Environmental Engineering Reference
In-Depth Information
Tundra and alpine
0·3
Desert and semi-desert
1
Aquatic
Open ocean
0·5
Reefs
0·6
Estuaries
0·7
Table 22.6 Production characteristics
LAI (m 2 m −2 )
Ecosystem type
% NPP consumed by herbivores
Ratio NPP/B
Tropical rain forest
6-16
7
0·04
Tropical evergreen
5-14
4
0·04
Temperate deciduous
3-12
5
0·04
Boreal forests
7-15
4
0·04
Savanna
1-5
15
0·23
Temperate grassland
5-16
10
0·33
Lakes
20
25
Open ocean
40
42
Reefs
15
2
Estuaries
15
2
with increasing turnover rates in these groups. Similarly the ratio NPP/B contrasts the
rapid turnover rates of lakes and oceans with the very low figures for terrestrial
ecosystems, especially forests.
PLANT NUTRIENTS
The essential nutrients needed for plant growth are eighteen in number. Three of them -
carbon, hydrogen and oxygen - comprise over 90 per cent of plant tissue and come from
water and atmospheric carbon dioxide and oxygen. The remaining fifteen nutrient
elements come largely from the soil, though there is the possibility of some absorption
through the stomata on leaves. The fifteen soil-derived nutrients can be classified into
major nutrients and minor nutrients on the basis of the amounts needed by plants.
Thus nitrogen, phosphorus, potassium, calcium, magnesium and sulphur are required in
large amounts. The minor nutrients are iron, manganese, copper, zinc, molybdenum,
boron, chlorine, cobalt and selenium. The minor nutrients are also known as trace
elements . There are two important features of nutrients which govern their cycling in
ecosystems and their behaviour in soils. These are, first, whether or not the element
participates in a cycle involving gaseous atmospheric components, and, second, the
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