A basic comprehension of the natural movement of N and P is critical to understanding their agriculture - environment relationship. A simplified version of the Nitrogen cycle is summarized in Figure 1, and the Phosphorus cycle is outlined in Figure 2.
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Nitrogen is often a limiting factor in the growth of plants. At the top of its cycle, nitrogen naturally exists within the atmosphere. Useless to plants and animals in this form, it enters the soil through a process called "fixation". Nitrogen-fixing bacteria in the soil, blue-green algae in water, clover and other legume plants "fix" nitrogen from the atmosphere into solid nitrate salts. A small amount of nitrogen is also fixed by lightning. Fixation may result in a direct transformation to nitrate (dissolved nitrogen in water, in combination with oxygen) or an indirect transformation via ammonia and ammonium salts.
These nitrate salts dissolve easily in soil water and are taken up by plant roots. The plants then convert the nitrates to larger nitrogen-containing protein molecules and other organic nitrogen molecules necessary for life. Some of these nitrogen-containing molecules are then transferred to plant-eating animals (herbivores) and eventually to other animals that feed on them (carnivores / omnivores). When plants and animals die, decomposers break down these large organic nitrogen molecules into ammonia gas and ammonium salts. Nitrogen re-enters the atmosphere primarily through growing plants and a natural denitrification process. Some nitrogen is temporarily lost from the cycle when soluble nitrate salts are eroded from the soil into rivers and eventually into the oceans (Miller, 1985).
Agricultural crop growth may be enhanced
within nitrogen deficient soils through the application of manufactured
fertilizer. The Haber Process converts nitrogen gas into ammonium
salts through a chemical reaction with hydrogen. Some nitrate salts
are also mined commercially.
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The phosphorus cycle is a "sedimentary" cycle in which the earth's crust is the major reservoir. On land, phosphate rock deposits are the primary source of phosphorus. Through natural and human induced erosion processes, phosphates from these rock deposits are washed into rivers, and eventually to the oceans, where they form shallow and deep ocean phosphate rock deposits.
Plants and animals play a role in the phosphorus cycle. As plant roots absorb phosphates from the soil, phosphorus is carried up through the food chain, eventually returning to the soil via animal waste and decay. However, these returns are small compared to the amount of phosphate which is continually eroding from the land to the oceans each year.
Phosphorus, more than any other element, can become the limiting factor for agricultural plant growth. Many tonnes of phosphate rock are mined each year in the production of fertilizers to replace some of the phosphates lost from farmland through erosion, crop production exports and lawns, and to make phosphate detergents (Miller, 1985).
Causes
for Concern
Both nitrogen (N) and phosphorus (P) have
been identified as having water quality impacts in certain regions where
concentrations become too high for the ecosystems in which they are found.
Population pressure, intensive agriculture, and industrial development,
have contributed to the contamination of surface water supplies .
In many areas, ground water problems also exist. N and P are problematic
once they are transformed into their water soluble forms: ammonia and nitrate
(N based) and phosphate (P based). This is an unfortunate irony,
because N and P are both nutrients which play a critical role in the aquatic
life cycle.
Human activities are contributing to increased
levels of nitrates and phosphates in our water systems. Sources include:
the release of nutrient-rich municipal wastes; excessive chemical fertilization
of land and crops in both urban and rural areas; and the release of excessive
amounts of agricultural and industrial wastes into surface and groundwater.
As with all other types of environmental contaminants, they may categorized
as follows:
| 1. | Point Source Pollution: coming from specific, identifiable sources/locations. |
| 2. | Non-Point Source: coming from a wide range of sources/locations. No one specific use may be traced as the sole source/location. |
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Eutrophication
Since the 1970s, public concern has emerged
regarding the environmental health of our lakes and streams. Eutrophication
is a natural process which results from the accumulation of nutrients in
lakes or other bodies of water. Human activities can accelerate eutrophication
by increasing the rate at which nutrients enter the water (Miller, 1985).
Fresh water eutrophication is typically caused by high phosphate concentrations. Depressed oxygen levels and the subsequent dominance of less desirable fish species are common results of eutrophication.
Eutrophication is evident through the excessive
growth of plants and algae which can develop into an unsightly scum along
the water surface, decreasing recreational value and occasionally resulting
in fish kills. Decaying mats of dead plants and algae can produce
foul tastes and odours in the water, while removing vital oxygen.
This is seen in the streams below.
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Excessive Nitrate levels in groundwater have resulted in serious animal and human health problems stemming from methemoglobinemia, a condition which disrupts oxygen transport through the loss of hemoglobin in the blood. Often referred to as "Blue Baby Syndrome", methemoglobinemia typically affects infants. The upper gastrointestinal tracts of very young infants contain pH levels greater than 4. This allows the conversion of nitrate to nitrite and its subsequent absorption into the bloodstream where methemoglbin forms (Cornclath and Hartment, 1948 in Henry and Meneley, 1993). The first reported connection between groundwater contamination by nitrates, and its subsequent effects occurred in 1945 (Comly in Henry and Meneley, 1993). It has since become clear that nitrate problems in well water were well established before the N fertilizer industry was a significant factor in North American agriculture - generally due to surface water contamination by human or animal waste (Henry and Meneley, 1993).
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