Page 130 - Soil and water contamination, 2nd edition

Nutrients 117

Nutrient cycling causes the nutrient concentrations in the dissolved phase in the soil

solution or surface water to vary greatly over the seasons. In the temperate climate zone, the
nutrients are absorbed in large amounts by plants at the beginning of the growing season
during springtime, so that the concentrations are considerably reduced. Therefore, nutrient
concentrations in surface water and the soil solution are generally lower in summer than in
winter. During autumn, when plants produce much detritus , the concentrations increase
again. The chemistry of nitrogen and phosphorus is quite different. This causes notable
spatial differences between nitrogen and phosphorus with respect to their occurrence,
speciation , mobility, and bioavailability . In the following sections the chemical properties of
nitrogen and phosphorus will be explained in more detail.

6.2 NITROGEN

6.2.1 Environmental role and occurrence of nitrogen

Nitrogen may occur in the +5, +3, 0, and -3 oxidation state s, and occurs in the environment
as the gaseous compounds free nitrogen (N ), nitric oxide (NO )) , nitrous oxide (N O)) ,
2
x
2
+
-
-
and ammonia (NH ), as nitrate (NO ), nitrite (NO ), ammonium (NH ), or as organic
3 3 2 4
nitrogen (in the form of plant material or other organic compounds). It is a fundamental
constituent of nucleic acids, proteins, and plant chlorophyll . The major dissolved forms of
nitrogen are ammonium and nitrate. Table 6.1 shows some typical ranges of concentrations
of nitrogen species in soil, groundwater, and surface water, which includes variations in both
space and time in unpolluted and polluted situations. This table demonstrates that there are
huge variations (several orders of magnitude) in the concentrations.
Ammonium is very soluble in water, but is also readily adsorbed to the negatively
charged cation exchange sites of colloidal particles (clay minerals and organic matter ).
Ammonium fixation is the process by which clay minerals (especially illite , vermiculite , and
montmorillonite ) capture ammonium tightly between the mineral lattices (see potassium
fixation, Sections 4.2.3 and 5.3). Most of this fixed ammonium is irreversibly bonded and
cannot be exchanged with the bulk solution. The amount of ammonium fixation is much
less compared to the quantities adsorbed to cation exchange sites. Ammonium is a weak acid
and under basic conditions is converted into volatile ammonia :
NH + NH + H + (pK a = 9.24) (6.1)
4 3
Like ammonium , nitrate is very soluble in water, but because nitrate does not interact
with soil particles, it is rather mobile and readily moves downstream with soil moisture,

Table 6.1 Typical concentration ranges of nitrogen species in soil, groundwater, and surface water.
Soil 1 Soil solution 2 Groundwater 3 Surface water 3 Wastewater 4
g N kg -1 mg N l -1 mg N l -1 mg N l -1 mg N l -1
Total N 0.8 – 9.7 - - 0.56–17 -
- - 1.3–280 trace–35 0.04–10.5 0.04–2.5
NO 3
- 1.4–28 trace–9 0.004–7.5 1.5–45
NH 4
1 Source: De Bakker (1979)
2 Source: Miller and Gardiner (2004)
3 Source: EEA (2006a)
4 Source: Van der Perk (1996)

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