Environmental Engineering Reference
In-Depth Information
nitrogen during denitrification with the collateral production of nitrous oxide. Decomposition
of manure from animal husbandry is another important source of nitrous oxide as well as
composting (EPA, 2006b).
The two major sources of methane emission in agriculture are animal husbandry and rice
cultivation followed by biomass burning. For animal husbandry see the section on Emission
of Greenhouse Gases from Animal Production in this chapter. Rice is produced around the
world almost exclusively in flooded fields. Organic matter present in the soil and water rapidly
decomposes and creates anaerobic conditions that promote the development of methanogenic
bacteria that produce methane. Most of that methane is oxidized by aerobic methanotrophic
bacteria in the soil; still, some methane escapes to the atmosphere through the rice plants or
diffusion through the water (EPA, 2006b).
Impact of monocultures
Monoculture is the preferred cropping system in industrialized agriculture. Some of the
advantages of monocultures are generation of high yields, allowance of mechanization, and
production of abundant food at relatively low cost.
On the downside, monocultures are more attractive to diseases and pests, reduce ecosystems
diversity, and do not recycle nutrients. As a result, monocultures are dependent on constant
fertilization and pest and disease control by application of biocides and mechanical energy to
run machinery. These practices make monocultures nonsustainable in the long run due to the
constant fossil fuel inputs for the manufacturing of fertilizers and chemicals and the running
of farm equipment (Powers and McSorley, 2000).
Impact of water use
The productivity of irrigated crops is about three times greater than those just watered by rain
(Food and Agriculture Organization [FAO], 2005). But irrigating crops does not come without
consequences. Even though water is abundant on the planet, 97 percent is unavailable for
direct use for irrigation or human consumption because it is seawater. From the remaining 3
percent, that can be surface or subterranean water, about 60 percent of the world's freshwater
is used for irrigation of crop fields (USGS, n.d.).
The extreme pressure from making water available for irrigation has created social and
environmental impacts. In arid or semi-arid areas where there is no surface water, the constant
extraction of subterranean water is causing the depletion of aquifers. A second effect is that
constant irrigation raises the water tables to the root level. The water is then transported to the
surface by capillary action, and once on the surface, it evaporates and leaves a salt residue in
a process called
salinification
. A similar phenomenon,
waterlogging
, happens in poorly
drained soils. Excessive irrigation in these types of soils soaks the dirt with water and prevents
the diffusion of oxygen that is vital for healthy root growth.
The direct impact of salinification and waterlogging is a drop in productivity. An indirect
effect of salinification is the contamination of streams, rivers, and lakes with salt that is carried
as a runoff, which produces a negative effect in ecosystems.
In areas of big rivers, large dams have been constructed to store water for energy production
and irrigation. Dams have a direct impact on local species, particularly on fish such as salmon
that migrate yearly to spawn. This has consistently decreased the availability of salmon and
impacted local communities.
Excessive extraction of irrigation water from rivers has made many rivers in the world
unable to reach the ocean. An unfortunate example is the disappearance of the Aral Sea located
between Kazakhstan and Uzbekistan, which has been constantly shrinking as a result of exces-
sive extraction of irrigation water from the rivers that once fed this internal large body of water.
