Geoscience Reference
In-Depth Information
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 G 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 second 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 have seen how inorganic nitrogen
ions ammonium (NH
4
+
) and nitrate (NO
3
-
) are produced by the mineralization of organic matter in the nitrogen cycle.
Most natural plant species and agricultural crops can use both these ions as a nitrogen source. However, NH
4
+
ions
are toxic to some plants, which must therefore take up their nitrogen as NO
3
-
ions. These are called nitrophilous
plants, and they tend to grow only on neutral and alkaline soils where most of the ionic nitrogen will occur as NO
3
-
.
In vegetation communities such as heaths and coniferous forests, where soils are acidic, ionic nitrogen occurs as
NH
4
+
, with virtually no NO
3
-
, as there are not enough nitrifying bacteria to convert the NH
4
+
to NO
3
-
. This presents
no problem to the heather and conifers, since they have evolved to tolerate high NH
4
+
levels, partially owing to
mycorrhizae fungi on their roots. Difficulties can occur when clear-cut harvesting removes all the trees and the ground
is replanted with coniferous seedlings. On cleared sites many plant species invade rapidly, soil pH rises and a new
population of soil micro-organisms appears, including nitrifying bacteria which convert the NH
4
+
to NO
3
-
. The planted
conifers are poor competitors for inorganic NO
3
-
nitrogen and the site becomes dominated by nitrophilous vegetation
such as ferns and herbs in the ground flora, and deciduous tree seedlings of birch and aspen. Coniferous seedlings
die from nitrogen starvation, and this is the main reason why in the forest industry of North America the failure of
replanted conifers to establish themselves is a considerable problem.
