While nitrogen is an element that is essential to life, it is an environmental scourge at high levels.
Humans are overloading ecosystems with nitrogen through the burning of fossil fuels and an increase in nitrogen-producing industrial and agricultural activities, according to a new study. Excess nitrogen from human activities pollutes fresh waters and coastal zones, and may contribute to climate change, according to the study. Nevertheless, such ecological damage could be reduced by the adoption of time-honored sustainable practices.
The nitrogen cycle — which has existed for billions of years — transforms non-biologically useful forms of nitrogen found in the atmosphere into various biologically useful forms that are needed by living things to create proteins, DNA and RNA, and by plants to grow and photosynthesize. This transformation is known as nitrogen fixation.
Mostly mediated by bacteria that live in legume plant roots and soils, nitrogen fixation and other components of the nitrogen cycle send nitrogen (in all its forms) weaving and winding through the atmosphere, plants, subsurface plant roots and soils.
Since pre-biotic times, the nitrogen cycle has gone through several major phases. The cycle was initially controlled by slow volcanic processes and lightning and then by anaerobic organisms, which can live without oxygen, as biological activity started. By about 2.5 billion years ago, as molecular oxygen appeared on Earth, a linked suite of microbial processes evolved to form the modern nitrogen cycle.
The human element
But by the start of the 20th century, human contributions to the nitrogen cycle began skyrocketing. "In fact, no phenomenon has probably impacted the nitrogen cycle more than human inputs of nitrogen into the cycle in the last 2.5 billion years," said study team member Paul Falkowski of Rutgers University in New Jersey.
"Altogether, human activities currently contribute twice as much terrestrial nitrogen fixation as natural sources, and provide around 45 percent of the total biological useful nitrogen produced annually on Earth," Falkowski said. Much of the human contributions of nitrogen into ecosystems come from an 800-percent increase in the use of nitrogen fertilizers from 1960 to 2000.
Another problem: Much of the nitrogen fertilizer that is used worldwide is applied inefficiently. As a result, about 60 percent of the nitrogen contained in applied fertilizers is never incorporated into plants and so is free to wash out of root zones, and then pollute rivers, lakes, aquifers and coastal areas through eutrophication, the researchers say. (Eutrophication is a process caused by excess nutrients that depletes oxygen in water bodies and ultimately leads to the death of animal life.)
In addition, some reactions involving nitrogen release nitrogen oxide into the atmosphere. Nitrogen oxide is a greenhouse gas that has 300 times (per molecule) the warming potential of carbon dioxide. In addition, nitrogen oxide destroys stratospheric ozone, which protects the Earth's surface and life from harmful ultraviolet (UV-B) radiation.
Nitrogen unloading
Mother Nature could right herself, at some point.
"Natural feedbacks driven by microorganisms will likely produce a new steady-state over time scales of many decades," Falkowski said. "Through this steady state, excess nitrogen added from human sources will be removed at rates equivalent to rates of addition, without accumulating."
But meanwhile, the Earth's population is approaching 7 billion people, and so ongoing pressures for food production are continuing to increase. [Read "Can Earth Survive?"]
"There is no way to feed people without fixing huge amounts of nitrogen from the atmosphere, and that nitrogen is presently applied to crop plants very ineffectively," Falkowski said.
So unless promising interventions are taken, the damage done by humans to the Earth's nitrogen cycle will persist for decades or centuries, the researchers said. These promising interventions, which would be designed to reduce the need to use fertilizers that add nitrogen to ecological systems, could include:
- Using systematic crop rotations that would supply nitrogen that would otherwise be provided by fertilizers;
- Optimizing the timing and amounts of fertilizer applications, adopting selected breeding techniques or developing genetically engineered varieties of plants that would increase the efficiency of nitrogen use;
- Using traditional breeding techniques to boost the ability of economically important varieties of wheat, barley and rye to interact favorably with the microbial communities associated with plant root systems and do so in ways that enhance the efficiency of nitrogen use.
"While the processes of eutrophication have been recognized for many years, only recently have scientists been able to begin placing the anthropogenic processes in the context of an understanding of the broader biogeochemical cycles of the planet," said Robert Burnap, an NSF program director.
The study is detailed in the Oct. 8 edition of the journal Science.
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