Nitrogen Management and the Future of Food: Lessons from the Management of Energy and Carbon
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Proc. Natl. Acad. Sci. USA Vol. 96, pp. 6001–6008, May 1999 Colloquium Paper This paper was presented at the National Academy of Sciences colloquium ‘‘Plants and Population: Is There Time?’’ held December 5–6, 1998, at the Arnold and Mabel Beckman Center in Irvine, CA. Nitrogen management and the future of food: Lessons from the management of energy and carbon ROBERT H. SOCOLOW* Center for Energy and Environmental Studies, Princeton University, Princeton, NJ 08544 ABSTRACT The food system dominates anthropogenic How the Nitrogen Cycle Works and How It Is Being disruption of the nitrogen cycle by generating excess fixed Disrupted nitrogen. Excess fixed nitrogen, in various guises, augments the greenhouse effect, diminishes stratospheric ozone, pro- The Nitrogen Cycle. Given that extensive introductions to motes smog, contaminates drinking water, acidifies rain, the biogeochemical nitrogen cycle are found elsewhere (1–5), eutrophies bays and estuaries, and stresses ecosystems. Yet, to a quick tour here may suffice. Nitrogen is found in three forms. date, regulatory efforts to limit these disruptions largely It is bound to itself in a two-atom molecule, dinitrogen, or N2; ignore the food system. There are many parallels between food this form is the most abundant, but it is almost unavailable to and energy. Food is to nitrogen as energy is to carbon. life because it is so stable that only a few specialized bacteria Nitrogen fertilizer is analogous to fossil fuel. Organic agri- (and lightning) can break it apart. Nitrogen is bound to carbon, culture and agricultural biotechnology play roles analogous to as organic nitrogen, in a magnificent variety of organic mol- renewable energy and nuclear power in political discourse. ecules, critical to life and present long after death, including Nutrition research resembles energy end-use analysis. Meat is proteins and their component amino acids. And it is bound the electricity of food. As the agriculture and food system neither to itself nor to carbon, in nitrogen nutrients. Nitrogen evolves to contain its impacts on the nitrogen cycle, several nutrients are relatively small molecules, both nitrogen ions and nitrogen gases. The principal nitrogen ions are ammonium lessons can be extracted from energy and carbon: (i) set the 1 2 goal of ecosystem stabilization; (ii) search the entire produc- (NH4 ) and nitrate (NO3 ). The nitrogen gases include am- tion and consumption system (grain, livestock, food distribu- monia (NH3); various oxides of nitrogen, including nitric oxide tion, and diet) for opportunities to improve efficiency; (iii) (NO), nitrogen dioxide (NO2), dinitrogen pentoxide (N2O5), implement cap-and-trade systems for fixed nitrogen; (iv) and nitrous oxide (N2O); and nitric acid vapor (HNO3). expand research at the intersection of agriculture and ecology, A specialized vocabulary describes the transformations from one form to another. Fixation is the process of making N2 into and (v) focus on the food choices of the prosperous. There are 1 important nitrogen-carbon links. The global increase in fixed nitrogen nutrients (largely NH4 ), and denitrification (in effect, unfixing) is the process of rebuilding N2 from nitrogen nitrogen may be fertilizing the Earth, transferring significant 2 nutrients (largely NO ). Nitrification oxidizes ammonium to amounts of carbon from the atmosphere to the biosphere, and 3 nitrate. (A complication: Side reactions of both nitrification mitigating global warming. A modern biofuels industry some- and denitrification produce N O.) Assimilation and immobi- day may produce biofuels from crop residues or dedicated 2 lization are the processes by which nutrients become organic energy crops, reducing the rate of fossil fuel use, while losses nitrogen (plants assimilate, microorganisms immobilize), and of nitrogen and other nutrients are minimized. mineralization is the process by which organic nitrogen is decomposed back into nitrogen nutrients. Assimilation, im- The agriculture and food system disrupts the biogeochemical mobilization, and mineralization are capabilities found widely nitrogen cycle at various spatial scales. Limiting the impact of in nature, but fixation and denitrification can be accomplished the agriculture and food system on the nitrogen cycle is only by specialized microorganisms. increasingly important, as that system grows to feed a larger Both air routes and water routes connect nutrient systems and more affluent world population. across large distances. Denitrification, mineralization, and Managing the food-nitrogen connection is likely to resemble nitrification all produce nitrogen gases. Once volatilized into managing the energy-carbon connection, a task that already the atmosphere, these gases undergo further chemical trans- has begun. The parallels between food and energy are myriad. formations before returning to the Earth’s surface by wet or In both the food and energy systems, alarms regarding a crisis dry deposition. Alternatively, nitrogen nutrients and organic of global supply were sounded in the 1970s, innovations and nitrogen can be leached into groundwater or carried in runoff adaptations followed that permitted growth to continue, and into surface water, then transported down waterways in solu- the focus now is on addressing adverse impacts of further tion or attached to solid particles. expansions of supply. Problems of scarcity share the stage with The nitrogen cycle is captured quantitatively by the magni- problems of abundance. tudes of the stocks of nitrogen in the various biological and This paper reviews the nitrogen cycle, its disruptions by geophysical ‘‘reservoirs’’ [measured in millions of metric tons human activity, and some of the adverse environmental con- of nitrogen, Mt(N), for example] and the flows of nitrogen y sequences of these disruptions. It then suggests principles, between pairs of reservoirs (in Mt(N) yr). The stock of N2 in extracted from the energy-and-carbon arena, that might guide the atmosphere is so large, 3.9 3 1015 Mt(N), as to be modification of the agriculture and food system to increase its responsiveness to nitrogen management objectives. Abbreviations: Mt(N), metric tons of nitrogen; Mt(C), metric tons of carbon; ha, hectare. *To whom reprint requests should be addressed. e-mail: socolow@ PNAS is available online at www.pnas.org. princeton.edu. 6001 Downloaded by guest on September 26, 2021 6002 Colloquium Paper: Socolow Proc. Natl. Acad. Sci. USA 96 (1999) effectively infinite. The stock of terrestrial organic nitrogen is 80 Mt(N)yyr is incorporated into synthetic nitrogen fertilizer, about 100,000 Mt(N); very little terrestrial fixed nitrogen is in and the rest is ‘‘consumed by chemical industries and lost the form of nutrient, because uptake by plants is rapid. Only during processing and transportation’’ (7). The third contrib- 4% of the 100,000 Mt(N) of terrestrial organic nitrogen is in utor is high-temperature combustion, estimated, also very living organisms, and the rest is in dead organic matter (5). Of roughly, at 30 Mt(N)yyr (8, 9). I examine each of these three the terrestrial dead organic matter, roughly 15% is labile, and in turn. 85% is recalcitrant, the distinction referring to the ease of Fertilizer. Nitrogen fertilizer is made from ammonia, and mineralization. The stock of fixed nitrogen in the ocean, about ammonia is made, in effect, fixed, from nitrogen in the air. An half nutrients and about half dead organic matter, is roughly ammonia factory requires very high pressures and moderately 10 times larger than the stock of terrestrial fixed nitrogen (5, 6). high temperatures to accomplish what bacteria accomplish at Evidence for Human Impact on a Global Scale. The con- ordinary pressures and temperatures. The single human ac- centration of nitrous oxide in the atmosphere gives indirect tivity of nitrogen fertilizer production provides more than half information about human impacts on the nitrogen cycle. of all anthropogenic fixed nitrogen. Fertilizer is the fossil fuel Records from ice cores reveal that the concentration of nitrous of food. oxide fluctuated only a few percent in the period from 2,000 Fig. 1A displays 35 years of global nitrogen fertilizer use years ago until about a century ago, when a statistically (1961–1995), disaggregated into 10 geographical regions (In- significant upward climb began. The current concentration, ternational Fertilizer Industry Association, http:yywww.fertil- about 310 parts per billion by volume (ppbv), is about 10% izer.org.). The rate of global nitrogen fertilizer use crossed 20 higher than the average value before this century, and the Mt(N) in 1965, 40 Mt(N) in 1973, and 60 Mt(N) in 1979. It current rate of increase is about 0.8 ppbv per year, or 0.3% per remained within a band from 75 Mt(N) to 85 Mt(N) from 1986 year, corresponding to a flow of 4 Mt(N)yyr (5). The stable to 1995, the consequence of continuous growth of consump- concentration in earlier times and the rising concentration in tion in Asia and a precipitous fall in consumption in the former the past century are presumed to be evidence that a stable Soviet Union and Eastern Europe. dynamic equilibrium governed the flows of nitrogen among The plausibility of both saturation effects and upward soils, waterways, oceans, and the atmosphere until human pressures complicates attempts to predict future consump- activity was boisterous enough to create a detectable signal. A tion of nitrogen fertilizer (10). The saturation in nitrogen similar story is told