Famine and Feast: Recent Changes to the Global Nitrogen Cycle

Nitrogen is the most abundant element in Earth’s atmosphere, yet millions of dollars are spent manufacturing nitrogen-based fertilizer every year.  What drives this lucrative industry? The answer is the form in which atmospheric nitrogen resides.  Nearly 80% of the atmosphere is composed of N₂ gas, which is unavailable to most land plants.  Nitrogen is a vital nutrient for biological organisms, and therefore plant growth tends to be limited by nitrogen in most natural environments.  Some plants have formed alliances with certain families of bacteria (i.e. Rhizobium), which possess key enzymes used to ‘fix’ N₂ into biologically available forms such as NO_x (NO₂, NO₃) and NH₄, to harvest nitrogen from the sky. Some forms of algae, such as Cyanobacteria can also fix nitrogen as well. But for the majority of plants, nitrogen is only available as a recycled organic matter – broken down leaves, wood, or animal scat. For the entire history of plants, nitrogen-fixing organisms controlled the abundance of bio-available nitrogen in the world, and whole ecosystems organized around the principle of recycling. Agricultural fields played by the same rules, and farmers had to carefully recycle crop and animal waste in order conserve nutrients in the soil.  

Vitousek et al. (1997) Comparative timing of a number of global changes. Alterations to the global nitrogen   cycle are on par with deforestation, population growth, and Carbon Dioxide release.

 

Haber Bosh process

N₂ + 3H₂ à 2NH₃

Things have changed recently. Without enzymes, nitrogen fixation takes a lot of energy and combustion engines in cars and coal fired power plants regularly reach temperatures hot enough to transform significant amounts of N₂ into NO_x. The nitrogen fixed from these processes is usually deposited near its source (roads, parking lots), or concentrated in areas downwind of coal plants.  The invention of the Haber-Bosch process, which uses energy from oil, rather than enzymes to fix N₂ into NH₃, harnessed nitrogen in a way that cars and coal did not. This invention revolutionized agriculture and increased crop yields across the world by as much as 30% in a period of agricultural history ironically coined the “Green Revolution”.  The vast toll of this prosperity for ecosystems is still being realized today.  Human activities have doubled the rate of nitrogen fixation globally¹, so much that this limiting nutrient has become over-abundant in agricultural fields and some forests.  Where more is added than plants and soil can absorb, nitrogen has leached into waterways at alarming rates tipped the delicate chemical balance of many estuary ecosystems.  In the past twenty years, efforts to restore estuaries and watersheds have curbed some agricultural nitrogen application, but a problem of this magnitude will undoubtedly take a long time to resolve because the only way to permanently remove nitrogen, is via a slow microbial process entitled denitrification. Scientists such as myself have dedicated careers to studying the controls on this elusive process, and are working to determine ways to enhance the natural ability of ecosystems to permanently remove nitrogen from waterways. 

Denitrification

2 NO₃⁻ + 10 e⁻ + 12 H⁺ à N₂ + 6 H₂O

Measuring nitrogen removal via denitrification in groundwater near streams, where nitrate concentrations are high and anaerobic microbes can convert NO₃^- back into N₂ gas. We use a stable isotope tracer to determine the rate of N₂ production in stream banks and created wetlands to identify "hot spots" of nitrogen removal in coastal watersheds. 

Citations

Vitousek, P.M. et al. Human alteration of the global nitrogen cycle: sources and consequences. Ecological Applications 7, 737-750 (1997).