Before I go on to the oceanic nitrogen cycle I would like to start with a few words about nitrogen and the nitrogen cycle in general for the benefit of those who may be less familiar with this topic. Here I have drawn the probably most simple nitrogen cycle diagram that I have ever seen.
The main players in the nitrogen cycle are:N2
- nitrogen gas. Nitrogen is harmless, it makes up about 79% of the atmosphere, and never the less we have no problems with breathing. In gas form it is inactive.NOx
, and others) - nitrogen oxides are greenhouse gases.NO
-nitric oxide or nitrogen monoxide. Although we need appropriate levels of nitrogen monoxide, too much is toxic, so it is a toxic air pollutant.NO2
- nitrogen dioxide is a poisonous air pollutant.N2O
- nitrous oxide is a general anaesthetic, commonly known as "laughing gas".Nitrates
are salts with combinations of one nitrogen and three oxygen atoms. Nitrates are nutrients and used in fertilisers. Excess levels in drinking water can cause "blue baby" disease (hindering haemoglobin in carrying oxygen).Nitrites
are salts that contain NO2
(one nitrogen and two oxygen atoms). Nitrite can be toxic (is extremely toxic to fish).NH3
- ammonia is a natural fertiliser, and an important source of nitrogen for living systems. "Household ammonia" is a solution of NH3
in water and used for cleaning. It stinks.
Simply said the nitrogen cycle is the natural circulation of nitrogen by living organisms via the atmosphere, the soil and the oceans. Most diagrams that you will see are mainly focused on the processes in the soil (and the oceans are sometimes “forgotten” - the geologist in me also miss the volcanoes and sediments in many diagrams, but that is yet another story). The nitrogen in N2
is unavailable to living organisms like humans or cows. Before we can take it up it has to be fixed. Nitrogen fixation is the process by which nitrogen is taken from its natural, relatively inactive form (N2
) in the atmosphere and converted into nitrogen compounds (such as ammonia, nitrate and nitrogen dioxide). This fixation can be performed by some bacteria in the soil and certain algae (blue-green algae aka cyanobacteria). Lupins for instance are known for their symbiosis with nitrogen fixing bacteria existing in the root nodules, a fact that some farmers are exploiting.
Until the 1860s the nitrogen cycle was relatively stable. Shortage of available nitrogen was however a limiting factor to agricultural activity. Human beings are however inventive, and artificial production of nitrogen (compounds) by the chemical industry has removed this restriction. Feeding hungry populations is of course a worthy goal, but we have on the other hand created new problems because more artificial nitrogen is now manufactured than is provided by natural sources. The equilibrium has been disturbed with things as dead zones
as a result. It has also lead to increased production of the greenhouse gas nitrous oxide, which traps heat 300 times more efficiently than carbon dioxide and also destroys ozone.
So now on to the oceanic nitrogen cycle
from WHOI you can often read very clear understandable articles. The recent Tracking Nitrogen's Elusive Trail in the Ocean
is one of them.
Tracking nitrogen as it gets incorporated into various chemical compounds in the air, in organisms, and not least in the ocean, is a real challenge. It is not easy to measure the chemical processes that are taking place. Carly Buchwald is working on a new method using a natural tag to follow nitrogen’s trail through its chemical life history: where it came from, what chemical reactions it has undergone, and how and where it ends up.
The tags are isotopes - light 14N and heavier 15N isotopes of nitrogen and two oxygen isotopes - 16O and 18O. The trick is a.o. the selectiveness of nitrite-oxidising microbes in their manipulation of isotopes, and these microbes are (in their converting of nitrite into nitrate) the only natural source of nitrate to the ocean.
And now I advice you to have a closer look at an excellent (interactive) diagram
of the oceanic nitrogen cycle (doesn’t work without flash!).
It is important to get a better understanding of how exactly human activities are affecting the nitrogen cycle.