Environmental Engineering Reference
In-Depth Information
Figure 22.10 The effects of forest clearance of nutrient cycles
in the Hubbard Brook study.
HUMAN MODIFICATIONS OF NUTRIENT CYCLES
human impact
Nutrient cycles are very dynamic systems which respond to human modifications in
complex ways. Direct modification is when human activity deliberately adds nutrients to
the soil to increase the output of plant growth. Indirect modification is when human
actions add nutrients to soil and water by pollution. The use of large amounts of artificial
fertilizer has become one of the cornerstones of modern agriculture. Essentially the
farmer is adding industrially manufactured nutrients to the topsoil, with the result that
increased growth yields increased crops. Without the use of fertilizers, it is doubtful
whether the global human population of 6 billion people could be adequately fed. In
altering the natural nutrient cycle, however, indirect effects arise, as in the case of the
nutrient nitrogen. This is the nutrient needed in largest amounts by crops, and the nutrient
most liberally applied to the soil by farmers. The colloids in soil are poor retainers of
anions such as nitrate (NO 3 ), and so after a single large application of nitrate much is
leached from the soil into rivers and ground water. This enrichment is the process of
eutrophication which has become such a major concern in recent decades.
Eutrophication promotes the excessive growth of algae and cyanobacteria in water, and
these organisms rapidly use up oxygen dissolved in the water, so endangering the fish
and other aquatic organisms which need to breathe oxygen. Phosphates (PO 3 3− ) from
fertilizers and industrial and domestic pollution are also involved in eutrophication.
The increased use of nitrogen fertilizers in agriculture also has serious effects at the
global level. Under natural conditions in the nitrogen cycle, denitrification is
approximately balanced by nitrogen fixation in the nitrogen cycle. With extra additions of
nitrate to the soil, increased reduction of nitrate (NO 3 ) to nitrous oxide gas (N 2 O) will
occur. Even small amounts of this gas are involved in two damaging processes. First, it
contributes to the destruction of atmospheric ozone (O 3 ), and secondly it absorbs
outgoing long-wave radiation from Earth and so increases global warming.
Organic farming techniques are often put forward as more sustainable alternatives to
modern farming. These systems do not use nitrogen fertilizers, and therefore must depend
on manure and/or legumes for their nitrogen. In the 'manure system', grass and legumes
are fed to farm animals, and the manure from the animals is returned to the fields as the
nitrogen fertilizer. In the 'legume system', leguminous plants, which fix atmospheric
nitrogen into organic compounds in the soil, are planted in the crop rotation. Under both
organic systems soil organic matter will increase, and will have beneficial effects on soil
structure, soil aeration and an active population of soil organisms. Soil organic matter
declines under continuous cultivation in modern agriculture. In terms of the nitrogen
cycle, conventional farming systems lose 50 per cent more nitrogen through leaching
than the organic systems. Nitrogen in organic form in the soil is released gradually, and
so leaching losses are minimized. In conventional farming, the timing and rates of
fertilizer applications could be better suited to plant uptake, and this would go some way
to avoid the problems of nitrate leaching.
The removal of trees in forestry also has important links with the nitrogen cycle We
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