Featured Image Credit: Peter Linforth; source: https://pixabay.com/photos/countryside-harvest-agriculture-2326787/
Reference: William H. Schlesinger. 2008. On the fate of anthropogenic nitrogen. PNAS. January 6, 2009. 106 (1) 203-208. https://doi.org/10.1073/pnas.0810193105
The chemical element nitrogen (N) is an essential building block of all life on Earth and represents the fourth most common element in biological organisms, including us. Because of its importance for plant growth and food production humans have doubled the natural input of available nitrogen to our ecosystems, with adverse effects on the environment and our health. This surplus of nitrogen led to the expansion of the dead zones in the Gulf of Mexico and Baltic Sea, the concentration of the potent greenhouse gas nitrous oxide is increasing in the atmosphere and infants suffer from high nitrate concentration in the drinking water. Schlesinger describes in his article where all the nitrogen ends up that we humans produce for fertilising our fields. He also warns that our knowledge of the nitrogen cycle is still limited and that nitrogen accumulation in unexpected places will lead to environmental deterioration.
Importance of nitrogen for life
We, like any other biological organisms need to build proteins for vital body functions like our metabolism, immune responses, and maintaining muscles and skin. One of the building blocks we need for that is nitrogen. We get nitrogen from our favourite food store or fast food place around the corner and consume it without thinking about it with our daily diet. But where does it come from in the first place?
The wheat our sandwiches are made of grows well because the farmers add plant available nitrogen (nitrate or ammonium) to the soil. Most of the nitrogen that is taken up by the plants and incorporated into the grain was previously extracted from air in a process called Haber-Bosch. This process uses the non-reactive nitrogen gas from the atmosphere to produce reactive nitrogen in the form of ammonia, which serves as a base for fertilisers. To grow more food, humans have doubled the natural input of nitrogen into the Earth’s ecosystems. Today this is approximately 150 million tons of nitrogen per year. Besides the immense value it brings in terms of a higher food production, it exerts adverse effects on our environment and health.
The fate of nitrogen after its use as fertiliser
Only a small part of the nitrogen applied as fertiliser ends up in our food. The vast majority is lost to the environment during food production or after consumption. Following fertiliser application, high losses due to runoff occur from the fields after heavy rain or in gaseous forms under drier conditions. About 23% (35 million tons) of the nitrogen is then transported to streams, lakes, and oceans where it leads to toxic algae blooms and oxygen-depleted dead zones. In the case of gaseous losses, nitrogen may be deposited on forests and other ecosystems where it can lead to the death of many plants that are relying on nutrient-poor soils.
What is not lost due to runoff or in gaseous forms leaches to the groundwater (10% or 15 million tons). In areas with high fertiliser use, the nitrogen (as nitrate and nitrite) in the groundwater gets into our drinking water and can lead to methemoglobinemia, a fatal condition for infants. The residence time of nitrogen in the groundwater can be thousands of years.
Through the process of denitrification about 11% (17 million tons) of reactive nitrogen is broken down by microorganisms in the soil and water to nitrous oxide or non-reactive nitrogen gas. The latter is non-critical for the environment. However, nitrous oxide is a potent greenhouse gas with 300 times the global warming potential of carbon dioxide on a 100-year time frame.
The largest part, 32% (48 million tons) of the available nitrogen produced by humans, is transported via the atmosphere to our oceans, causing environmental degradation in coastal regions and probably also in the open ocean.
The author traces back the yearly flux of 124 million tons of nitrogen to its sinks. The majority of the 150 million tons produced per year ends up in the oceans, transported via the rivers or the atmosphere. Smaller amounts accumulate in groundwaters and as nitrogen or nitrous oxide gas in the atmosphere. However, he points out that we still cannot account for the remaining flux of 26 million tons per year. We do not know were all our nitrogen fertiliser will end up, but for sure a large portion of it will cause significant environmental damage.