Geoscience Reference
In-Depth Information
ux
H
6
) occupies a principle place in the cycle of nitrogen, especially on the territories
with cultural vegetation. For instance, an increase of the share of legumes in
agriculture can raise H
6
up to 35 t km
−
2
year
−
1
. Therefore a consideration of this
fl
Fixation of nitrogen by plants directly from the soil via the root systems (
fl
flux in the model is necessary and can be realized in the following form:
H
6
¼
r
j
R
j
l
j
=r
ij
where
ʼ
ʺ
is the constant.
The rate of assimilation of nitrogen by the roots of plants is known to depend
much on the soil temperature regime, decreasing a little with the temperature
lowering to 8
C. The motion of
nitrogen from the roots to the upper parts of the plants slows down, too. The
formula for H
6
-
10
°
C and dropping at temperatures below 5
-
6
°
re
fl
ects this regularity through the respective reactions of the plants
'
productivity with the time lag neglected.
On land, plants assimilate annually about 30
10
6
t N from the atmosphere and
×
10
6
t N directly from the soil. Approximate estimates of the
productivity of various types of vegetation average R
ʺ
= 710
-
3243 t km
−
2
year
−
1
.
Hence, we have
more than 5.3
×
10
−
4
.
On global scales, the ways of nitrogen migration include the transport of its
compounds between land and oceans due to water run-off. The annual input of
nitrogen from land into the World Ocean is estimated at 38.6
10
−
5
ʼ
ʺ
= 0.134
×
to 0.506
×
10
6
t. If the total
sink to the ocean from land is described by the function W
SO
, then the nitrogen
×
fl
ux
H
11
can be approximated by the expression
H
11
¼
k
N
N
S2
1
exp
k
N
W
SO
½
ð
Þ
;
ʻ
N
and k
N
are the coef
where
cients. The functional form foresees that the nitrogen
fl
flux from land to the ocean is equal to zero in the absence of the run-off and its
stabilization at a level
ʻ
N
, with the run-off volume considerably increasing. To
estimate the parameters
ʻ
N
and k
N
, it is necessary to take into account the spatial
heterogeneity of the types of soil-vegetation formations, relief, and other geo-
physical parameters. In particular, the content of nitrogen compounds in water
differs as a function of the run-off territory. The river waters in the forest regions
with the temperate climate contain 0.4 mg
'
−
1
of nitrates, for the arid climate this
'
−
1
. The concentration of nitrates increases sharply in drainage
waters of the irrigation systems (5.5 mg
value is 1.45 mg
'
−
1
), in the river waters of thickly popu-
'
−
1
), and reaches a maximum in the soil solutions of the salted
irrigates soils (200 mg
lated regions (25 mg
'
−
1
). Ground waters contain from 10 to 100 mg
'
−
1
of
10
3
km
3
,
nitrates. The total run-off of water into the World Ocean reaches 50
×
30 % of which being the underground run-off, hence, the total
flux of nitrogen per
unit area of the ocean will be 0.107 t year
−
1
. Assuming W
SO
= 0.337 m and that a
95 % level of the sink saturation is reached at a
fl
fivefold increase of W
SO
, we obtain
k
N
= 1.367,
ʻ
N
= 0.708.
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