Geoscience Reference
In-Depth Information
[
3
] (see section 2.1.2 in Chapter 2 of [MON 14b]). On land, nitrogen
is largely regenerated in soils.
Production of NH
4+
by ammonification [
4
]
.
The organic compounds
synthesized by autotrophs circulate in the aquatic and terrestrial food
webs, where several heterotrophic organisms excrete nitrogenous
compounds, including ammonia (NH
3
; aquatic invertebrates, bony fish
and amphibians), urea (cartilaginous fish, including sharks, land
amphibians and most mammals) and uric acid (insects, reptiles and
birds). Nitrogen compounds of metabolic origin as well as from organic
detritus are transformed into NH
4
+
by a process called ammonification.
Nitrification by bacteria in water or soil [
5
]. During the next stage,
called nitrification, NH
4
+
is transformed into nitrite (NO
2
-
) and nitrate
(NO
3
-
) by bacteria that live in water or soil:
2 NH
4
+ 3 O
2
→ 2 NO
2
-
+ 2 H
2
O + 4 H
+
[4.18]
2 NO
2
-
+ O
2
→ 2 NO
3
-
[4.19]
Use of nitrogenous nutrients [
6
]. Nitrogenous macronutrients
(NO
3
-
, NO
2
-
and NH
4
+
) are used by autotrophic organisms (e.g. marine
phytoplankton in sea-surface waters) for the synthesis of new organic
matter. In the sea or on land, the rate of production of autotrophic
organisms (phytoplankton, vegetation) strongly depends on the rate of
replenishment of nitrogenous nutrients.
In the ocean, when the rate of replenishment of nitrogenous nutrients
is low, especially when stratification of the water column prevents or
slows down the return to the surface of nutrients that have been
regenerated at depth [
3
], autotrophic production is low (for example, in
the center of the large ocean gyres or during the summer in temperate
regions). In contrast, when the rate of replenishment of nitrogenous
nutrients is high, for example in coastal regions that receive inputs from
rivers flowing through areas of intensive agriculture and therefore
transporting large amounts of NO
3
-
[
7
], the production of
phytoplanktonic or benthic algae may be so high that the ecosystem
