托福阅读真题Official 53 Passage 2（七）

托福阅读真题Official 53 Passage 2（七）

Rain Forest Soils

On viewing the lush plant growth of a tropical rain forest,most people would conclude that the soil beneath it is rich in nutrients.However,although rain forest soils are highly variable,they have in common the fact that abundant rainfall washes mineral nutrients out of them and into streams.This process is known as leaching.Because of rain leaching,most tropical rain forest soils have low to very low mineral nutrient content,in dramatic contrast to mineral-rich grassland soils.Tropical forest soils also often contain particular types of clays that,unlike the mineral-binding clays of temperate forest soils,do not bind mineral ions well.Aluminum is the dominant cation(positively charged ion)present in tropical soils;but plants do not require this element,and it is moderately toxic to a wide range of plants.Aluminum also reduces the availability of phosphorus,an element in high demand by plants.

High moisture and temperatures speed the growth of soil microbes that decompose organic compounds,so tropical soils typically contain far lower amounts of organic materials(humus)than do other forest or grassland soils.Because organic compounds help loosen compact clay soils,hold water,and bind mineral nutrients,the relative lack of organic materials in tropical soils is deleterious to plants.Plant roots cannot penetrate far into hard clay soils,and during dry periods,the soil cannot hold enough water to supply plant needs.Because the concentration of dark-colored organic materials is low in tropical soils,they are often colored red or yellow by the presence of iron,aluminum,and manganese oxides;when dry,these soils become rock hard.The famous Cambodian temples of Angkor Wat,which have survived for many centuries,were constructed from blocks of such hard rain forest soils.

Given such poor soils,how can lush tropical forests exist.The answer is that the forest’s minerals are held in its living biomass—the trees and other plants and the animals.In contrast to grasslands,where a large proportion of plant biomass is produced underground,that of tropical forests is nearly all aboveground.Dead leaves,branches,and other plant parts,as well as the wastes and bodies of rain forest animals,barely reach the forest floor before they are rapidly decayed by abundant decomposers—bacterial and fungal.Minerals released by decay are quickly absorbed by multitudinous shallow,fine tree feeder roots and stored in plant tissues.Many tropical rain forest plants(like those in other forests)have mycorrhizal(fungus-root)partners whose delicate hyphae spread through great volumes of soil,from which they release and absorb minerals and ferry them back to the host plant in exchange for needed organic compounds.The fungal hyphae are able to absorb phosphorus that plant roots could not themselves obtain from the very dilute soil solutions,and fungal hyphae can transfer mineral nutrients from one forest plant to another.Consequently,tropical rain forests typically have what are known as closed nutrient systems,in which minerals are handed off from one organism to another with little leaking through to the soil.When mineral nutrients do not spend much time in the soil,they cannot be leached into streams.Closed nutrient systems have evolved in response to the leaching effects of heavy tropical rainfall.Evidence for this conclusion is that nutrient systems are more open in the richest tropical soils and tightest in the poorest soils.

The growth of organisms is dependent on the availability of nutrients,none of which is more important than nitrogen.Although there is an abundant supply of nitrogen in Earth’s atmosphere,it cannot be absorbed by plants unless it is“fixed,”or combined chemically with other elements to form nitrogen compounds.Nitrogen-fixing bacteria help tropical rain forest plants cope with the poor soils there by supplying them with needed nitrogen.Many species of tropical rain forest trees belong to the legume family,which is known for associations of nitrogen-fixing bacteria within root nodules.Also,cycads(a type of tropical plant that resembles a palm tree)produce special aboveground roots that harbor nitrogen-fixing cyanobacteria.By growing above the ground,the roots are exposed to sunlight,which the cyanobacteria require for growth.Nitrogen fixation by free-living bacteria in tropical soils is also beneficial.

Question 13 of 14

Look at the four squares[■]that indicate where the following sentence could be added to the passage.In many contexts—grasslands and most other types of forest,for example—that inference would be correct.Where would the sentence best fit?Click on a square[■]to add the sentence to the passage.

正确答案：A

题目详解

题型分类：插入句子题

题干分析：被插入句子中的that inference,一定在前文出现过“某种推断”。

选项分析：

A位置正确，前面的most people would conclude that..就是“某种推断”。

B位置错误，后面的this process和前面的内容关系紧密，无法插入。

C、D位置错误，前面都没有出现“某种推断”。

Question 14 of 14

Directions:An introductory sentence for a brief summary of the passage is provided below.Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage.Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage.This question is worth 2 points.

Although tropical rain forest plants grow vigorously,the clay soils of tropical rain forests are very low in mineral nutrients.

Answer Choices:

A.

Clays with poor mineral-binding capacities plus low levels of organic compounds that result from rapid decomposition produce compacted soils that are highly vulnerable to rain leaching.

B.

Plants are able to tolerate the high levels of aluminum in tropical rain forest soils only because of bacteria and fungi that neutralize aluminum while absorbing and transferring phosphorus.

C.

Because water does not penetrate deep into tropical rain forest soils,most trees growing in such soils have shallow feeder roots that spread over large areas in search of adequate moisture.

D.

The delicate fungal hyphae that extend throughout most tropical rain forest soil help maintain soil quality by transferring excess nutrients from plant tissues back into the soil.

E.

Tropical rain forests have evolved closed nutrient systems,in which plants and their fungal partners transfer mineral nutrients among organisms with little leaching through to the soil.

F.

Nitrogen-fixing bacteria in the soil and within the roots and root nodules of many plants provide rain forest plants with the nitrogen needed to help compensate for poor soils.

正确答案：EFA

题目详解

题型分类：文章总结题

题干分析：选择概括性的正确选项。

选项分析：

A选项(Clays with)正确，说的是泥土较差的矿物结合能力和较低的有机物含量导致泥土容易受到雨水冲刷的破坏。对应原文第一段内容。

E选项(Tropical rain)说热带雨林形成封闭式营养系统，营养从一个植物体转移到另一个植物体，没有漏到土壤中，对应第三段内容，正确。

F选项(Nitrogen-fixing)说植物根上和土壤中的固氮细菌可以帮助植物获得氮，对应第四段内容，正确。

B选项(Plants)错误，说植物能忍受土壤中的较高铝含量，因为细菌在吸收磷的时候可以中和铝。原文说植物无法忍受较高的铝含量，且铝会影响磷的吸收，与原文相反。

C选项(Because)说因为水不会渗到地下，所以植物的根不会往下长，而是长出feeder roots。原文意思是植物需要feeder roots吸收地表面的矿物质，而不是吸水。

D选项(The delicate)说菌丝将矿物营养物从植物中转移到土壤中，原文第三段说的是细菌帮助营养物质从一种森林植物转移到另一种，封闭营养系统，没有转移回土壤中，与原文相反。