Page 134 - Soil and water contamination, 2nd edition

Nutrients 121

• Wet deposition: wash-out of gases and aerosols from the atmosphere in solution by
precipitation.
• Occult deposition: removal of gases and aerosols from the atmosphere through diffusion
across the interface between air and wet surface, in fog and dew drops.

In contrast to nitrogen dioxide, the transport range of ammonia in the atmosphere is limited
and dry deposition is mainly concentrated around local emission sources: about 20 percent
of the emitted ammonia is deposited within 5 km of the source (Erisman et al., 1988).
Atmospheric deposition is locally affected by aerodynamic surface roughness controlled by
local surface topography and land cover. The presence of hills and transitions in the height and
structure of vegetation cover causes airflow perturbations, which alter the rates of dry and wet
atmospheric deposition . Because forest canopies have a large roughness and deposition surface,
the deposition rate is higher on forests than on other vegetation surfaces. The deposition
rate is especially enhanced where sharp transitions occur in surface roughness: for example,
in the zone from 50 to 100 m from the forest edge the rate of atmospheric deposition of
N may be 50 percent higher (Ivens et al., 1988; Draaijers, 1993) (see Section 16.3.3). The
nitrogen deposited by dry and occult deposition reaches the soil surface as a consequence of
rainfall via throughfall (i.e. the precipitation that falls through or drips off of the plant canopy)
and stemflow (i.e. the precipitation intercepted by plants that flows down the stem to the
ground). Furthermore, atmospheric deposition rates follow local weather patterns. Obviously,
the deposition rates increase with increasing amounts of precipitation. Due to more stable
atmospheric conditions and lower wind speeds, the deposition rates are lower at night than
during the day. During springtime, when large amounts of manure are applied, the ammonia
deposition is largest. During autumn and winter, as deciduous trees shed their leaves, the
deposition surface is reduced and, accordingly, so are the atmospheric deposition rates.
As soon as ammonium reaches the soil, it is nitrified to nitrate , producing nitric acid .
For every ammonium ion that is nitrified, 4 protons are released (see Equation 6.2). As a
consequence, the process of deposition of ammonium aerosols contributes substantially to
the acidification of natural ecosystems. In poorly buffered ecosystems, such as coniferous
forests, heaths, and oligotrophic lakes and peat bogs, the processes of eutrophication and
acidification are closely interrelated due to the atmospheric nitrogen inputs.

6.3 PHOSPHORUS

6.3.1 Environmental role and occurrence of phosphorus

Phosphorus is the second key plant nutrient. In living organisms, phosphorus makes up part
of critical proteins, such as DNA. Phosphate to phosphate ester bonds in ATP (adenosine
triphosphate) and ADP (adenosine diphosphate) are the major energy storage and energy
transfer bonds in cells. ATP is a high-energy nucleotide that has a ribose sugar (adenosine)
and three phosphate groups. The breakdown of ATP releases a great deal of energy which the
cell uses for its various activities. ADP has a ribose sugar and two phosphate groups. ADP
is used to synthesise ATP with the energy released in cell respiration . When ATP is used for
cellular activities, ADP is re-formed. In plant cells, ATP is produced in the mitochondria and
chloroplasts. Besides the energy transformation within cells, phosphorus plays a role in cell
growth, the stimulation of early root growth, and in fruiting and seed production. In animals
and humans, phosphorus is essential for the growth of bones and teeth, which are primarily
made from calcium phosphate.
In the environment, phosphorus occurs as organic phosphorus, i.e. as part of living or
dead organic materials, or as inorganic orthophosphates, i.e. phosphoric acid (H PO ) and
3 4

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