Page 75 - Introduction to Agriculture by: Aqleem Abbas

Introduction to Agriculture Notes prepared by: Aqleem Abbas

Effects on human activity

Human beings have been altering the water cycle for thousands of years. Irrigation channels are constructed to bring water to dry
land. Wells are dug to obtain water from the ground. Excessive pumping from wells has drastically lowered the water table,
depleting some ancient water supplies irreversibly and causing the intrusion of salt water into groundwater in densely populated
low-lying coastal regions. Levees are built to control the course of rivers, and dams are built to render rivers navigable, store
water, and provide electrical power. Evaporation of water behind dams is a serious source of water loss. Increasing urbanization
has led to more severe flooding because rainwater reaches streams more rapidly and in greater quantity from areas where the
ground has been paved.

As human population continues to grow, effective use and management of the planet’s water resources have become essential.
Careful management of waterworks has alleviated many problems, but limits to the water supply place limits on the sustainable
population of an area and can play an important part in the politics of some regions, as in the Middle East.

Nitrogen Cycle,

natural cyclic process in the course of which atmospheric nitrogen enters the soil and becomes
part of living organisms, before returning to the atmosphere. Nitrogen, an essential part of the
amino acids, is a basic element of life. It also makes up 78 percent of the Earth’s atmosphere, but
gaseous nitrogen must be converted to a chemically usable form before it can be used by living
organisms. This is accomplished through the nitrogen cycle, in which gaseous nitrogen is
converted to ammonia or nitrates. The high energies provided by lightning and cosmic radiation
serve to combine atmospheric nitrogen and oxygen into nitrates, which are carried to the Earth’s
surface in precipitation. Biological fixation (see Nitrogen Fixation), which accounts for the bulk
of the nitrogen-conversion process, is accomplished by free-living, nitrogen-fixing bacteria;
symbiotic bacteria living on the roots of plants (mostly legumes and alders); cyanobacteria
(formerly known as blue-green algae); archaebacteria (also known as archaea) in deep-sea
hydrothermal vents and other geothermal environments; certain lichens; and epiphytes in tropical
forests.

Nitrogen “fixed” as ammonia and nitrates is taken up directly by plants and incorporated in
their tissues as plant proteins. The nitrogen then passes through the food chain from plants to
herbivores to carnivores. When plants and animals die, the nitrogenous compounds are broken
down by decomposing into ammonia, a process called ammonification. Some of this ammonia is
taken up by plants; the rest is dissolved in water or held in the soil, where microorganisms
convert it into nitrates and nitrites in a process called nitrification. Nitrates may be stored in
decomposing humus or leached from the soil and carried to streams and lakes. They may also be
converted to free nitrogen through denitrification and returned to the atmosphere.

In natural systems, nitrogen lost by denitrification, leaching, erosion, and similar processes is
replaced by fixation and other nitrogen sources. Human intrusion in the nitrogen cycle, however,
can result in less nitrogen being cycled, or in an overload of the system. For example, the
cultivation of croplands, harvesting of crops, and cutting of forests all have caused a steady