Human activity has skewed the balance in the earth’s nitrogen cycle. But how did the modern nitrogen cycle evolve? A recent review published in Science tries to answer that question and make suggestions about the future.
How did the nitrogen cycle of the earth start?
Most of the nitrogen in our atmosphere came from the mantle (which still has a large amount of nitrogen stored in it, see image) in the chemically reduced ammonium (NH4+) form. The earliest organisms had nitrogen delivered to them from the earths core. Ultraviolet oxidation of ammonium salts to nitrogen gas helped increase the concentration of nitrogen in the atmosphere.
To balance the amount of nitrogen in the atmosphere it is suggested that in the early earth atmosphere, heat shock associated with lightning and high-energy meteorite impacts produced nitrous oxide (N2O) which was converted tonitrite (NO2-) by photochemical and aqueous phase reactions. Nitrite was then reduced to the ammonium form by Fe2+ rich seas.
Somewhere around 2.7 billion years ago major changes began on earth, which put the current nitrogen cycle more or less in place. Two new types of organisms evolved: those with the ability to fix atmospheric nitrogen (gaseous N2) into terrestrial forms (such as NH4+ or NO2-); and those whose metabolism involve the reverse of this process (denitrification). With every generation the microbes increased the efficiency of these processes.
What about the future?
In trying to feed our growing population, we now add twice as much nitrogen to the soil (after chemically ‘fixing’ it to make fertilisers) as microbes fix from the atmosphere. This sudden rise in the terrestrial nitrogen could spell doom for our efforts to curtail climate change (RSC members can read this Chemistry World feature article for more information).
It is estimated that the natural feedback driven by microbes will eventually produce a new steady state, but the time scale for that to occur is in decades. With an estimated world population growing to 9 billion by 2050, there will be an increased demand for fixed nitrogen for food crops. But all hope is not lost, as there are steps we can take to help manage the nitrogen cycle - for example making more efficient use of the available nitrogen already in soil and minimising fertiliser use. This will in turn allow the microbial processes to ensure that a new balance is reached, says the review.