Geoscience Reference
In-Depth Information
organisms have easy access to it. This is, however, not the case, as
explained afterwards in this section.
Despite the high abundance of gaseous nitrogen (N
2
) in the
environment, most living organisms cannot directly fix this form of
nitrogen. Indeed, most autotrophic organisms must use nitrate (NO
3
-
)
or ammonium (NH
4
+
) as sources of nitrogen, which chemical forms
may be lacking in certain areas or at certain times of the year, and
most heterotrophic organisms must obtain their nitrogen from organic
matter, which is not always available in sufficient quantities. Hence
despite the abundance of N
2
, nitrogen may limit the development of
many living organisms and ecosystems, since the forms of nitrogen
they need may be in short supply.
As mentioned above, nitrogen is the most abundant constituent of
the atmosphere. The abundance of N relative to other chemical
elements in other reservoirs than the atmosphere is as follows:
- in sea-surface waters, nitrogen is the most abundant dissolved
gas since the solubility in seawater of the four main atmospheric gases
at 25°C is 383 (N
2
), 207 (O
2
), 10.1 (Ar) and 9.6 (CO
2
) µmol kg
-1
.
However, the total mass of nitrogen in seawater is much lower than
that of hydrogen or oxygen because these two chemical elements
make up the water molecules and are therefore the most abundant in
the ocean (Table 4.2 and section 4.1.2);
- in living matter, nitrogen is the fourth most abundant chemical
element (expressed in terms atoms). For example, the four most
abundant chemical elements in the human body (as percentages of the
total number of atoms) are: H (63%), O (24%), C (12%) and N (1%).
Another example is provided by marine plankton, where the atomic
ratio O:C:N (the ratio does not take H into account, this element being
the most abundant in atoms) is 13.3:6.6:1.0 (these values are known as
the Redfield ratio);
- in deep-ocean waters and marine sediments, organic matter is
decomposed by heterotrophic bacteria. This releases into environment
nitrogen contained in organic matter, primarily in the form of
nitrogenous nutrients. The same phenomenon occurs in soil.
