Nitrifying Our Earth

by James Shipp, PhD student. Originally published in issue eight of Resonance.

The nitrogen cycleNitrogen is vital for healthy growth of plants and crops. Artificial fertilisers are added to our crops to supply nitrogen, but could we be driving nitrification of soils too far?

One of the key cycles in the natural world is responsible for conversion of nitrogen into various products through the atmosphere, soil and oceans. The key steps in the nitrogen cycle are:

Fixation and Ammonification
This is performed via capture of N2 by symbiotic bacteria in the root nodules of plants (known as diazetrophs). These organisms possess a nitrogenase enzyme which uses hydrogen and nitrogen to produce ammonia (NH3), the starting material for the production of key amino acids and other organic compounds. When a plant dies, bacteria in the soil convert organic matter back into ammonium (NH4+), releasing fixed nitrogen into the environment where it can be used by other organisms.

Nitrification
The conversion of ammonium to nitrite (NO2−) is performed by ammonium oxidising bacteria (AOB), two key examples being Nitrosomonas and Nitrosococcus. Other species, such as Nitrobacter and Nitrospira, perform the oxidation required to convert nitrite to nitrate (NO3−).

Denitrification
The reduction of nitrates by anaerobic bacterial species such as Pseudomonas and Clostridium back into N2 gas is the step that completes the cycle. These species use such nitrate anions in place of oxygen during respiration, the inert, gaseous N2 produced then diffuses back into the atmosphere, where the cycle repeats.

Nitrosomonas

Nitrobacter

Pseudomonas

Figure 2: From left to right – Nitrosomonas, Nitrobacter and Pseudomonas bacteria.

Artificial Nitrogen Fixation – the Haber-Bosch Process

The Haber-Bosch ProcessNitrogen fixation is not just limited to natural processes, in fact, artificial fixation is critical for modern life. The Haber-Bosch process, which replaced using mined nitre as a nitrogen source, produces 450 million tonnes of nitrogen based fertilisers per year. The majority of these are anhydrous ammonia, ammonium nitrate, and urea. They are produced in a reaction between nitrogen and hydrogen at high temperature in the presence of a catalyst, typically iron doped with K2O, CaO, SiO2, and Al2O3.

The original catalysts suggested by Fritz Haber were osmium, or uranium. However the use of these was not widely implemented because of how rare and toxic these metals are. Artificial nitrogen fixation has had a dramatic impact on our ability to grow food, and can be thought of as one of the main reasons for the population explosion we are experiencing to this day.

It has been estimated that without these fertilisers we would require almost four times more agricultural land than we do today to produce the food we need.

Impact of Artificial Fertilisers

However, the use of artificial fertilisers on crop land has led to significant environmental problems, which, if allowed to continue, could have severe consequences for our soil, rivers, and oceans. The rate-limiting step of the nitrogen cycle is nitrification; therefore there is a natural limit to the amount of nitrites and nitrates in the soil at any time. Adding a large excess of these ions to soils can lead to leaching of these ions into rivers. Due to both their high solubility in water, and the inability of soils to retain these excess anions, nitrites and nitrates are prone to being washed out of soils into the groundwater.

In time, the ions washed out of agricultural soil will end up in oceans. The enrichment of the sea with nitrogen based anions leads to rapid growth of algae within the water, a process known as eutrophication. This occurs because nitrogen levels are normally the limiting factor in marine ecosystems, so when an excess is available the habitat can support a much larger population of algae, leading to agal blooms.

Algal bloom 1

Algal bloom 2

Algal bloom 3

Figure 4: Three examples of algal blooms in different environments.

An algal bloom will absorb most of the sunlight at the surface of the water. It causes large changes in the amount of oxygen available in the water. These changes lead to the suffocation of fish and other marine animals. Moreover, some algae release neurotoxins, such as domoic acid, which are very harmful to livestock or humans. This can be problematic for the food industry if shellfish and other edible species absorb the toxins.

Domoic acidThe nitrogen cycle is a carefully balanced sequence of natural processes which regulates nitrate concentrations in our soils. The use of artificial fertilisers could tip the balance, leading to huge excesses of these ions in the oceans. If not properly controlled, or if new sustainable methods of farming are not developed, this could lead to significant problems in the near future, as our oceans become dominated by toxic algal blooms.

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Image credits: TEM Image of Nitrobacter winogradskyi str. Nb-255.jpg, Powai Lake Summer.JPGTM1544: An Algal River Gipping