L’azote est un des principes les plus abondamment répandus dans la nature. Combiné avec le calorique, il forme le gaz azote ou la mofette, qui entre environ pour les deux tiers dans le poids de l’air de l’atmosphère. Il demeure constamment dans l’état de gaz au degré de pression et de température dans lequel nous vivons ; aucun degré de compression ni de froid n’ont encore pu le réduire à l’état liquide ou solide.
Antoine Lavoisier, Traité Élémentaire de chimie, p. 150 (1789)
Then the idea flashed across my mind like lightning, as I was savouring the last slice of Parma ham (prosciutto di Parma), one of life’s little guilty pleasures. I had bought it on impulse because I felt that I needed to stimulate my neural reward system to counterbalance the sombre feelings knocking at the door of my empty flat, a place I would have to learn to call home after an improvised, unplanned relocation. So there sat I, on an overinflated exercise ball (my only chair), gobbling down slice after slice of Parma ham, the scattered remains of my first supper taking up all the surface of a small folding table. My eyes could not help dwelling on the jagged cardboard boxes lying around me, all of them bursting with books, clothes, heirlooms of past lives looking for a present reincarnation.
Then the idea struck me, as the Parma ham was making its way into my stomach: packing and relocating, this is what a biogeochemical cycle is all about. An element becomes incorporated in a molecule, in a crystal, and then it contributes to building a rock, a complex scaffolding, a living organism, taking up temporary residence, a transitory permanence, like a tenant on a short-term let. I remembered my previous flat, the chinaware kept tidily in kitchen cupboards waiting for guests that never showed up at tea-time, all the framed photographs that enlivened the living room: that life seemed like a tenancy bound to last forever. Then, the hour comes, time is up, the contract expires, and this fragile stability is dismantled: decay sets in, things end up in disarray, and transformations occur; the crockery is protected in bubble wrap – some might break during the removal! – the pictures are rolled and stored, and all the belongings are packed in boxes, then stored in a room or shipped to a new place. Yet, this transient phase is not a death, is just an intermediate step before a new reassembly, a new beginning, although the waiting could be a long one. In a biogeochemical cycle, elements are similarly locked away in reservoirs, packed and temporarily immobilised in a form that is not readily available for chemical reactions, or easily accessible for organisms, like books in a cardboard box. No matter how long it takes, the element will be set free again sooner or later, popping out of the reservoir like a jack-in-the-box, ready to transform again into something different. Likewise, there will come a day when my books packed in boxes full like a tin of sardines find their shelves again, and my flat will finally be a cosy place I can certainly call home.
I came round after all this day-dreaming and I realised that the salty ham had made me thirsty. Rapidly, I gulped down a glass of water and just a second later I was shaken by loud hiccups: so ravenous my appetite for a new life was, what a glutton I had been!
Nitrogen cycle: the ham…
To start with, there is something scary about the nitrogen cycle, and that has to do with the reservoir molecule itself, N2: this diatomic species surrounds us, floods us with every breath we take; it is invisible, it is odourless, but it is ubiquitous and it can turn into a ruthless killer, despite being a non-toxic gas. In fact, N2 is an asphyxiant gas, and oxygen deficiency is one of the main hazards arising from the use of liquid nitrogen: as soon as the oxygen content of the atmosphere decreases below 11 %, fainting can take place without prior warning. Like many other elements, nitrogen is profoundly ambivalent, and this is reflected even in the two names this element is known under: there is nitrogen 1 and its positive nuance of “generator of nitre (aka KNO3)”; then, Lavoisier’s own choice was much more sombre: azote 2, the “lifeless”. There would even be a third name, now mostly forgotten, but equally evil-sounding: Rutherford’s noxious air, which is self-explanatory.
There is no better embodiment of the nitrogen cycle than a slice of ham; in this case, however, I had better just refer to a generic cooked ham (prosciutto cotto), rather than the prized delicacy from Parma I was savouring the other day. Nitrogen is found in ham mostly as a component of the amino acids assembled into the 19.8 g of proteins per 100 g of generic cooked ham3. These proteins, part of the muscles of the pork leg processed into cooked ham, derive from the metabolism of the unlucky pig that met an untimely end. For animals to fatten faster and faster, highly nutritious fodder is required, which can be provided by intensive agriculture: that is where nitrogen is again heavily involved…
…I recall, long time ago, a train journey across the plain of northern Italy (Pianura Padana), scattered farmhouses amid the maize fields stretching on both sides of the railway line: I was young, and it seemed that the summer would draw on forever, an endless sequence of glorious sunshine basking in the scorching light of the dog days. That is holiday-time, when sultry afternoons tempt to laze around, hoping that the autumn rains will never fall. Come August, the fields are lush, maize stalks are full-grown, like dark green bristles of a giant brush. Yet very little of this maize bonanza will end up as coarse, yellow cornflour for polenta: most of it will be harvested and processed into animal fodder, and some of it will feed pigs. This highly-productive, intensive agriculture would not be feasible without agricultural machinery and synthetic fertilisers. The example par excellence of a high-nitrogen fertiliser is ammonium nitrate (NH4NO3), an oxymoronic salt entailing two antipodal moieties, the two extremes of the nitrogen cycle, nitrate and ammonium, +5 and −3 in terms of oxidation state. This is no peaceful coexistence, as ammonium nitrate can quite easily explode, but there is another “explosive” side to this salt: its use can boost agricultural yields, helping to feed the growing world population, and as a consequence the demand for nitrogen-containing fertilisers has skyrocketed.
The story of the development of synthetic fertilisers is a tale of ingenuity, fruitful collaboration between academia and industry, but also nationalism and warfare, and an early example of a dual-use technology4. The chemical process that allows us to “mine” the atmosphere for nitrogen, transforming inert N2 into a form of nitrogen both available for plants and useful for the chemical industry, like ammonia (NH3), is called the Haber-Bosch process. Its chemical reaction is:
N2 + 3H2 → 2NH3
This process was made feasible by a combination of engineering advances (i.e. how to perform chemical reactions at high pressure) and the development of an active, yet affordable, catalyst, in this case Fe (plus trace amounts of components that help the reaction further, i.e. promoters). What a stroke of luck! Fe is one of the most abundant metals in the Earth’s crust! This is the reason why the original catalyst developed in Germany in 1909 is still largely in use today. Ammonia is, though, just half of the ammonium nitrate, the other being HNO3, which is produced by “burning” (well, oxidising…) ammonia itself by means of the so-called Ostwald process, another breakthrough of the German industrial chemistry of the early 1900s. When it comes to nitrogen chemistry (as in so many other fields, say football, the governance of the Eurozone, reliable cars and top-class Formula 1 drivers), Germany rules.
So, ammonium nitrate can be seen as the final product of two veritable “pillars” of chemical industry; on the other hand, when a poetical state of mind carries me away, I rather think of NH4NO3 as the recombination, though unstable, of the very two indistinguishable nitrogen atoms once bound by a triple wedlock, and separated by the brute force of an ingenious chemical process…in John Donne’s own words, from A Valediction: Forbidding Mourning:
Our two souls therefore, which are one,
Though I must go, endure not yet
A breach, but an expansion,
Like gold to airy thinness beat.
This ion pair, unstable reunification of what was once impossible to tell apart, also features in this artist’s impression, taken from the back cover of my PhD thesis5
But if we just stick to chemistry and think in terms of all the chemical reactions mentioned so far, Lavoisier would remind us that “nothing is lost, nothing is created, everything is transformed”6; thus, we could as well state that ham is a slice of sky, noxious air turned into tasty meat.
Yet, there is more than just pork in a slice of ham. Nitrogen comes into play also in the form of controversial but key additives which are of paramount importance in terms of food safety. In the first place, there is nitre itself, aka saltpetre or potassium nitrate, the “E 252” additive appearing on the list of ingredients. Curious readers can read the full story elsewhere7; in short, saltpetre, a minor component of the salt mixture which was traditionally used to cure meat, turned out to be a boon to ward off botulism, which is often fatal. In fact, halophiles bacteria surviving the salting process feast on nitrate transforming it into nitrite (NO2−), which effectively inhibits the growth of Clostridium botulinum, along with other microorganisms8. Additionally8, nitrite contributes to the development of the typical flavour associated with cured meat, favours the long-term conservation of the cured meat and is instrumental in developing an intense, eye-catching red colour (which is also a desirable “marketing side-effect”). Nitrite can be directly added as potassium (E 249) or sodium nitrite (E 250).
There has been an intense debate about the threats to human health arising from the intake of nitrite from diet. I have had a quick look at the literature and one of the most recent reviews8 states that earlier conclusions linking consumption of nitrite-containing processed meat and (stomach) cancer are “largely based on epidemiological associations of weak magnitude and have been challenged by researchers both associated and independent of the meat and poultry industry”, despite adding that “[t]he intense debate will continue”. That said, data reported in the same review point out that the nitrite content of spinach is slightly higher than that of cured meat…should Popeye worry? Most likely not, or at least not yet (as long as Olive Oyl does not grow tired of his boring diet!). In the meantime, those among you who can read Italian should have a look at this hilarious post by Bressanini about cured meat and the misuse of the word “chemistry” as a synonym of “non-natural”.
…and the evil
Nitrites are definitely not the evilest molecules in our food, and yet there is indeed a worrying aspect of the nitrogen cycle. As the Nobel laureate in Chemistry Paul Crutzen remarked, we live in an age that can be called Anthropocene, and the nitrogen cycle has not escaped human influence.
Let us go back again to the slice of ham, let us talk about feeding the planet, arguably a “hot topic” since it is the focus of the upcoming EXPO 2015 in Milan. Recent estimates have indicated that a whopping 48% of the world population in 2008 was fed thanks to nitrogen fertilisers4: one could say that almost half of the world population depends on our ability to turn atmospheric nitrogen into ammonia, and this is no hot air! Here is a quick back-of-the-envelope calculation: the amount of nitrogen consumed in food equals 15 G m3 of pure N2 under standard ambient temperature and pressure conditions: this corresponds to the volume of 82 billion footballs! Half of this “edible air” is man-made, through the Haber-Bosch process. Unfortunately, the efficiency in fertiliser use is very poor, which means that most of the nitrogen added to crops is not incorporated in plants, but it is lost to the atmosphere, the soil, or groundwater. This enormous injection of “reactive” forms of nitrogen sets in a cascade of harmful environmental consequences: nitrogen atoms are recycled very fast and a single nitrogen atom can wreak havoc in many different ways, from the depletion of the ozone layer to algal blooms in coastal areas. Scientists have raised significant concern about the unwanted fallout of the nitrogen bonanza4.
So, sadly, it is time for ammonium nitrate to part again. It never rains but it pours, and nitrate is lost, washed away, but nor does ammonium reach the stage of the nitrogen cycle I like the most, the slice of ham. This cation can sometimes experience an ethereal afterlife: under certain conditions, it rises up to the atmosphere as NH3, still largely unchanged after emerging from the flames and pressures of that veritable Hephaestus’s forge called Haber-Bosch process. Back to the same settings it hailed from, NH3 whirls around, experiencing the breeze, the freedom of an airborne life. Loneliness does not befit NH3, however, and every cloud has a silver lining, because it is amid water and particles that ammonium meets nitrate again: it is a hazy affair, however, since airborne salts like this are part of the aerosol that blurs blue skies, that much-feared particulate matter. This lasts a cloud’s lifetime; during their tryst, the ions hasten to tell about their past lives. Ammonium listens as its loved anion narrates an eventful adventure that started in another hellish environment, the combustion chamber of a Diesel engine:
“In my million-year-long previous life as N2 I always used to like watching the harvest. This time the two of us were flying above lush maize fields among the swallows and their acrobatic, unpredictable flight. One of them breathed us in, but I knew we would be freed again soon, my partner and I. Yet, the bird nosedived to pursue a fly, and no sooner had it expired us than we found ourselves, close, too close to the air intake of the combined harvester that was cutting its way through the cornfield. A spray of fuel choked me unconscious. Nothing, I can remember nothing else at all. Then when I woke up I was drifting alone, dizzy, with a sour aftertaste in my mouth, and oxygen atoms all around me”.
Fossil fuel combustion, in Diesel engines in particular, injects in the atmosphere large amounts of reactive nitrogen as nitrogen oxides, the infamous NOx, which are eventually transformed into nitrate, or they contribute to ozone formation as part of “photochemical smog”. Come rain, the fate of nitrate incorporated into aerosol, for example as ammonium nitrate, is that of a precipitous descent onto the ground, and a new separation. There is no easy way back to the atmosphere: the higher you fly, the harder you fall…
…Thunderbolts and lightning during a late-summer downpour, the harbinger of autumn. Ammonium nitrate crashes onto the ground in the heavy rain, the ions dissolve, and bacteria in the well-aerated soil feast on ammonium, oxidising it into nitrate. This process, called nitrification, is a key step of the nitrogen cycle:
NH4+ + 2O2 →(aerobic, autotrophic bacteria) → 2H+ + H2O + NO3−
Regardless of its source, nitrate is is the most mobile nitrogen-containing inorganic molecule: soils tend to have an overall negative electric charge, as nitrate, which is then repulsed and easily leaks away from fields into groundwater, into rivers, towards estuaries and the sea. It is sadly ironical that the overuse of fertilisers, so instrumental in the production of the large amount of fodder required to feed animals and, in turn, to satisfy the gluttony of meat-loving affluent societies, will eventually result in an undesired, sumptuous banquet for aquatic plants. In fact, some ecosystems feature nitrogen-poor conditions: if they are thrown off-balance by a large influx of nitrogen (as nitrate for example), algal blooms will occur, the well-known “eutrophication”. This name itself looks like another ironic pun: eutrophic, from the Ancient Greek εὖ and τροφή , means “well nourished”… Wetlands and coastal areas are particularly vulnerable: “dead zones” can develop, which risk becoming hotbeds of disease-carrying insects, with a potentially frightening impact in tropical areas: imagine densely populated coastal regions bordered by mosquito-ridden eutrophic marshes.
However, ecosystems display a certain ability to cope with some extra influx of nitrogen as nitrate, thanks to naturally-occurring denitrifying bacteria.
2NO3−+ 10e− + 12 H+ →(facultative anaerobic bacteria) → 6H2O + N2
This reaction takes place under oxygen-poor conditions, think of subsurface water or sediments at the bottom of a river. By sheer chance – what a luck! – the two nitrate molecules that are absorbed by one of these bacteria dwelling in the sediments of a coastal wetland contain exactly those two nitrogen atoms originally bound together in the N2 molecule that reacted in the Haber-Bosch process. Romantic as this encounter might sound, denitrification is a matter of survival for these bacteria, which normally would use oxygen for their metabolism – just as our cells do – but that are forced, in the absence of O2, to switch on this second-choice metabolic pathway, dumping electrons from catabolism onto nitrate molecules.
Aptly enough, it was in the depth of an oxygen-deprived sediment layer that the two nitrogen atoms were reunited again into N2. The molecule floats, rises, emerges from the salty sea, takes off into the air and ascends to the place where it belonged. Millions of years will pass before these nitrogen atoms will be split apart again to take another ride on the merry-go-round of this biogeochemical cycle. The diatomic couple spins like ice skaters, one atom seemingly fixed while the other, triply holding fast, circles around, like the compass in the poem by John Donne:
A temporary epilogue
Romance aside, are we really between the hammer and the anvil? It is hard to answer, in particular if we want to answer in the future tense.
In a nutshell, the Haber-Bosch process has already “mobilised” a huge amount of nitrogen by transforming the inert N2 that constitutes the reservoir into highly-reactive molecules that are also extremely mobile, inducing a cascade effect through repeated cycling between various species, as we have seen for ammonium and nitrate. Neglecting for a moment the contribution from fossil fuel combustion, ammonia synthesis itself has had the dual effect of providing mankind with a cheap, abundant source of fertilisers while offsetting the balance of the nitrogen cycle. The large inefficiency in fertiliser use compounds this problem: nitrate leaks to vulnerable ecosystems, and, under certain circumstances, bacteria cannot even convert nitrate all the way to N2, stopping in their tracks at N2O. To add insult to injury, this “laughing gas” is a treacherous molecule, endowed with the double-whammy of being a potent greenhouse gas and of being able to destroy ozone. The loss of this precious stratospheric sunshade, in fact, is currently mostly due to N2O released from agricultural soils…think about it next time you slice some ham: the ozone layer is being sliced too!
Scientists have tried to model the future global demand for nitrogen-containing fertilisers from now to 2100 4. Factors that contribute to increase the use of fertilisers are:
- growth of world population
- expansion of cropland for biofuel production
- increased worldwide meat consumption
On the other hand, the global demand for fertilisers would be curtailed by the following factors:
- decrease of global fertility rates (i.e. a constant world population in the long run)
- increasing yields with an increased fertiliser efficiency
- optimisation of “protein efficiency” in the human diet (i.e. an increase of the share of animal proteins coming from dairy products with respect to meat: milk proteins incorporate nitrogen more efficiently than meat)
If the latter factors counterbalance the former three in the coming years, then the global demand for nitrogen fertilisers should level off soon. On the other hand, an increased biofuel production would almost single-handedly double the global consumption of nitrogen-containing fertilisers.
Whatever the scenario, ingenious, out-of-the-box solutions could arise from the same carefree curiosity (and chemical ingenuity) that inspired Primo Levi’s unsuccessful attempts to prepare an ingredient for lipstick (alloxan) from…excrement, which
because of its nitrogen content, it is highly valued as a fertilizer for truck gardens[…] I devoted a day to a coarse sifting of the chicken shit, and another two trying to oxidize the acid contained in it into alloxan. The virtue and patience of ancient chemists must have been superhuman, or perhaps my inexperience with organic preparations was boundless. […] The shit remained shit and alloxan and its resonant name remained a resonant name. […] Best to return among the colorless but safe schemes of inorganic chemistry
Primo Levi, Nitrogen, in The Periodic Table
(English translation by Raymond Rosenthal), Penguin Books, 2000,
1 French nitrogène, from the Greek νίτρον + -γενής, meaning “begetter of nitre”.
2 French azote, from the Greek ἄζωτος, meaning “lifeless”.
3 According to the data reported by the Istituto Nazionale di Ricerca per gli Alimenti e la Nutrizione.
4 J.W. Erisman et al., “How a century of ammonia synthesis changed the world”, Nature Geoscience
5 For the records, the thesis is entitled Electrocatalysis of the nitrite reduction – a mechanistic study (Leiden University, 2012). The drawing Ammonium and nitrate engaging in a fertile but explosive relationship, is by C.B.
6 “Car rien ne se crée, ni dans les opérations de l’art, ni dans celles de la nature, et l’on peut poser en principe que, dans toute opération, il y a une égale quantité de matière avant et après l’opération ; que la qualité et la quantité des principes est la même, et qu’il n’y a que des changements, des modifications“. Antoine Lavoisier, Traité élémentaire de chimie, 1789, available online at http://www.lavoisier.cnrs.fr/
7 H. McGee, On Food and Cooking, Hodder & Stoughton, London, 2004, pages 173-174
8 J.J. Sindelar and A.L. Milkowski, “Human safety controversies surrounding nitrate and nitrite in the diet”, Nitric Oxide.