Nitrogen, which comprises 78 percent of Earth's atmosphere, is necessa

Nitrogen, which comprises 78 percent of Earth's atmosphere, is necessary for plant nutrition. But agriculture and industry have doubled the rate at which nitrogen is "fixed" and in so doing may have raised a serious environmental threat. Despite the natural abundance of nitrogen, most of it is in the form of an inert gas, N2, which cannot be used by living things. To nourish plants, it must be fixed—bonded with hydrogen or oxygen. In nature, some of this fixing is done by lightning, but most of it is the work of certain algae and bacteria. Human activity has greatly increased the supply of usable nitrogen. The largest artificial source of fixed nitrogen is chemical fertilizer. The cultivation of plants such as legumes that carry nitrogen-fixing bacteria on root nodules is another major source. Automobiles, factories, and power plants release significant amounts of fixed nitrogen by bu ming fossil fuels such as oil and coal. Other sources include forest fires and sewage treatment.

It is uncertain what effect these changes have had so far, but the potential for harm is considerable. Burning forests and fossil fuel creates fixed-nitrogen gases that increase smog and acid rain, damage the ozone layer, and add to global warming by trapping heat in the atmosphere. Synthetic fertilizers, along with this airborne fixed nitrogen, cause plants that thrive on nitrogen to drive out other species, which may become extinct along with the animals that depend on them. Also, since many bacteria and fungi feed on nitrogen, these nitrogen-rich plants decompose faster than other plants. This prevents the plant community from storing larger amounts of carbon. The failure of plant communities to absorb more carbon dioxide and store it as carbon may become another contributor to global warming.

Finally, fixed nitrogen in the air settles and adds to the fixed nitrogen already in the soil from chemical fertilizers. This fixed nitrogen runs off, or seeps into rivers and streams, where, with fixed nitrogen from sewage treatment plants, it flows into lakes and oceans. Besides making drinking water unsafe, this can lead to algae blooms that kill fish. It can also spur the growth of small aquatic plants that cloud the water and deprive larger plants of sunlight. When these larger plants die, they are consumed by bacteria that multiply and deplete the water of oxygen, rendering it incapable of sustaining life.

1.According to the passage, which of the following may be an effect of artificial fertilizers?

I. Increased acid rain
II. Extinction of some plant species
III. Algae blooms

A) I only
B) II only
C) I and II only
D) II and III only
E) I, II, and III

2. It can be inferred that the author would be most likely to agree with which of the following statements?

A) Most of the fixed nitrogen in the environment today is of human origin.
B) Nitrogen in the form of N2 is relatively harmless.
C) Excess fixed hydrogen has done irreversible harm to the environment.
D) The plants that are grown for food are not the ones that do best in a high-nitrogen environment.
E) Algae blooms are caused by a loss of oxygen in the water.

3. Which of the following areas would probably be hardest hit by the effects of fixed nitrogen?

A) a region where thunderstorms are frequent
B) the part of a river upstream from a sewage treatment facility
C) an area where legumes are grown
D) a bay at the mouth of a major river that flows past farms that use chemical fertilizers
E) a region where fungi are grown commercially


4. Which of the following statements, if true, would most weaken the author's claim?

A) Forest fires generate more fixed oxygen than do automobiles.
B) Fixed-nitrogen gasses are not the only gasses that trap heat in the atmosphere
C) The demand for food grown without chemical fertilizers has been increasing.
D) Earth's forests have enormous untapped potential to absorb and store carbon dioxide.
E) Most of the bacteria and fungi that consume nitrogen are harmless to humans.