Māris kļaviņŠ, azamat azizov, JĀnis zaļoksnis environment, pollution, development: the case of uzbekistan

Figure 4.21. 
Carbon 
cycle. 
Amounts stated in PgC, 
flows – in PgC/y. 

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4. HUMANS AND THE ENVIRONMENT
 69
reacts with oxygen in the atmosphere, but these reactions require 
high energy that is generated only during the lightning discharges or 
is found at the top layers of the atmosphere.
Chemical reactions in the atmo sphere and the activity of living 
organisms ensure nitrogen fixation (its conversion into chemically 
reactive and biologically available compounds). 
The  main  process  in  the  nitrogen  cycle  is  its  fixation  (assimi-
la tion)  –  the  reactions  through  which  micro-organisms  fix  the 
atmospheric nitrogen, converting it into ammonium. Bacteria and 
blue-green algae are able to fix nitrogen. Micro-organisms produce 
1-5 kg N/ha, nitrogen fixing bacteria – 100-300 kg N/ha.
In the process of assimilation, nitrogen organic compounds 
are produced. They decompose and, in turn, produce ammonia or 
ammonium salts (ammonification). The latter are further converted 
into  nitrates  and  nitrites  (nitrification).  Nitrification  can  also  be 
considered to be the oxidation of organic and inorganic nitrogen 
compounds.
The final stage of the nitrogen cycle is its return to the atmo-
sphere by means of the production of N
2
  (denitrification).  This 
process takes place in the soil. N
2
O can also be produced in the 
process of denitrification.
Nitrogen(I) oxide (N
2
O) is naturally found in the atmo sphere as 
a product of various biological and photochemical transformations. 
The anthropogenic emission amounts of N
2
O are small; therefore, 
nitrogen(I)  oxide  pollution  –  also  taking  into  account  its  low 
toxicity – is not hazardous to living organisms. This substance does 
not have any odour or taste and is chemically inert. N
2
O enters 
into the atmo sphere mostly by means of denitrification processes, 
when the nitrogen compounds used in agriculture leach into the 
N
2
Nitrates (NO
3
)
Leaching
Organic matter
NH
4
+
Nitrification
Nitrification
Nitrites (NO
2
)
Decomposition products
of living organisms 
Assimilation
Ammonification
N
2
O
N
2 
+ O
2
        2NO
Denitrification
Precipitation
Nitrogen fixing
bacteria