ammonium (NH 4 +) and N(5+) nitrate (NO 3 - ) are the most important. Nitrogen
gas dissolved in natural waters cannot be utilized as a nitrogen source by most
plants and algae because they cannot break its strong triple bond (see Section
2.3.1). Specialized ‘nitrogen-fixing’ bacteria and fungi do exist to exploit N 2 , but
it is not an energetically efficient way of obtaining nitrogen. Hence, these
microorganisms are only abundant when N 2 is the only available nitrogen source.
Nevertheless, along with fixation of N 2 by lightning, nitrogen-fixing microor-
ganisms provide the major natural source of nitrogen for rivers. Increases in both
the area and the intensity of agricultural activity are probably responsible for the
increased NO 3 - concentrations seen in British (Fig. 5.13), other European and
North American rivers. Globally, human activity has doubled the natural river-
ine transport of reactive nitrogen, mostly as NO 3 -.
Biological processes use nitrogen in the 3- oxidation state, particularly as
amino functional groups (see Table 2.1) in proteins. This is the preferred oxida-
tion state for algal uptake (although NO 3 - and NO 2 - can be taken up) and also the
form in which nitrogen is released during organic matter decomposition, largely
as NH 4 +. Once released into soils or water, NH 4 +being cationic may be adsorbed
on to negatively charged organic coatings on soil particles or clay mineral sur-
faces. Ammonium is also taken up by plants or algae, or oxidized to NO 2 - and
ultimately NO 3 - , a process that is usually catalysed by bacteria.
In contrast to NH 4 +, NO 3 - —the thermodynamically favoured species in oxy-
genated waters—is anionic, soluble and not retained in soils. Therefore, NO 3 -
from rainwater or fertilizers, or derived from the oxidation of soil organic matter
The Chemistry of Continental Waters 1677505002500Nitrate (mmol l–1)1930 1940 1950 1960 1970 1980
YearWHO limitFig. 5.13Mean annual dissolved nitrate concentrations in the River Thames, UK,
1930–1980. WHO limit refers to the World Health Organization recommended maximum
safe nitrate concentration in drinking water. Data courtesy of UK Department of the
Environment National Water Council, Crown Copyright 1984.