enzyme DMSP-lyase (Malin et al. 1992; Liss et al. 1997). The capacity of phytoplankton to produce DMSP varies between species, with prymnesiophytes considered to be the most prolific (Malin et al. 1992 ; Liss et al. 1997 ; Watson and Liss 1998).

Hydrological processes exert strong control over biological and climatic processes in every ecosystem. They are particularly important in the boreal zone, where the average annual temperatures of the air and soil are relatively near the phase-change temperature of water (Chapter 4). Boreal hydrology is strongly controlled by processes related to freezing and thawing, particularly the presence or absence of permafrost. Flow in watersheds underlain by extensive permafrost is limited to the near-surface active layer and to small springs that connect the surface with the subpermafrost groundwater. Ice-rich permafrost, near the soil surface, impedes infiltration, resulting in soils that vary in moisture content from wet to saturated. Interior Alaska has a continental climate with relatively low precipitation (Chapter 4). Soils are typically aeolian or alluvial (Chapter 3). Consequently, in the absence of permafrost, infiltration is relatively high, yielding dry surface soils. In this way, discontinuous permafrost distribution magnifies the differences in soil moisture that might normally occur along topographic gradients. Hydrological processes in the boreal forest are unique due to highly organic soils with a porous organic mat on the surface, short thaw season, and warm summer and cold winter temperatures. The surface organic layer tends to be much thicker on north-facing slopes and in valley bottoms than on south-facing slopes and ridges, reflecting primarily the distribution of permafrost. Soils are cooler and wetter above permafrost, which retards decomposition, resulting in organic matter accumulation (Chapter 15). The markedly different material properties of the soil layers also influence hydrology. The highly porous near-surface soils allow rapid infiltration and, on hillsides, downslope drainage. The organic layer also has a relatively low thermal conductivity, resulting in slow thaw below thick organic layers. The thick organic layer limits the depth of thaw each summer to about 50–100 cm above permafrost (i.e., the active layer). As the active layer thaws, the hydraulic properties change. For example, the moisture-holding capacity increases, and additional subsurface layers become available for lateral flow. The mosaic of Alaskan vegetation depends not only on disturbance history (Chapter 7) but also on hydrology (Chapter 6).

Since the days of the IBP, there has been a strong emphasis on research about the biogeochemistry of shortgrass steppe ecosystems (e.g., Clark, 1977; Woodmansee, 1978). A major theme has been seeking to understand spatial and temporal patterns and controls of biogeochemical pools and fluxes at scales that span from several centimeters to hundreds of kilometers, and from hours to millennia. The synthesis of this work has resulted in a conceptual framework regarding the biogeochemical dynamics of the shortgrass steppe, with two key components:… 1. Spatial and temporal patterns are controlled by five 1. major factors: climate, physiography, natural disturbance, human use, and biotic interactions. Plants are the most important biotic component. The interaction of these factors as they change in time and space determines the distribution and size of biogeochemical pools and the rates of biogeochemical processes. 2. Carbon (C), nitrogen (N), and other associated biologically active elements are overwhelmingly located belowground, with more than 90% found in soils (Burke et al., 1997a). This distribution determines the biogeochemical sensitivity of the shortgrass steppe to perturbations…. These ideas have been synthesized in the development of the CENTURY ecosystem simulation model, originally developed for grasslands and agroecosystems in the shortgrass steppe region of the western Great Plains (Parton et al., 1987, and chapter 15, this volume). The model represents complex interactions among the five controlling factors to simulate C and N cycling, and has served as an organizing framework for developing hypotheses and for evaluating questions that are dif. cult to address in the field (Parton et al., chapter 15, this volume). The objectives of this chapter are to describe how nutrient pools and fluxes are distributed in the shortgrass steppe, to characterize how the five controlling factors interact to create spatial and temporal patterns, and to evaluate the potential future changes to which the biogeochemistry of the shortgrass steppe may be particularly vulnerable.

Ecological modeling has played a key role in scientific investigations of the SGS LTER during the past several decades. The SGS LTER site, focused initially on the Central Plains Experimental Range (CPER), was the main grassland research site for the Grassland Biome component of the U.S. IBP effort (Lauenroth et al., this volume, chapter 1). Initial development of ecosystem models occurred from 1 970 to 1 975 as p art of t he I BP . All the U.S. I BP projects (grassland, tundra, desert, deciduous forest, and coniferous forest biomes) included research on the development of ecosystem models, with the goals of using models to help formulate and interpret field experiments, and of projecting the impact of changes in management practices on ecosystem dynamics. Models were developed as part of the Grassland Biome project (Bledsoe et al., 1971; Innis, 1978), and included modeling specialists who worked with research biologists on the development and formulation of the ecosystem models. The modeling activities of t he U.S. IBP Grassland Biome project included developing the ELM Grassland model (Innis, 1978). The ELM model was a complex process-oriented model that was intended to be used at all the Grassland Biome sites in the United States. This model was developed by postdoctoral fellows who were to formulate the different submodels, and then link the submodels using software that was developed as part of the program. The submodels included a plant production submodel, a cattle production submodel, a linked nutrient cycling and soil organic matter submodel, a grasshopper dynamics submodel, and a soil temperature and water submodel. Biophysical and biological data from the different sites were collected to develop and test the model. Model development was constrained by lack of knowledge about the biological processes that control ecosystem behavior, and by lack of appropriate data to test the ability of the model to simulate ecosystem responses to changes in grazing and fertility management practices. However, the ELM Grassland model was quite successful at investigating the interactions of different components of the ecosystem, and at helping to formulate new research efforts.