Nitrogen Cycle and World Food Production Vaclav Smil Distinguished Professor in the Faculty of Environment, University of Manitoba, Winnipeg, CANADA

09 6/4/11 12:20 Page 1 scientific Nitrogen cycle and world food production Vaclav Smil Distinguished Professor in the Faculty of Environment, University of Manitoba, Winnipeg, CANADA. http://www.vaclavsmil.com Summary PIn the biosphere nitrogen has the most complex cycle of all circulating elements. Its incessant reuse makes life on Earth possible. Traditional agricultures supplied limited amounts of the nutrient by recycling organic wastes and by planting leguminous crops. Only the Haber-Bosch synthesis of ammonia, first commercialised in 1913, removed this key constraint on crop productivity. Global agriculture has become steadily more dependent on synthetic nitrogenous compounds without whose applications we would not be able to produce roughly half of today’s world food. This high, and rising, dependence exacts a considerable environmental price, as the losses of nitrogen fertilisers lead to contamination of waters, eutrophication, and excessive atmospheric deposition and emissions of a potent greenhouse gas. Inefficient use of nitrogenous fertilisers is also an obvious economic loss and a waste of natural gas, the prime feedstock and energiser of ammonia synthesis. There is no single measure that could substantially cut these losses but they can be reduced by careful agronomic management and (most effectively, but controversially) by moderate consumption of animal foodstuffs. In any case, more nitrogen will be needed to feed the additional 1.5-2 billion people that will be added to the global population before its growth levels off later in this century. Glossary and phosphorus (P) which results in activity. excessive plant (principally algae) Eutrophication: degradation of Troposphere: lowest layer of the growth and decay. water quality owing to enrichment earth’s atmosphere, below the by nutrients, primarily nitrogen (N) Anthropogenic: produced by human stratosphere. 9 12 6 Abbreviations Gt, gigatonne, or 10 tonnes, 10 kg. Mt, megatonne, or 10 tonnes. Introduction constituent of all living matter structural compounds. The three Life on Earth would be (typically 45-50% of dry cycles are remarkable because of impossible without incessant weight). Proteins cannot be their complexity, the cycling of key elements that made without nitrogen, and importance of microbes in their make up biomass. Three cycles – disulphide bridges make the functioning and because the those of carbon, nitrogen and proteins three-dimensional, cycled elements are transported sulphur – are particularly ready to enter into myriads of by both air and water and hence noteworthy: carbon is, of enzymatic reactions. Proteins able to move far away from course, the dominant also act as signalling and their sources. Relative neglect and many references to papers published (about 150 Mt N) as is formed by in scientific journals whose content will natural processes (biofixation, existential necessity never become widespread public lightning). There is no possibility that oncerns about rapid global knowledge. we will be able to produce food warming – anthropogenic without nitrogen. Therefore, the Cemissions of carbon dioxide This attention disparity is curious nitrogen cycle and human interference (CO2) and the resulting planet-wide because human interference in the in its functioning should get at least as rise of tropospheric temperatures and nitrogen cycle is incomparably more much attention as our current (and I massive than our perturbance of the alteration of carbon cycle – have come would argue exaggerated) carbon cycle. This is because most of to dominate not only media attention preoccupation with global climate the disruption is due to that most but also scientific debates in general change. fundamental of all energy conversions, and environmental concerns in the production of food. In absolute particular. A simple search of Google terms anthropogenic emissions of Understanding the cycle News reveals the information gap. In carbon from fossil fuel combustion Studies of the nitrogen cycle go back January 2011 the site listed more than (nearly 9 Gt C/year) are less than 10% to the period of pioneering research 700 items for the carbon cycle but of the annual carbon uptake by that laid the foundation of modern life fewer than 90 items for the nitrogen terrestrial photosynthesis. In contrast, sciences during the 19th century (Smil cycle. I would argue that the real human activities (fertilisers, planting of 2001). Justus von Liebig, one of the public awareness gap is much greater legumes and emissions of nitrogen founders of organic chemistry, than this, an order of magnitude oxides from combustion) now release formulated the law of the minimum difference, as the comparison includes about as much reactive nitrogen (plant growth is limited by the WORLD AGRICULTURE 9 10 6/4/11 12:20 Page 1 scientific ‘With average crop yields remaining at the 1900 level the crop harvest in the year 2000 would have required nearly four times more land and the cultivated area would have claimed nearly half of all ice-free continents’ element that is present in the soil in inherently low, typically less than 0.5% America. Eventually China, the world’s the least adequate amount), published in cereal straws, 2-4% in animal most populous country, became, and the first concepts of global biospheric manures, and nitrogen losses before remains, the largest user as well as the cycles and recognised the importance and after their application are high. largest producer of synthetic nitrogen of nitrogen in crop production by These facts necessitate massive labour- (FAO 2011). contrasting it with forestry: Agriculture intensive applications of farmyard differs essentially from the cultivation manures in amounts commonly in The role of synthetic nitrogenous of forests, inasmuch as its principal excess of 10 t/ha. Bird guano has a fertilisers – initially a variety of object consists in the production of higher N content but its resources compounds, most recently dominated nitrogen under any form capable of were limited. Chilean nitrates contain by urea – is perhaps best illustrated by assimilation; whilst the object of forest about 15% N but their exports could comparing typical grain yields in 1900 culture is confined to the production satisfy only a small share of the with the year 2000. In 1900 traditional of carbon (Liebig 1840). potential global demand. Moreover, cultivars received virtually no synthetic the first synthetic products, nitrogen fertilisers, whereas in the year Jean-Baptiste Boussingault (1802- ammonium sulphate from coking and 2000 high-yielding cultivars were 1887) found that the nutritional value grown with an adequate nitrogen synthetic cyanamide (CaCN2) were of fertilisers is proportional to their also available only in limited quantities supply (Smil 2011). This, together nitrogen content and demonstrated and were rather expensive. with advances in breeding (producing that legumes can restore nitrogen to short-stalked cultivars with high the soil. John Bennet Lawes (1814- harvest indices) and chemical 1900) and Joseph Henry Gilbert Synthetic fertilisers and protection (herbicides and pesticides) (1817-1901) initiated the first food production has more than tripled the average US continuous experiments with crops wheat yields during the 20th century All this changed with unexpected receiving different amounts of and the analogical multiples are 5.8 rapidity just before the outbreak of fertilisers and demonstrated, beyond for France and 3.8 for China. in the World War I. In early July 1909, after any doubt, that nitrogen fertilisers USA average yield of corn, the years of intensive experiments, Fritz (followed by phosphates) are the key country’s most important field crop, Haber (1868-1934) succeeded in to higher grain crop yields. In 1877 rose more than five-fold during the synthesising ammonia from its Théophile Schloesing (1824-1919) 20th century, from 1.6 t/ha to 8.5 elements with the help of an iron- proved the bacterial origins of t/ha. Japan’s average rice yield, already based catalyst (Smil 2001; nitrification, in 1885 Ulysse Gayon relatively high in 1900 (2.2 t/ha), Stoltzenberg 2004). Within four years (1845-1929) and his assistants closed increased nearly three times before of this laboratory bench the cycle by finding denitrifying reaching a plateau between 5.8-6.5 bacteria that can reduce nitrates and, demonstration, BASF, Germany’s t/ha during the 1980s. Nationwide largest chemical company, had a via NO and N2O, return N2 to the yield maxima for rice are 6-8 t/ha, in atmosphere. In 1886 Hermann commercial process ready: Carl Bosch East Asia and California. Hellriegel (1831-1895) and Hermann (1874-1940) was the engineer Wilfarth (1853-1904) solved the responsible for this remarkable With average crop yields remaining mystery of leguminous nodules as the achievement. Haber-Bosch synthesis of at the 1900 level the crop harvest in sites where biofixation (conversion of ammonia made it possible to mass- the year 2000 would have required produce inexpensive nitrogenous atmospheric N2 into NH3) takes place. nearly four times more land and the fertilisers but their use remained cultivated area would have claimed The nitrogen cycle (Fig 1) and its key limited during the inter-war years. nearly half of all ice-free continents, role in agricultural productivity were Ammonia synthesis took off only after rather than under 15% of the total thus well understood by the end of World War II, its global output rising land area that is required today (Smil the 19th century and this very from 3.7 Mt N in 1950 to 85 Mt N by 2011). A different perspective has knowledge led to growing concerns the year 2000 and to about 133 Mt in been quantified by Burney, Davis and about future harvests: where will the 2010, with about 75% of the total Lobell (2010): they calculated that countries with growing populations used as fertiliser (Table 1). European, between 1961 and 2005 increased and with rising demand for better North American and Japanese crop yields had a net effect of nutrition get the additional nitrogen agricultures were the first beneficiaries avoiding emissions of up to 161 Gt C.

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