Environmental Engineering Reference
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to the land in this way, and presumably similar if less spectacular returns are made in all
coastal areas. However, terrestrial phosphorus is still lost to the marine environment; one
calculation is that, whereas the world's rivers discharge 14 million tonnes of phosphorus
into the oceans annually, sea birds can return only about 70,000 tonnes (0ยท5 per cent).
This 'leak' of the nutrient is a further aspect of the 'phosphorus problem'.
BIOGEOCHEMICAL CYCLING OF BASE CATIONS
Base cations are those essential plant nutrients which are absorbed as the positively
charged ion (cation) and which have no gaseous phase. From Table 22.7 we see that the
main store is in rocks and minerals, and that the group contains the elements potassium
(K + ), calcium (Ca 2+ ) and magnesium (Mg 2+ ). Other minor nutrients or trace elements
which cycle in a similar way are iron (Fe 2+ or Fe 3+ ), manganese (Mn 2+ ), copper (Cu 2+ ),
zinc (Zn 2+ ) and cobalt (Co 2+ ). The term 'biogeochemical' reminds us that vegetation, soil,
rocks, atmosphere and wildlife must never be considered separately, in isolation; each is
part of a continuously interacting ecosystem. The biogeochemical cycle is a good
example of a process-response system discussed in Chapter 1. In some respects the
biogeochem-ical cycles of the base cations are the simplest cycles, as they do not have a
gaseous component, although there are atmospheric inputs of precipitation, dust and
aerosols. A general model of nutrient cycles for cations is shown in Figure 22.8.
Considerable attention was paid to the study of biogeochemical cycles by the Hubbard
Brook ecosystem
Figure 22.8 The general model of nutrient cycles for cations
(e.g. calcium Ca ++ , magnesium Mg ++ , potassium K + ).
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