It doesn’t offer as catchy a label as “global warming,” but human-induced changes in the global nitrogen cycle pose engineering challenges just as critical as coping with the environmental consequences of burning fossil fuels for energy.
The nitrogen cycle reflects a more intimate side of energy needs, via its central role in the production of food. It is one of the places where the chemistry of the Earth and life come together, as plants extract nitrogen from their environment, including the air, to make food. Controlling the impact of agriculture on the global cycle of nitrogen is a growing challenge for sustainable development.
Nitrogen is an essential component of amino acids (the building blocks of proteins) and of nucleotides (the building blocks of DNA), and consequently is needed by all living things. Fortunately, the planet’s supply of nitrogen is inexhaustible — it is the main element in the air, making up nearly four-fifths of the atmosphere in the form of nitrogen molecules, each composed of two nitrogen atoms. Unfortunately, that nitrogen is not readily available for use by living organisms, as the molecules do not easily enter into chemical reactions. In nature, breaking up nitrogen requires energy on the scale of lightning strikes, or the specialized chemical abilities of certain types of microbes.
Such microbes commonly live in soil, and sometimes live symbiotically in roots of certain plants. The microbes use enzymes to convert nitrogen from the environment into the forms that plants can use as nutrients in a process called fixation. Plants turn this fixed nitrogen into organic nitrogen — the form combined with carbon in a wide variety of molecules essential both to plants and to the animals that will eat them.
The opposite of this process is denitrification, in which organisms use nitrogen nutrients as their energy source and return nitrogen molecules to the atmosphere, completing the cycle. Denitrification also produces some nitrogen byproducts that are atmospheric pollutants.
Until recent times, nitrogen fixation by microorganisms (with an additional small amount from lightning strikes) was the only way in which nitrogen made its way from the environment into living organisms. Human production of additional nitrogen nutrients, however, has now disrupted the natural nitrogen cycle, with fertilizer accounting for more than half of the annual amount of nitrogen fixation attributed to human activity. Another large contribution comes from planting legumes, including soybeans and alfalfa, which are attractive hosts for nitrogen-fixing microbes and therefore enrich the soil where they grow. A third contributor is nitrogen oxide formed during burning of fuels, where the air becomes so hot that the nitrogen molecule breaks apart.
Such human activity has doubled the amount of fixed nitrogen over the levels present during pre-industrial times. Among the consequences are worsening of the greenhouse effect, reducing the protective ozone layer, adding to smog, contributing to acid rain, and contaminating drinking water.
Ammonia factories supplement the enzymatic magic of microbial nitrogen fixation with the brute forces of temperature and pressure, extracting close to 100 million metric tons of nitrogen from the atmosphere each year. Nitrogen removed from the air by human activity adds seriously to a number of environmental problems. Fertilizer for agricultural fields is the major source of nitrous oxide, a potent greenhouse gas. One nitrous oxide molecule, in fact, traps heat about 200 times more effectively than each molecule of carbon dioxide, the most plentiful greenhouse gas. Nitrous oxide also remains in the air for a long time — on the order of a century — because it does not dissolve easily in water and resists reacting with other chemicals. Consequently it eventually reaches the stratosphere where sunlight breaks it into nitric oxide, a key link in the chain of reactions that damages the Earth’s protective ozone layer.
At the same time, other fixed-nitrogen gases released from fertilizers contribute to producing ozone in the lower atmosphere, where it is a pollutant rather than a protector. This reactive nitrogen can also lead to production of aerosols that can induce serious respiratory illness, cancer, and cardiac disease when in the air we breathe. Yet another pollution problem, acid rain, is fueled in part by nitrogen oxides from fertilizer.
Other forms of fixed nitrogen that are applied during fertilization, particularly nitrite ions, also exacerbate water pollution problems. High nitrate concentrations in drinking water are a direct human health problem, causing “blue baby syndrome.” Additional ecological concerns arise from the role of fixed nitrogen compounds in over-enriching aquatic ecosystems, producing large amounts of phytoplankton (small water plants) that deplete oxygen supplies in the water and lead to “dead zones.”
“Globally, until nitrogen fixation is balanced by denitrification, the amount of excess fixed nitrogen in the world will grow relentlessly, with increasing consequences for ecosystems and the public health,” writes Robert Socolow of Princeton University. [Socolow, p. 6005]
Maintaining a sustainable food supply in the future without excessive environmental degradation will require clever methods for remediating the human disruption of the nitrogen cycle. Over the past four decades, food production has been able to keep pace with human population growth thanks to the development of new high-yielding crop varieties optimally grown with the help of fertilizers.
Engineering strategies to increase denitrification could help reduce the excess accumulation of fixed nitrogen, but the challenge is to create nitrogen molecules – not nitrous oxide, N2O, the greenhouse gas. Similarly, technological approaches should be improved to help further control the release of nitrogen oxides produced in high-temperature burning of fuels.
A major need for engineering innovation will be in improving the efficiency of various human activities related to nitrogen, from making fertilizer to recycling food wastes. Currently, less than half of the fixed nitrogen generated by farming practices actually ends up in harvested crops. And less than half of the nitrogen in those crops actually ends up in the foods that humans consume. In other words, fixed nitrogen leaks out of the system at various stages in the process – from the farm field to the feedlot to the sewage treatment plant. Engineers need to identify the leakage points and devise systems to plug them.
For instance, technological methods for applying fertilizer more efficiently could ensure that a higher percentage of the fertilizer ends up in the plants as organic nitrogen. Other innovations could help reduce runoff, leaching, and erosion, which carry much of the nitrogen fertilizer away from the plants and into groundwater and surface water. Still other innovations could focus on reducing the gas emissions from soils and water systems.
Efficiency gains could also come from recycling of organic waste. Manure has always been regarded as an effective fertilizer, but the distances separating cattle feedlots and dairies from lands where crops are planted makes transporting manure expensive. Moreover, manure and food wastes have their own set of environmental challenges, including their roles as sources of potent greenhouse gases like methane and nitrous oxide. Engineering challenges include finding ways of capturing those gases for useful purposes, and converting manure into pelletized organic fertilizer. Solutions that focus on integrated ways of reducing greenhouse and other gas emissions from wastes, while at the same time improving their potential as economically transported fertilizer, are needed.
In addressing the nitrogen cycle problem, experts must remember that fertilizers and farming have played a central role in boosting worldwide food production, helping to avoid mass starvation in many areas of the world. Efforts to mitigate the agricultural disruption of the nitrogen cycle might have the effect of raising the cost of food, so such steps must be taken in concert with efforts to limit their effects on people living in poverty.
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