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United Nations Environment Programme
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Newsletter and Technical Publications
Freshwater Management Series No. 5

Guidelines for the Integrated Management of the Watershed
- 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).

Nitrogen

The most important route of nitrogen transport from river catchment areas is groundwater, and the principal source of nitrogen is agriculture (Omernik 1977).

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.

SEASONAL DYNAMICS OF NITROGEN COMPOUNDS IN THE SOIL
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.
In summer, it rapidly decreases as plants utilise nitrate together with other food components during their growth and development.
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 organic matter.

Phosphorus

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 settlements.

PHOSPHORUS TRANSFORMATIONS
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 1979).
The active forms of phosphorus in soil solution assimilated by plants are comprised of H2PO4, HPO42-, 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 al. 1992).
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).

 

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