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氮平衡（nitrogen balance)

氮平衡(nitrogen balance)

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发布时间：2020-11-16

更多“氮平衡（nitrogen balance)”相关的问题

第1题

The cause of the decline of North Africa is popularly attributed to climatic changes, the

theory being that the area became hotter and drier and the people were forced to abandon a thriving civilization. However, some geologists, after carefully weighing the evidence, have challenged the conclusion that the climate has changed in any important way since Roman times. Although climatological factors may also have contributed, it was man who destroyed the balance of nature responsible for maintaining soil fertility and moisture.

Why is it that under certain circumstances, the plant cover is unable to regenerate itself, thus setting the stage for serious erosion (腐蚀)？ To answer this question, let us take an extreme example, the destruction of tropical rain forests. In general, the equatorial (赤道附近的) soil is poor: forests can exist in these regions only because they are part of a balanced cycle. All that the forest produces is returned to the forest. The organic matter that falls from the trees constitutes the humus (腐殖土) that the forest requires. When people cut down all the trees to make room for crops, the soil is laid bare and, deprived of shade, heats up. An increase in soil temperature increases the rate of decomposition (分解) of organic matter. Organic nitrogen (氮) is converted into soluble ammonia (氨) and nitrates (硝酸盐), which the rains quickly leach (沥滤，溶滤)away. It has been calculated that a rise in temperature from 77 to 78.8 may increase the loss of nitrogen by fifteen to twenty pounds per acre per year. With the fertility of soil destroyed, an irreversible change from forest to desert can be initiated.

What do people usually think contributed to the decline of North Africa？

A．Human factors.

B．Economic factors.

C．Climatic factors.

D．Population factors.

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第2题

挥发性盐基总氮（total volatile basic nitrogen，TVBN)

挥发性盐基总氮(total volatile basic nitrogen，TVBN)

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第3题

账面平衡（Book Balance)

账面平衡(Book Balance)

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第4题

The blue of the sea is caused by the scattering of sunlight by tiny particles suspended in

the water. Blue light, being of short wavelength, is scattered more efficiently than light of longer wavelengths. Although waters of the open ocean are commonly some shade of blue, green water is commonly seen near coasts, especially in tropical or subtropical regions. This is caused by yellow pigments being mixed with blue water. Phytoplankton are one source of the yellow pigment. Other microscopic plants may color the water brown or brownish-red. Near the shore, silt or sediment in suspension can give water a brownish hue. Outflow of large rivers can often be observed many miles offshore by the coloration of suspended soil particles.

Marine phytoplankton (Greek for "plant wanderers") are microscopic single-celled plants that include diatoms, dinoflagellates, coccolithophorids, green algae, and blue-green algae, among others. The growth of these organisms, which photosynthesize light, depends on a delicate balance of nutrient enrichment via vertical mixing, which is often limited by the availability of nitrogen and light. Diatoms are one-celled plants with patterned glass coverings. Each glass, or silicon dioxide box, is ornamented with species-specific designs, pits, and perforations making them popular with microscopists and, more recently, electron scanning microscopists.

Green water near coastlines is almost always caused by ______.

A．sand color

B．red pigments in coastal waters

C．blue pigment

D．reflected light and yellow pigment from plant life

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第5题

work-life balance （工作与生活的平衡).（）

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第6题

From the passage, we know that man must ______.A．do nothing to keep the balance （平衡)of n

From the passage, we know that man must ______.

A．do nothing to keep the balance (平衡)of nature

B．try his best to change the balance of nature

C．try his best to keep the balance of nature

D．do his best to stop the balance of nature

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第7题

Eyes play an important role in body balance (平衡) and it is important to ______ as m

Eyes play an important role in body balance （平衡) and it is important to ______ as m

uch information from the environment as possible.

A.absord

B.involve

C.invent

D.conquer

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第8题

There is now no doubt that the world is getting warmer. Data from both the Southern and No

rthern Hemispheres(地球的半球)shows a(n)【C1】______trend in average temperatures over the【C2】______hundred years. At the moment most scientists are not prepared to stick their necks out and【C3】______a cause to this trend. It is possible that it represents a natural climatic change caused by a【C4】______increase in the sun's radiation output. But these same scientists are【C5】______aware that recent temperature changes are at the limit of known natural【C6】______: it seems increasingly likely that the cause is a(n)【C7】______greenhouse effect.

The basic theory of the greenhouse effect is quite simple. The earth's atmosphere consists【C8】______of oxygen and nitrogen,【C9】______there are small concentrations of various "greenhouse" gases-【C10】______carbon dioxide, water vapor, and methane(沼气)-- which play a very important role in【C11】______the planet's "heat balance". As their name suggests these gases have a similar effect on glass in a greenhouse: They let heat from the sun【C12】______, and keep it in.

The atmosphere is more or less transparent to the【C13】______solar radiation which warms the earth(ground or oceans)on which it falls.【C14】______it has been heated, the earth is warmer than space and gives【C15】______energy in the form. of in visible long-wave infrared radiation. This【C16】______for nighttime cooling. Greenhouse gases【C17】______some of this long-wave radiation, and reradiate it in the lower atmosphere.

The gases【C18】______act rather like a blanket by preventing some infrared radiation from leaving the earth-atmosphere system.【C19】______certain limits, the more greenhouse gases present, the more infrared radiation will become【C20】______, and the higher the surface temperature of the earth.

【C1】

A．current

B．downward

C．upward

D．disturbing

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第9题

在网络规划中考虑系统平衡（system balance)时，必须假设（)。

A.手机和BTS的灵敏度相同

B.手机和BTS的路径损耗相同

C.手机和BTS的天线损耗相同

D.手机和BTS的合路器损耗相同

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第10题

Radiocarbon DatingNowadays scientists can answer many questions about the past through a t

Radiocarbon Dating

Nowadays scientists can answer many questions about the past through a technique called radiocarbon(放射性碳), or carbon-14, dating. One key to understanding how and why something happened is to discover when it happened.

Radiocarbon dating was developed in the late 1940s by physicist Willard F. Libby at the University of Chicago. An atom of ordinary carbon, called carbon-12, has six protons(质子) and six neutrons(中子) in its nucleus. Carbon-14, or C-14, is a radioactive, unstable form. of carbon that has two extra neutrons(原子核). It returns to a more stable form. of carbon through a process called decay(衰减). This process involves the loss of the extra neutrons and energy from the nucleus.

In Libby's radiocarbon dating technique, the weak radioactive emissions(放射) from this decay process are counted by instruments such as a radiation detector and counter. The decay rate is used to determine the proportion of C-14 atoms in the sample being dated.

Carbon-14 is produced in the Earth's atmosphere when nitrogen(氮)-14, or N-14, interacts with cosmic rays(宇宙射线). Scientists believe since the Earth was formed, the amount of nitrogen in the atmosphere has remained constant. Consequently, C-14 formation is thought to occur at a constant rate. Now ratio of C-14 to other carbon atoms in the atmosphere is known. Most scientists agree that this ratio is useful for dating items back to at least 50,000 years.

All life on Earth is made of organic molecules(分子) that contain carbon atoms coming from the atmosphere. So all living things have about the same ratio of C-14 atoms to other carbon atoms in their tissues(组织). Once an organism(有机体) dies it stops taking in carbon in any form, and the C-14 already present begins to decay. Over time the amount of C-14 in the material decreases, and the ratio of C-14 to other carbon atoms goes down. In terms of radiocarbon dating, the fewer C-14 atoms in a sample, the older that sample is.

Nowadays many scientists depend on radiocarbon for dating age-old objects.

A．Right

B．Wrong

C．Not mentioned

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