The Nitrogen Cycle

Figure 1.2 illustrates the global nitrogen cycle. The amount of nitrogen in the various pools and the transfer flows are mainly based upon the figures from Bolin and Cook (1983). As seen from Fig. 1.2 many of the numbers are indicated as ranges due to uncertainty in the estimation. The figures have steadily been adjusted due to new measurements and new knowledge gained during the last two decades. Further changes of our knowledge about the nitrogen cycle can be expected in the coming years.

The cycle is not in balance due to human activity. The production of nitrogen fertilizer the conversion of gaseous nitrogen as dinitrogen (N2) into ammonia and nitrate, which are deposited in the lithosphere. The major part is washed out to the hydrosphere, where no major global change of the concentration of nitrogen is observed, but where the nitrogen together with phosphorus may cause eutrophication on a local or regional scale; see also Section 1.4.

Another unbalance in the nitrogen cycle is the transfer of nitrogen by combustion from the lithosphere to the atmosphere. These fluxes would influence the atmospheric concentrations of inorganic nitrogen radically, if the inorganic nitrogen was not deposited in the hydrosphere and lithosphere. It implies that also from this source there is a net, diffuse input of nitrogen to the lithosphere and hydrosphere, contributing to the eutrophication of aquatic ecosystems on a local or regional scale.

Figure 1.3 shows another important nitrogen cycle, namely in soil and ground water. All nitrogen compounds and in particular nitrate show an increasing concentration in the root-zone, due to the increasing nitrogen input to the lithosphere from dry and wet deposition and from industrial fixation; see also Fig. 1.2. A part of this nitrate will leach to the groundwater and an elevated nitrate concentration in this potable water source is observed as a result of the above mentioned unbalance in the nitrogen cycle. As the time needed for the nitrate to reach the ground water table is several decades, the final results of the increasing nitrogen pollution during the last 25 years have not yet been reflected in the nitrate concentration of ground water. Elevated nitrate concentrations are already a threat to the ground water quality in many industrialized countries due to the increased

ATMOSPHERE

ATMOSPHERE

Layers The Earth Lithosphere
Figure 1.2. The global nitrogen cycle is shown. Values in compartments are in Pg N, while the fluxes are in Tg N / yr.

nitrogen consumption and pollution over hte last 30-40 years, but we can expect that the problem will increase very rapidly in the coming years due to the above mentioned time lag.

Figure 1.4 gives the nitrogen cycle in many aquatic ecosystems. The increasing inflow of nitrate and ammonium to the aquatic ecosystems due to the in-

Dentrification

Dentrification

Leaching to the ground water

Figure 1.3. The nitrogen cycle in soil and ground water. Note that the processes causing the global unbalance are included in the figure: the industrial production of fertilizers and the wet and dry deposition. These two processes are causing the nitrate pollution of the ground water sources.

Leaching to the ground water

Figure 1.3. The nitrogen cycle in soil and ground water. Note that the processes causing the global unbalance are included in the figure: the industrial production of fertilizers and the wet and dry deposition. These two processes are causing the nitrate pollution of the ground water sources.

creased production of fertilizers and the increase in nitrogenous emission from human activities in general to the atmosphere and further on to the lithosphere and atmosphere, accelerate directly the growth of plants. This process, named eutrophication, may cause several other problems as will be touched upon in the next sections, dealing with the sources and effects of nitrogen pollution.

DENIT

DENIT

Figure 1.4. The nitrogen cycle in an aquatic ecosystem. The processes are: 1) uptake of inorganic nitrogen by algae (and plants), 2) photosynthesis, 3) nitrogen fixation, 4) grazing, 6) predation 5) and 7) loss of undigested matter 8) mortality, 9) mineralization, 10) and 11) settling, 12) excretion of ammonia, 13) release of ammonium from sediment, 14) nitrification, 15) 16) and 18) input / output, and 17) denitrification.

Figure 1.4. The nitrogen cycle in an aquatic ecosystem. The processes are: 1) uptake of inorganic nitrogen by algae (and plants), 2) photosynthesis, 3) nitrogen fixation, 4) grazing, 6) predation 5) and 7) loss of undigested matter 8) mortality, 9) mineralization, 10) and 11) settling, 12) excretion of ammonia, 13) release of ammonium from sediment, 14) nitrification, 15) 16) and 18) input / output, and 17) denitrification.

0 0

Post a comment