Newsletter and Technical Publications
Freshwater Management Series No. 5
Guidelines for the Integrated Management of
- Phytotechnology and Ecohydrology -
C. Nutrients cycles in river basins
The water cycle (hydrological cycle) is driven by solar
energy, which regulates the processes of evaporation and conveyance of water
molecules within the atmosphere. Specific atmospheric conditions bring about
condensation of water vapour and formation of clouds, which eventually form
rain. Water in the form of atmospheric precipitation returns to the soil
surface, where a part evaporates back into the atmosphere and part forms
surface and subsurface flows, which intensify flows into water bodies. The part
infiltrates into the soil percolates into deeper groundwaters forming
underground flow and storages. A part of the precipitation also is absorbed by
plants and is later transpired into the atmosphere. The balance is deposited in
the form of snow and ice and, in part, is retained on the land surface and for
particular periods of time completely cut off from the water cycle.
Water, recycling in ecosystems
and including surface runoff, is the principle factor that determines the
spread and distribution of chemical substances within ecosystems. The use of
phytotechnologies is especially recommended for improving water quality by
reducing nutrient (nitrogen and phosphorus) and pesticide concentrations, which
are retained within plant biomass.
The principle processes spreading pollution are: erosion, surface flows,
atmospheric precipitation, groundwater flows, drainagewayes, and stormwater
runoff from municipal areas (Bechrendt 1999). The typical composition of
surface washings consist of suspended solids, as well as herbicides,
pesticides, and biogenic substances (mainly nitrogen and phosphorus).
The most important route of nitrogen transport from river catchment
areas is groundwater, and the principal source of nitrogen is agriculture
According to the work of Bechrendt (1999), nitrogen makes up 48% of the
total biogenic compounds entering large rivers, while the role of erosion,
surface runoff, and atmospheric precipitation in contributing nitrogen to
aquatic systems is each estimated at about 2% of the total inflow.
Investigations carried out in relation to surface runoff, as well as ground
water flow, have shown that 64% of the nitrogen is lost from fields with
cultivated crops, 9.2% is lost due to surface washoff, and 26% is leached into
the ground water.
Nitrogen occurs in both inorganic and organic forms. In water, it is
found mostly in the active, mineral forms: nitrate, nitrite and ammonium. In
contrast, the nitrogen present in runoff from arable land is 95 to 99% organic,
while only 1 to 5% is in the combined mineral forms of ammonium and nitrate (NH4+,
NO3-). Nitrate is not adsorbed onto soil particles. As a
result, it is easily washed away from the soil (Freeze and Chery 1979).
Ammonium is a more stable form of mineral nitrogen, which strongly attaches to
soil particles, especially aluminosilicates (Boatman and Murray 1982).
The processes of mineralisation and immobilisation are responsible for
fluctuations in the amounts of NH4+
and NO3- in the soil, and are a part of the
soil-plant nitrogen. It is connected with the larger nitrogen cycle which
includes atmospheric nitrogen, N2O5 - which reacts with
water to form HNO3 that is carried by rain water onto the soil
surface at a rate of between 0.18 to 23.5 kg/ha/y.
The amount of nitrogen in the soil fluctuates
during the year (Figure 5.4.) (Pedersen 1990). It reaches its maximum level
immediately prior to the start of the vegetative growing season as a result
of fertilisation and partial mineralisation.
|SEASONAL DYNAMICS OF NITROGEN COMPOUNDS IN THE SOIL
In summer, it rapidly decreases as plants
utilise nitrate together with other food components during their growth and
In autumn, the amount of nitrate in the soil
increases again, and from the beginning of late autumn, into winter, and
during the spring ice-melt, is flushed into deeper levels of the soil
profile so it moves beyond the radical zone. This is the most dangerous
period for underground waters, which can be polluted by heavy nitrate loads
during this period.
The transportable forms
of nitrogen make up several percent of total nitrogen, and comprise
compounds that are remobilised from the remains of plants by microbial
degradation and nitrogen that remains in the active fraction in soil
The principal sources of phosphorus in water bodies are
municipal and industrial sewage, discharges from urban and agricultural
drainage channels, and the dumping of untreated sewage from villages and small
The transformation processes affecting phosphorus compounds in the soil
comprise: adsorption, desorption, precipitation, and dissolution.
The adsorption of phosphates takes place on the
surface of hydrated iron oxides and clays, clay minerals
(aluminosilicates), organic substances, calcium carbonates, and humic
compounds saturated with iron, clay, or calcium cations. Over-adsorbed
phosphates on iron and clays, together with freshly precipitated amorphous
calcium phosphates, constitute the mobilisable forms of phosphorus.
Immobilised forms are present in bottom sediments in complexes with clays
and iron in acidic environments, or with calcium in alkaline ones. They can
be easily released in anaerobic conditions, as evident during the
degradation of organic matter when the redox potentials change. Released
compounds are rapidly utilised by algae, bacteria, and fungi, and can be
stored in form of polyphosphates for use in times of phosphorus deficit
(Starmach et al. 1976). That is
why, despite the scarcity of the assimilated forms of phosphorus,
phosphorus transformations play an important role in appearance and
intensity of the symptoms of eutrophication in aquatic ecosystems (Kajak
The active forms of phosphorus in soil solution
assimilated by plants are comprised of H2PO4,
and PO43-. As a result of the strong sorption of
phosphorus by soil particles, its concentration in ground and drainage
waters seldom exceeds 0.1 mg/l (Fotyma et
With time, the precipitated phosphates undergo
crystallisation and become more inaccessible to plants, as a consequence of
what is called "phosphorus ageing".
The mobilisation of phosphorus into rivers is connected mainly with soil
erosion events during heavy rainfalls or periods of snow melt. However, in
well-aerated sandy soils, phosphorus leaching from fertiliser applications
phosphorus may be significant, depending upon the dislocation of colloidal
particles containing this element. Atmospheric conditions that influence the
rate of surface run-off, therefore, indirectly determine the amount of
pollution that is associated with phosphorus compounds (Ryszkowski 1990).