Nitrogen Atoms Live On The Wild Side

by Gord Leathers Pity the poor nitrogen atom, a sad and lonely creature who craves the company of other atoms. It likes to dance with hydrogen and will willingly embrace three of them in a compound called ammonia. From there it will take two of its hydrogen friends into a carbohydrate and form an amino acid, the basis of life as we know it. Hydrogen and oxygen is a wild crowd that likes a lot of action and, when channeled constructively, they can can do a lot of good things together. Ammonia is a very effective fertilizer and nothing builds tissue better. In the wrong neighborhood though, the relationship can be explosive. Ammonium nitrate is an important component of munitions as well as fertilizers and it was a cocktail of ammonium nitrate and diesel fuel that ripped apart the Alfred P. Murrah Federal Building in Oklahoma City killing 169 people and injuring many others back in 1995. What nitrogen really likes is to enter a stable, monogamous relationship with another atom of nitrogen. Once consummated the bond is strong (it's a triple bond) and extremely difficult to tear asunder. It takes a lot of persuasion to coax nitrogen back to the wild side where it dances with hydrogen. So this is the nitrogen conundrum. Diatomic nitrogen is the single largest component of the atmosphere and it provides a stable, inert soup that delivers oxygen into a living organism. If it weren't for its inherent stability when bolted to itself, life on this planet could not exist. On the other hand, in its less stable form it provides a very important building block in the structure of life itself. If it weren't for its potentially explosive dance with hydrogen, oxygen and carbon, life on this planet could not exist. We have to have it both ways.

When we first started practicing agriculture ten thousand years ago, we found that fields could only grow cereal grains for two or three years. As it turns out this was because any usable nitrogen was exhausted. Replenishing soil nitrogen was done by letting a field rest for several years allowing the soil organisms to bring the fixed nitrogen back up to the levels needed. At some point we discovered that mixing manure into the field and adding legume crops to the rotation made it productive again in less time. The real breakthrough in modern industrial agriculture happened around 1913 when the first nitrate plant began making air into ammonia by using coal and water. Fritz Haber was a German chemist and an expert in nitrogen gas reactions. Up until then, the biggest source of nitrates was sodium nitrate from a quarry in Chile. With World War One looming on the horizon, the British Navy blockaded Germany from Chilean minerals so Haber was charged with finding a way to produce ammonium nitrate using local materials. Haber knew that nitrogen and hydrogen would form ammonia but the reaction was very slow and produced little usable compound. By raising the temperature and pressure he was able to get the reaction to move much faster and much more efficiently. A further reaction with nitric acid produced ammonium nitrate. Discovery of this process won Haber the Nobel Prize for chemistry in 1918. The modern industrial process puts nitrogen and hydrogen together at 500 degrees Celsius and 200 atmospheres pressure (that's like parking the full weight of a John Deere Combine with the hopper filled to capacity on your right foot).

The next major breakthrough will come when biochemists find a way to fix nitrogen efficiently at room temperature and atmospheric pressure the way Nature has been doing it for several billion years. Rhizobium is a bacterium that's able to do just that and it's been doing it in and around the roots of certain species of plants. The plant provides the chemical energy rhizobium needs to break the triple bond between nitrogen atoms. In exchange the plant absorbs the ammonia produced by rhizobium and uses it to build tissue. Rhizobium does it with an enzyme, nitrogenase, that helps break apart elemental nitrogen. If we could get Rhizobium to live with cereal grains in the same way it lives with legumes it could revolutionize nitrogen soil fertility. Better still, we could persuade grains to manufacture nitrogenase and fix nitrogen itself. There are at least 20 genes that have been identified in the manufacture and function of nitrogenase and transferring all that into a higher plant such as cereal grains is extremely complicated. In addition, nitrogenase is very sensitive to oxygen and is immediately destroyed on contact even at low concentrations. Technology like this is still many years away but it would pose huge advantages in a protein hungry world. It would cut the amount of nitrogen based fertilizer manufactured and released into the environment and in so doing, cut the amount of natural gas used in the manufacturing process. It would place a source of fixed nitrogen right at the root of the plant itself where it's needed and the mobile fixed nitrogen would stay with the plant rather than leach out of the soil. It would also reduce the amount of nitrogen oxide gas, an important greenhouse gas, lost into the atmosphere through denitrification.