Increasingly, we hear of nitrogen pollution affecting the water quality of rivers, the productivity of estuaries, and the particulate matter in the air we breathe. Nearly all nitrogen pollution stems from agriculture, which strives to feed more than 7 billion of us with low-cost abundance. Compared to industry, we’ve been reluctant to point the finger at farmers, but now it’s time.

A recent paper on the nitrogen flow though the city of Paris crystallized the problem for me. The data are expressed in kilograms of nitrogen per person per year for each citizen of Paris. For instance, averaged over the entire population, 21.4 kilograms (46.6 lbs) of nitrogen fertilizer are used to grow the food for Paris—about 90% of this is used to grow feed for animals. Animal nutrition is hugely inefficient, inasmuch as only 4.6 kg of N per year makes it to the food in the grocery stores of Paris. The other 87% of nitrogen provided animals is lost to the environment.

In contrast, about 77% of the nitrogen applied to vegetable and grain crops is delivered to the food supply of Paris.
Some nitrogen fertilizer is lost to the atmosphere as ammonia, where it forms small particles known as PM2.5. These are easy to breathe, and as an air pollutant they are the cause of a variety of health problems and mortality in downwind regions.

When nitrogen is applied in excess, much of it is lost as nitrate (NO3) to surface waters, leading to water quality problems. For the city of Paris, about 55% of the nitrogen applied as fertilizer is lost to water as nitrate. Most of this occurs in the agricultural sector; only about 10% of the nitrogen applied to farm fields makes its way to food for humans, and into their personal excretion. Similar results are found in the U.S., where the Mississippi River carries huge quantities of nitrogen to the Gulf of Mexico, largely from fertilized farms in the Midwest.

Several conclusions jump out of the Paris report. First, a switch from animal foods to vegetal foods would lead to a dramatic reduction in the nitrogen pollution of the environment. Second, nitrogen is clearly “over applied” in agricultural production. A judicious use of nitrogen fertilizer, including application of slow release fertilizers that provide nitrogen at the optimal time for plant uptake, could reduce nitrogen losses. Third, the maintenance of wetlands and other areas that harbor denitrifying bacteria could dramatically increase the return of nitrogen to the atmosphere as nitrogen gas. (Denitrifying bacteria convert nitrate to N2 which naturally comprises 78% of our atmosphere).

These are not new observations, but the analysis from Paris is a striking exposé of them.

References
Baker, L.A., et al. 2001. Nitrogen balance for the central Arizona-Phoenix ecosystem. Ecosystems 4: 582-602.

Billen, G., et al. 2009. The food-print of Paris: Long-term reconstruction of the nitrogen flows imported into the city from its rural hinterland. Regional Environmental Change 9: 13-24.

Esculier, F., et al. 2019. The biogeochemical imprint of human metabolism in Paris megacity: A regionalized analysis of a water-agro-food system. Journal of Hydrology 573: 1028-1045.

Grobe, F., Katja Fennel and A. Laurent. 2019. Quantifying the relative importance of riverine and open-ocean nitrogen sources for hypoxia formation in the northern Gulf of Mexico. Journal of Geophysical Research—Oceans doi: 10.1029/2019JC015230

Paulot, F. and D.J. Jacob. 2014. Hidden cost of U.S. agricultural exports: Particulate matter from ammonia emissions. Environmental Science and Technology 48: 903-908.

Schlesinger, W.H. and E.S. Bernhardt. 2013. Biogeochemistry: an analysis of global change. Academc/Elsevier, San Diego.