Download AKUBOH OLIVIA 13/SCI03/001 BCH 413 How Humans Affect

AKUBOH OLIVIA
13/SCI03/001
BCH 413
How Humans Affect Nitrogen Cycle
The nitrogen (N) cycle is a natural cycle that moves nitrogen through different compounds in the
ecosystem. The nitrogen cycle is important because all organisms require nitrogen in order to
live. Virtually everywhere prehistoric people lived they modified the environment for their
benefit by the use of fire. By physically removing the land's cover of vegetation, fire enhanced
the transfer of N from the land by wind and water erosion and by leaching of dissolved N.
Reduced shade and a blackened surface made the surface warmer (and by reducing
evapotranspiration) often made the land wetter. This stimulated biological nitrification and
denitrification, which, in turn, further enhanced losses of N from the land to the hydrosphere and
atmosphere, respectively. Similarly, fire chemically stimulated mineralization of soil organic
matter to NH4+, much of which the highly alkaline ash left behind by fire converted to NH3 gas,
further enhancing loss of soil N to the atmosphere. Additionally, fire itself chemically converted
much of the N contained in the burnt vegetation to N2 gas in a process called pyrodenitrification.
Conversely, fire preferentially stimulated the growth of plants which supported symbiotic and
mutualistic N fixation as well as asymbiotic populations of N-fixing bacteria. In spite of fireinduced enhanced losses of soil N to the hydrosphere and atmosphere, the effect of fire in
enhancing N fixation is so great, the fired landscape generally gained in N as a result of fire. In
Illinois, the prehistoric use of fire by Native Americans aided to generally double the content of
soil N by maintaining most of the land in prairie rather than in forest. These prairies (and the
burned forest remnants) supported an animal population whose effect on the Native Americanshaped ecology was to further enhance transfers of N to the atmosphere and hydrosphere. But, as
with fire, the animal kingdom enhanced N fixation more than it enhanced N transfers.
As more conventional civilizations developed in historic times, human influences on the N cycle
shifted with the changing land-use practices. Fire was still used; however, it was used in
combination with other techniques. Fired grasslands and woodlands supported domesticated
animals. The N-rich manure and urine of these animals were mixed and composted with
relatively N-poor plant materials to produce a slow release organic fertilizer of remarkablyconsistent biochemical properties. This organic fertilizer was applied to tilled and hoed cropland.
The physical disturbance of tillage hastened the mineralization of N in organic matter to NH4+
and the aeration produced by tillage hastened the conversion of this mineralized N to NO3-, both
forms of inorganic N being readily used by crops. In humid temperate climates, such as in
Illinois, about two acres of pasturage was required to support every acre of plowed cropland;
some of this cropland was used to feed farm animals used for labor whose manures were
recycled back to the cropland. Nevertheless, cropland lost fertility relative to its so-called virgin
state. For example, in the Midwest Cornbelt, tilled croplands lost almost half of their N content
in the first 60 years. Wetland agricultures were developed by civilizations in both the New and
Old World. The inherently high N-fixing capacity of wetland ecosystems meant that these
wetland agricultures required little or no augmentation to sustain fertility.
As civilizations became industrialized, human influences on the N cycle shifted yet again.
Animal power was replaced by machine power; manure was replaced by chemical fertilizers and
use of improved N-fixing legumes. Today's intensive crop production requires application of N
fertilizer greater than that applied in the mid-1900s. The more N fertilizer applied the greater is
the potential for losses of N to the atmosphere and the hydrosphere. Recognizing these fertilizerrelated problems, agriculture is developing improved best management practices (BMPs) such as
slow-release chemical fertilizers and water table management (and other) techniques to decrease
rates of nitrification/denitrification during periods of low plant N demand. Also agriculture is
shifting to a combination of rebuilding the soil organic N reservoir and enhanced plant
rhizosphere N fixation and N uptake to decrease dependency on chemical-N fertilizers.
Energy generation releases geologic N stored in fossil fuel and also fixes atmospheric O2 and N2
under conditions of high temperature and pressure. Engineering solutions have and continue to
reduce the amount of fixed N produced per unit of energy produced.
The major human activities that today influence the global N cycle are fossil fuel combustion,
the production and use of chemical fertilizer, and the growing of N-fixing crops. These activities
are reported to have doubled the magnitude of N fixation over continents. The major
environmental issues associated with this enhancement are global climate change, stratospheric
ozone depletion, regional smog, visibility degradation, acid rain, water-use impairment, and
eutrophication.