The
Nitrogen Cycle
The complex ecology of the earth is epitomized in
the circuitous cycle of nitrogen from the atmosphere to the soils and waters
and creatures of the earth and back again. Nitrogen as the diatomic gas N2
comprises 79 percent of the atmosphere and therefore 79 percent of every
inhaled breath taken by every living thing. The irony of nitrogen is that every
exhaled breath is also 79 percent nitrogen; it is essentially inertly
unreactive as a gas. However, the
element nitrogen is vital to life. It is a key constituent of the amino acids
(amines are nitrogen-hydrogen NH2 containing compounds) that build
protein molecules that do everything from transporting oxygen in the blood
(hemoglobin) to regulating glucose metabolism (insulin) and for the constituent
nucleotides of DNA. Nitrogen makes up about 2.5 percent of the human body, the
fourth most abundant element after carbon, hydrogen and oxygen. None of the
nitrogen in our bodies comes directly from the air we breathe. It is provided
by the nitrogen cycle. The decay of organic matter consequent to the death of
organisms yields the bulk of usable nitrogen; a global recycling carried out by
several types of soil bacteria that decompose proteins into ammonia (NH3)
that is subsequently converted to nitrite (NO2) and finally, the
familiar nitrate (NO3). The
first part of the cycle is appropriately called ammonification ant the second
part nitrification. The bacteria that carry out this crucial conversion do so
for the energy released by the oxidation of ammonia. There are many losses of
nitrogen in the recycling of extant nitrogen from the death and decay of living
things. Among these are the harvesting of crops, soil erosion, and the return
of the various monatomic nitrogen compounds back to the atmosphere in the form
of the diatomic nitrogen gas; this process is called denitrification. The
replenishment of lost nitrogen has been the bane of civilization since the
advent of agricultural and pastoral societies supplanted the hunter-gatherers
about 10,000 years ago and humans began to deplete the soils of nitrogen to
continually grow and harvest food.
The nitrogen cycle is sustained by the decomposition
of the gas N2 into elemental atomic nitrogen N so that it can be
used in chemical compounds. The decomposition of the nitrogen gas into
elemental nitrogen is called nitrogen fixation. There are three ways for this
to occur; all require energy. The first
is atmospheric, comprising about 5 percent of elemental nitrogen production. Lightning,
the raw energy of nature, courses through and electrifies the air to create
elemental nitrogen from the gas; the resultant compounds in the form of
nitrates (NO3) fall to earth with the attendant rain. The second
source of liberated nitrogen is provided by the telluric engines of nitrogen
fixation: bacteria, and their distant cousin archaea; referred to generically
as nitrogen-fixing bacteria or diazotrophs. Before the industrial age,
atmospheric and nitrogen-fixing bacteria were the only sources of the elemental
nitrogen necessary for all living things. Artificial
nitrogen fixation, known as the Haber-Bosch process, was developed in the early
20th Century and is now responsible for about 30 percent of all
elemental nitrogen in the form of fertilizers; it is the third source of
elemental nitrogen.
The problem with transforming gaseous nitrogen in
the form N2 into the monatomic N is that the bond between the two
nitrogen atoms is very strong. In a simplistic sense, atoms join together to
form compounds in order to reach a stable low-energy state that occurs when the
outmost shell of each atom is filled. The inert gases mark the points on the
periodic table where this occurs. Helium with 2 electrons is the first inert
gas, neon with 10 electrons is second continuing through argon, krypton, xenon
and radon. The quantum energy levels of the electrons around the nucleus of the
atom are assigned the principal quantum numbers 1, 2, 3 … or, alternatively K, L,
M … (the letters are an artifact of the spectroscopy terminology by which they
were first discovered). In each principal quantum level, there are intermediate
sublevels that determine the geometric shape of the electron's possible
locations. These are called subshells as they represent different energy levels
within the principal quantum level. The ‘s’ subshell
has 2 electrons, the 'p' subshell has 6 electrons, the 'd' subshell has 10
electrons and the 'f' subshell has 14 electrons. Chemical bonds occur between atoms as the
electrons in the outer or valence subshell seek to establish a stable state,
which means that their outer subshell is filled. The nature of chemical bonding
was first posited by the American chemist Gilbert Lewis in 1923; the eponymous
Lewis theory has four fundamental tenets: (1) elements enter into compounds so
as to share or exchange electrons; (2) in some cases the electrons are
transferred from one atom to another (an ionic bond); (3) in some cases the
electrons are shared between the two atoms (a covalent bond); and (4) each of
the constituent atoms ends up with an "inert gas" outermost, or
valence, electron shell.
Nitrogen has an atomic number of 7 to indicate the
number of protons and an atomic weight of 14 to indicate that it also has 7
neutrons. This also means that it has 7 negatively charged electrons to balance
the positive electrical charge of the protons.
Nitrogen seeks the stability of the inert neon, which has 2 electrons in
the 1s subshell, 2 electrons in the 2s subshell and 6 electrons in the 2p
subshell. Since nitrogen only has 3 electrons in its 2p subshell and needs 3
more for stability. Therefore, one
nitrogen atom preferentially bonds with another nitrogen atom so that they
share the 3 outer electrons, each seeming to have 6 in the 2p subshell. It is
very stable in this configuration; so much so that enormous amounts of energy
are required to break the three di-nitrogen covalent bonds. It is easy to see
how lightning provides this energy; one discharge releases about 3 billion
joules of energy, the electrical usage of an average American in a month. It is
not easy to see how bacteria, prokaryotes with no cell nuclei and the simplest
of all living things could do this; it is one of the wondrous curiosities of
nature.
Most of the nitrogen fixing bacteria are those in
the genus Rhizobium – they are
generically called rhizobia - that live in a mutualistic symbiotic relationship
with plants in the Legume family, which includes peas, beans, clovers, alfalfa
and vetch. The bacterium sends out a
molecule called a nodulation factor that is specific to the receptors on the
root hairs of the appropriate legume – i.e. there are specialized bacteria for
specific legumes. The correct relationship having been established, the
bacterium is permitted entry to the root cell cortex to establish a bacterial
colony that is manifest in physical nodule formation. The benefit to the legume
in its association with the bacteria is perhaps obviously the monatomic
nitrogen in the form of ammonia that is necessary for the plant to grow. The
bacteria are provided the nutrients required for growth and multiplication from
their plant host. The legume also provides the enzyme nitrogenase, a catalyst
that is required for the nitrogen reduction reaction to occur; it has the
physical topography to align the required reactants. The energy needed to break
the three covalent bonds to make monatomic nitrogen from the diatomic gas is 420
kilojoules/mole. The rhizobia provide this energy from the conversion of
adenosine triphosphate (ATP) to adenosine diphosphate (ADP), the fundamental
mechanism that cells use to convert chemical energy for metabolic processes.
Each conversion releases 30.5 kilojoule/mole and releases phosphorus (P). The
overall reaction is:
N2 + 8H+ + 8e- + 16ATP à 2NH3 + H2 + 16ADP + 8P
The
nitrogenase, which consists of an iron protein and a molybdenum-iron protein,
provides the electrons, which are thought to be the rate-limiting step for the
reaction. Molybdenum and iron are thus critical elements to the fixation
process, as are the 16 ATP to ADP conversions that release 488 kilojoules to
provide enough energy to drive the reaction to the right. There are other
nitrogen fixing bacteria in the soil that create ammonia, but they are of
lesser import.
For millennia, the process of nitrogen fixation by
the diazotrophs with the small addition of lightning induced atmospheric
nitrate was sufficient to supply the soils with adequate nitrogen to ensure
fertility. Early hominids had little impact on the nitrogen supply, as they
subsisted near the top of the food chain as hunter-gatherers. It was when the
cultivation of crops and the domestication of animals led to settlements that
soil fertility problems first became manifest. It is simply not possible to
grow crops other than legumes in the same location and harvest them every year,
as each harvest removes about 25 pounds of nitrogen. There is no record of the first
realization that farming in the same location led to reduced yields, but it is
certain that it happened. There is also no record of the first recognition that
some plants, the legumes, would enrich the soil. By the time of the Greek and
Roman empires, it was well established that there was a need to replenish soil
fertility. Theophrastus (371 – 287 BCE), a student of Aristotle and the father
of botany documented the importance of
soil fertility and recognized the significance of bean legume plants in his
eight volume compendium On the Causes of Plants. Gaius Plinius Secundus, better known as Pliny
the Elder (23-79 CE) provides details on crop rotation and the need to plow the
green growth of fallow fields under to maintain fertility in his The Natural
History (Latin: Naturalis Historia),
which was published from 77 CE until his untimely death in Pompeii when
Vesuvius erupted in 79 CE. This was not just true in Europe: the Chinese
rotated soybeans, the Indians lentils and the Siamese mung beans. It was thus well established that there were
limits to soil fertility, and that this was related to land use, and,
ultimately to human population.
Late in the 19th Century, it was evident
that eventually the nutritive needs of ever expanding population would become
problematic relative to the amount of arable land that was available for
growing food. The Reverend Thomas Malthus (1766 – 1834) famously wrote in An
Essay on the Principle of Population that “"The power of population is
so superior to the power of the earth to produce subsistence for man, that
premature death must in some shape or other visit the human race …… gigantic
inevitable famine stalks in the rear, and with one mighty blow levels the
population with the food of the world.”
His views are often expressed in the notion that populations increase
geometrically (1, 2, 4, 8 …) while food supply can only grow arithmetically (1,
2, 3, 4 …). Crop rotation and the growth of leguminous crops were not enough. There
was simply too great a demand for the food that increasingly infertile fields could
provide, each harvest resulting in a diminution of nitrogen. Farmers sedulously
gathered manure to spread on the fields to increase fertility, taking advantage
of the recycled nitrates that they harbored. By 1800, the larger population
centers in Europe had started to experience inadequacies in food production
even with the optimum utilizations of manure and crop rotation. The short term
answer was nitrates from South America, first the bird guano of the Peruvian
Chincas Islands, and then, when that was depleted, the refined nitrates from
the caliche deposits of the Chilean Atacama Desert. The bird guano was the
result of millions of years of sea bird habitation; it was removed by the
shipload for fertilizing the fields of Europe and depleted in the two decades
between 1840 and 1860. The nitrate
demand for fertilizer was supplemented by their demand for explosives with the
invention of nitroglycerine and dynamite by Alfred Nobel in the 1860’s. The
Atacama Desert became the most valuable natural resource in the world; the
British, Germans, French and Americans has staked out claims by 1900. The world needed the nitrogen of nitrate for
fertilizer to make food and for dynamite to make war.
As related in The Alchemy of Air by Thomas
Haber, the threat to the world food supply due to the imbalance between
population and food production, a new and improved Malthusian prognostication,
reached the halls of the British Academy of Sciences in 1898. Sir William
Crookes, its incoming president, began his inaugural speech with the
asseveration that “England and all civilized nations stand in deadly
peril.” He proceeded to explain his
calculations: the depletion of the South American nitrates would result in
world-wide famine by the 1930’s, a scant three decades away. “We are drawing on
the earth’s capital, and out drafts will not be honored perpetually.” He concluded with his prescription; that the
world’s scientists embark on an unprecedented enterprise: to find a way to fix
elemental nitrogen synthetically directly from the atmosphere; to make N from N2. The challenge, though not insurmountable,
was daunting. What was needed was enough energy confined to a small enough
space to apply to the nitrogen bond-breaking problem. Initial efforts,
primarily in the United States and Norway, were directed at electric arcs,
emulating the natural atmospheric lightning process. These methods were
eventually abandoned due to the scarcity of electrical generating capacity and
collection and distribution problems attendant with the nitric acid product
that resulted. The Germans took a different approach, one based on chemistry,
metallurgy, and mechanical engineering.
The motivation to fix nitrogen from the air was a
matter of national interest for Germany, which only added to any humanitarian
aspects of the challenge posed by the British Academy of Science. Germany had
emerged as a major European land power following the consolidation of its many
individual and fractious states under the aegis of the Kingdom of Prussia and
its success in the Franco-Prussian War of 1870. It was a matter of great concern to the
Germans that they depended on the nitrate deposits of Chile for the manufacture
of explosives, the lifeblood of military force projection. The British
domination of the seaways could inevitably result in their enervation due to a
naval blockade should a conflict arise. Dr. Fritz Haber was a professor of physical
chemistry at the University of Karlsruhe just south of Heidelberg when he set
out to fix nitrogen from the air. The basic process consisted of combining
atmospheric nitrogen with hydrogen in the presence of a catalyst to produce
ammonia according to the relatively simple:
N2 + 3H2 à 2 NH3
The problem was that a temperature of ~ 600°C and a pressure
of ~ 200 atmospheres (about 3,000 pounds per square inch or psi) were required
to break the nitrogen bonds – something that nitrogen fixing bacteria do at
room temperature (25°C and 1 atmosphere – 14.7 psi). These extreme conditions
greatly exceeded the capabilities of any industrial or laboratory equipment
available at the turn of the last century.
The laboratory autoclave that Haber designed and built was made entirely
of quartz; a metal pressure vessel would have exploded. The catalyst that was
needed to physically align the nitrogen with the hydrogen to facilitate the
reaction was a matter of trial and error that included iron, platinum and nickel
until osmium, the densest and least abundant of the earth’s crustal elements,
was found to result in the highest level of ammonia generation. It worked well
enough (at 125 milliliters per hour) to
approach the German chemical firm BASF for purposes of scaling up the rate of
ammonia (NH3) generation necessary for industrialization of nitrate production,
no mean task.
Carl Bosch was a chemist with a background in
metallurgy employed by BASF who thought that he could engineer a renitent oven
that would meet industrial capacity needs. BASF promptly (and quietly) cornered
the world market in osmium and provided Bosch with unlimited resources. The
challenges were prodigious. Pure nitrogen gas was obtained by liquefying air
and slowly heating it so that only nitrogen evaporated. Pure hydrogen gas was
extracted from water by heating steam with coke (baked coal). A readily
available catalyst was created after experiments with thousands of elements and
combinations of elements in the form of an alloy of iron, aluminum and calcium.
But the real challenge was the design of the ovens, which had to operate at a
temperature at which iron glows red and a pressure that was twenty times higher
than the boiler in a steam engine. Bosch approached Krupps, Germany’s premier
armaments manufacturer, to make the first oven - an eight-foot tall cylinder
with inch thick walls. It ran for three days before it burst. The walls of the
oven were sectioned and, when examined microscopically, revealed the presence
of many tiny cracks permeated with hydrogen. Bosch had discovered a phenomenon
now well known among materials engineers, the hydrogen embrittlement of high
strength steels. He solved the problem in a uniquely elegant way - by having an
inner steel liner that would be subject to the high pressure hydrogen and
embrittle and an outer shell that would be subject to a lower pressure. Ammonia
was being produced at the rate of two tons a day by early 1911. A source of
man-made fertilizer was in statu nascendi. Fritz Haber moved on to become the
head of the Kaiser Wilhelm Institute in Berlin and led the team that developed
and deployed the deadly chlorine gas in World War I. He won the Nobel Prize in
chemistry in 1918. Carl Bosch was one of the founders and the first head of the
German chemical conglomerate IG Farben. He won the Nobel Prize in chemistry in
1931. By 1913, the Haber-Bosch process was making 20 tons of ammonia a day and
furnished the Germans with ample nitrates for war munitions though 1918.
It is now 100 years since the first
industrialization of nitrogen by Bosch at BASF; the Haber-Bosch
process, a Gesamkunstwerk of German engineering, will produce about 500 million
tons of fertilizer in 2013. The hydrogen necessary to react with nitrogen that
Haber and Bosch got from water is now derived from methane (CH3),
the consumption of which for nitrate production comprises 5 percent of the
annual global natural gas production.
The electrical power necessary to pressurize and heat the ammonia ovens
requires about 2 percent of the world’s total annual electricity generating
capacity. The fertilizer derived from the process is responsible for feeding
about one third of the earth’s population. Had it not been for the advent of manufactured
nitrates, Malthus would likely have been right. As fossil fuels are depleted
over time, and absent any technological breakthroughs, the nexus between energy
and food security will again become manifest, and Malthus may well be noted for
his prescience. While this is in and of itself a troubling state of affairs, it
is not the only problem with artificial fertilization. Nitrogen compound
run-off from agricultural farmlands results in pelagic dead zones. The
acidification of freshwater due to nitric acid results from the reaction of
soil microbes with nitrates (NO3-) and ammonium (NH4-). Gaseous emissions of nitrous oxide (N2O),
which is also a by-product of nitrogen related soil microbial activity,
contributes to the greenhouse effect.
Eutrophy is the healthy action of nutrition
functions in an organism or system. It has taken a pejorative connotation in
its application to the mostly neritic coastal areas at the mouths of major
rivers and bays. Eutrophication is the over-abundance of nutrients due
primarily to the run-off of artificial fertilizers into river estuaries that
ultimately reach the ocean. While the increase in nutrients will generally have
some ameliorative short term effect such as increasing the fish population, the
long term result is hypoxia, the decrease in dissolved oxygen. The quid pro quo
of increased nutrients that results in regions where nothing can live is a bit
counterintuitive. The dead zone phenomenon occurs because excess nitrogen nutrients
cause surface algal growth to explode, the so-called algal bloom, blocking
sunlight from penetrating the underlying waters. The cascading detrimental
effect of blocking sunlight to plants in the lower regions of the water column
is their inability to photosynthesize glucose and produce oxygen. Dissolved
oxygen is necessary for the respiration of heterotrophic sea animals and its
lack results in death in the dead zone. Nitrous oxide (N2O) is the
least known of the three "major" greenhouse gases (the others are
carbon dioxide and methane); its provenance usually listed as "agricultural
soil management," mostly from fertilizers. Nitrous oxide constitutes about
8% of the total greenhouse gas composition, but, since it has a Global Warming
Potential (GWP) of 310, it is over three hundred times worse than CO2.
The Nitrogen Cycle is vital to life on earth. Though
we are surrounded by a dense blanket that is almost 80 percent gaseous
nitrogen, we cannot use it. It must first be changed into nitrogen compounds
that we can use. For the billions of years that preceded the appearance of
hominids about 5 million years ago, nitrogen compounds were created by nitrogen
fixing bacteria and lightning and recycled by the process of death and decay to
sustain the cycle. The growth of populations of Homo sapiens incident to the discovery of agriculture eventually
subverted the cycle. More nitrogen was being taken out in the form of crops
than was being returned. For the hundred years of the 19th Century,
nitrogen from bird droppings and mineral deposits from South American deserts
sufficed to make up the difference. For the hundred years of the 20th
Century, the manufacture of artificial fertilizer using gargantuan quantities
of methane and electrical energy as input for the Haber-Bosch process enabled
the Earth’s population to double between 1950 and 2000. The agribusiness trends
of the 21st Century with increased reliance on monoculture crops
sustained by manufactured fertilizers do not auger well for the future. The
eutrophication of the oceans, the acidification of the lakes and the increases
in nitrous oxide in the atmosphere are all indicative of inchoate problems that
must be addressed. As fossil fuels deplete over the next century, it is not at
this time clear how the world’s population will be sustained in the absence of
industrial fertilizer. Perhaps a latter day Fritz Haber and Carl Bosch will
figure it out. The future viability of a global human society depends on it.