Geoscience Reference
In-Depth Information
The surface part of the nitrogen cycle contains a constant process of removal of
nitrogen from the biosphere into deposits, in particular, as a result of accumulation
of saltpetre on the Earth surface through erosion and alkalifying. From the available
estimates, H
21
3
:
9
10
4
t/km
2
=
year, with H
21
\
H
22
, but H
21
þ
H
13
H
1
þ
H
22
þ
H
23
. This relationship follows from the fact that during the geological period
the loss of nitrogen had been balanced by its input. Of course, in the present
biosphere, with the changing intensity of most of the
fluxes enumerated in
Table
1.11
, this balance has started breaking due to increasing
fl
uxes H
9
and H
19
.
Finally, one should mention the fact that in connection with the persistent C/N
ratio for different types of soils and climatic zones for nitrogen
fl
fl
fluxes it is important
to
regulation on a
regional level. Quite special here are arid regions where soils are poor in micro-
organisms, and the regime of moisture cycle is determined by division into dry and
wet seasons. In this connection, Austin et al. (2004) shown that the episodic nature
of water availability in arid and semiarid ecosystems has signi
find correlations between factors of biogeochemical cycles
'
cant consequences
on belowground carbon and nutrient cycling. Pulsed water events directly control
the C/N of microbially available substrate. A level of this control depends on the
spatial and temporal heterogeneity of vegetation cover, topographic position and
soil texture. Seasonal distribution of water pulses
finally lead to the change in
biogeochemical cycling in water-limited ecosystems. Schematic outline of bio-
geochemical cycles of C and N in arid and semiarid ecosystems under dry season,
and after a rainfall pulse is given in Fig.
1.33
.
In the seawater, nitrogen is present in the form of dissolved gas, ions of
ammonium NH
4
þ
, nitrite NO
2
, nitrate NO
3
, and in the form of various organic
compounds. Inorganic nitrogen compounds are assimilated by algae and phyto-
plankton and thus transfer into organic forms that serve as food for living organ-
isms. An expenditure of inorganic nitrogen supplies is compensated by atmospheric
precipitation, river run-off, and mineralization of organic remains in the process of
the living organisms functioning and dying-off. According to Ivanov (1978),
nitrogen
fluxes in sea water can be schematically shown in Fig.
1.35
. Of course, not
all nitrogen
fl
fluxes available in nature have been taken into account. A diversity of
the ways of nitrogen transformation in water has been studied inadequately, though
the available information is suf
fl
cient for the global model. The processes, such as
replenishing the nitrogen supplies in water due to lysis of detritus and functioning
of living organisms, the nitrogen exchange between photic and deep layers of the
ocean, as well as nitrogen
cation, have been
thoroughly studied and described in literature. Also, there are rough estimates of
nitrogen supplies in the ocean, according to which one can assume, on the average,
that N
U
=N
P
= 0.77
fixation at photosynthesis and denitri
10
4
tkm
−
2
and N
L
=N
F
=10
5
tkm
−
2
. More detailed spatial
distributions of nitrogen supplies in the hydrosphere can be calculated from the data
on biomass, dissolved organic matter, and concentration of dissolved oxygen. The
volume relationships of dissolved nitrogen are related to the volume of oxygen as
m
'
N
2
/l = 1.06 + 1.63 m
×
'
O
2
/l.
Search WWH ::
Custom Search