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在观察热带雨林茂盛的植物生长时，大多数人会得出这样的结论：雨林下面的土壤营养丰富。

On viewing the lush plant growth of a tropical rain forest, most people would conclude that the soil beneath it is rich in nutrients.

然而，尽管雨林土壤变化很大，但它们都有一个共同的事实，那就是大量的降雨将矿物质养分冲出土壤，流入河流。

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.

由于雨水浸出作用，大多数热带雨林土壤的矿物养分含量不是低就是极低，这种情况与富含矿物的草原土壤形成了鲜明的对比。

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.

热带森林土壤也经常含有特殊类型的粘土，不像温带森林土壤的矿物结合粘土，它们不能很好地让矿物离子结合。

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.

铝是热带土壤中的主要阳离子（带正电荷的离子）；但植物不需要这种元素，而且它对多种植物有中度毒性。

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.

铝还会减少磷的供应，而磷是植物需求量很大的一种元素。

Aluminum also reduces the availability of phosphorus, an element in high demand by plants.

较高的湿度和温度提升了分解有机化合物的土壤微生物的生长速度，因此热带土壤通常比其他森林或草原土壤含有的有机物质(腐殖质)要少得多。

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.

由于有机化合物对于疏松致密的粘土，保持水分和结合矿物养分都能起到帮助，所以热带土壤中相对缺乏有机物质的情况对植物是有害的。

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.

植物根系不能深入硬粘土，在干旱时期，土壤不能保持足够的水分来满足植物的需要。

Plant roots cannot penetrate far into hard clay soils, and during dry periods, the soil cannot hold enough water to supply plant needs.

由于深色有机物质在热带土壤中的浓度较低，它们常因铁、铝和锰氧化物的存在而呈红色或黄色；当干燥时，这些土壤就会变得如岩石般坚硬。

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.

柬埔寨著名的吴哥窟寺庙，已经存在了许多世纪，它是由这样坚硬的雨林土壤建造的。

The famous Cambodian temples of Angkor Wat, which have survived for many centuries, were constructed from blocks of such hard rain forest soils.

鉴于如此贫瘠的土壤，茂盛的热带森林怎么可能存在呢?

Given such poor soils, how can lush tropical forests exist?

答案是，森林里的矿物质存在于它的生物群落中——树木、其他植物和动物。

The answer is that the forest’s minerals are held in its living biomass—the trees and other plants and the animals.

与大比例植物生物量产生于地下的草地不同，热带森林的草地几乎都是地上的。

In contrast to grasslands, where a large proportion of plant biomass is produced underground, that of tropical forests is nearly all aboveground.

枯叶、枯枝和其他植物部分，以及热带雨林动物的废物和尸体，在被大量的分解者——细菌和真菌——迅速腐烂之前，几乎不能到达森林地面。

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.

腐烂释放出的矿物质被众多浅而细的树根迅速吸收，并储存在植物组织中。

Minerals released by decay are quickly absorbed by multitudinous shallow, fine tree feeder roots and stored in plant tissues.

许多热带雨林植物（和其他森林一样）都有菌根（真菌根）伙伴，它们脆弱的菌丝散布在大量的土壤中，它们从土壤中释放和吸收矿物质，并将它们运送回寄主植物，以换取所需的有机化合物。

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.

真菌菌丝能吸收植物根系自身无法从极稀的土壤溶液中获得的磷，且其能够将矿物养分从一种森林植物转移至另一种森林植物。

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.

因此，热带雨林一般有着被称作封闭营养系统的东西，在这个系统中，矿物质从一个生物体传递到另一个生物体，很少渗入土壤。

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.

当矿物质营养物质在土壤中停留的时间不长时，它们就不能被过滤到溪流中。

When mineral nutrients do not spend much time in the soil, they cannot be leached into streams.

由于热带强降雨的浸出作用，已形成封闭的营养系统。

Closed nutrient systems have evolved in response to the leaching effects of heavy tropical rainfall.

这一结论的证据是，营养系统在最肥沃的热带土壤中更开放，而在最贫瘠的土壤中更紧密。

Evidence for this conclusion is that nutrient systems are more open in the richest tropical soils and tightest in the poorest soils.

生物体的生长依赖于营养物质的供应，而营养物质中没有一种比氮更重要。

The growth of organisms is dependent on the availability of nutrients, none of which is more important than nitrogen.

尽管地球大气中有丰富的氮供应，但除非被“固定”，或与其他元素化学结合形成氮化合物，否则植物无法吸收氮。

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.

固氮细菌为热带雨林植物提供所需的氮，帮助它们应对贫瘠的土壤。

Nitrogen-fixing bacteria help tropical rain forest plants cope with the poor soils there by supplying them with needed nitrogen.

热带雨林中的许多树种属于豆科植物，这种植物以根瘤内固氮细菌的组合而出名。

Many species of tropical rain forest trees belong to the legume family, which is known for associations of nitrogen-fixing bacteria within root nodules.

此外，苏铁（一种类似棕榈树的热带植物）产生特殊的地上根，其中含有固氮蓝藻。

Also, cycads (a type of tropical plant that resembles a palm tree) produce special aboveground roots that harbor nitrogen-fixing cyanobacteria.

通过在地上生长，根暴露在阳光下，这是蓝藻生长所需要的。

By growing above the ground, the roots are exposed to sunlight, which the cyanobacteria require for growth.

生长在热带土壤中的自由生活细菌所带来的固氮同样是有益的。

Nitrogen fixation by free-living bacteria in tropical soils is also beneficial.

Select the TWO answer choices that, according to paragraph 1, help explain the relatively low mineral nutrient content of most tropical rain forest soils. To receive credit, you must select TWO answers.

According to paragraph 1, one reason that the relatively high levels of aluminum in rain forest soils present a problem for plants is that aluminum

Paragraph 2 supports the idea that, as compared with rain forest soils, grassland soils

Why does the author mention “Angkor Wat”?

Paragraph 2 suggests the idea that compared with grassland soils, tropical rain forest soils have all of the following characteristics EXCEPT

The word “multitudinous” in the passage is closest in meaning to

According to paragraph 3, the main advantage of a closed nutrient system is that such a system

According to paragraph 4, why can getting enough nitrogen be a problem for plants?

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.

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.

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.