Official 10 Set 5

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Phosphorus Cycle

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What is the main purpose of the lecture?
  • A. To discuss environment phenomena that have changed the phosphorus cycle

  • B. To illustrate how interrupting the phosphorus cycle can affect the environment

  • C. To describe how phosphorus ends up in the atmosphere

  • D. To explain how phosphorus gets recycled in the environment

显示答案 正确答案: D

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    NARRATOR:Listen to part of a lecture in an ecology class.

    FEMALE PROFESSOR:So we've been talking about nutrients, the elements in the enviroment that are essential for living organisms to develop, live a healthy life and reproduce.Some nutrients are quite scarce; there just isn't much of them in the environment, but fortunately, they get recycled.When nutrients are used over and over in the environment, we call that a nutrient cycle.Because of the importance of nutrients and their scarcity, nutrient recycling is one of the most significant ecosystem processes that we'll cover in this course.The three most important nutrient cycles are the Nitrogen cycle, the Carbon cycle, and the one we are going to talk about today, the Phosphorus cycle.

    So the phosphorus cycle has been studied a lot by ecologists because, like I said, phosphorus is an important nutrient, and is not so abundant.The largest quantities are found in rocks and at the bottom of the ocean.

    How does Phosphorus get there?Well, let's start with the Phosphorus in rocks.The rocks get broken down into smaller and smaller particles as they’re weathered—they’re weathered slowly by rain and wind over long periods of time.Phosphorus is slowly released as the rocks are broken down and it gets spread around into the soil.Once it's in the soil, plants absorb it through their roots.

    FEMALE STUDENT:So that's the reason people mined rocks that contain a lot of phosphorus to help with the agriculture?

    FEMALE PROFESSOR:Uh-huh [yes]. They mine the rock, artificially break it down and put the phosphorus into the agriculture fertilizers.So humans can play a role in the first part of the phosphorus cycle—the breaking down of rocks and the spreading of phosphorus into the soil—by speeding up the rate at which this natural process occurs, you see?

    Now … after the phosphorus is in the soil, plants grow, they use phosphorus from the soil to grow.And when they die, they decompose, and the phosphorus is recycled back into the soil. Same thing with the animals that eat those plants … or eat other animals that have eaten those plants.We call all of this the land phase of the Phosphorus cycle.But a lot of the Phosphorus in the soil gets washed away into rivers by rain and melting snow. And so begins another phase of the cycle.Can any one guess what it's called? Nancy.

    FEMALE STUDENT:Well, if the one is called the land phase, then this has to be call the water phase, right?

    FEMALE PROFESSOR:Yes. [sarcastically] That’s such a difficult point, isn’t it?In a normal water phase, rivers eventually empty into oceans, and once in the oceans, the Phosphorus gets absorbed by water plants like algae.Then fish eats the algae, or eat other fish that have eaten those plants.

    But the water phase is sometimes effected by excessive fertilizers.If not all of the phosphorus gets used by the crops and large amounts of phosphorus gets into the rivers, this could cause a rapid growth of water plants in the river, which can lead to the water ways getting clogged with organisms, which can change the flow of the water.Several current studies are looking at these effects, and I really do hope we can find the way to deal with this issue before these ecosystems are adversely affected.

    OK? Of course, another way that humans can interrupt the normal process is fishing.The fishing industry helps bring Phosphorus back to land.In the normal water phase, the remaining phosphorus makes its way—settles—to the bottom of the ocean and gets mixed into ocean sediments.But remember, this is a cycle: the phosphorus at the bottom of the ocean has to somehow make its way back to the surface … to complete the cycle, to begin the cycle all over again.

    After millions of years, powerful geological forces, like underwater volcanos, lift up the ocean sediments to form new land.When a underwater volcano pushes submerged rock to the surface, a new island is created.Then over many more years, the Phosphorus-rich rocks of the new land begin to erode, and the cycle continues.

    MALE STUDENT:What about, well, you said that the Nitrogen cycle is also an important nutrient cycle.And there is a lot of nitrogen in the atmosphere, so I was wondering: is there a lot of Phosphorus in the atmosphere too?

    FEMALE PROFESSOR:Good question, George.You are right to guess the Phosphorus can end up in the earth's atomosphere.It can move from the land or from the oceans to the atomosphere and vice versa.However there's just not substantial amount of it there, like there is with Nitrogen. It's a very minimal quantity.

  • 旁白:听一段生态学的课堂讲解。

    教授:我们一直在讲养分,就是环境中对生物发展、健康生存并繁殖的必要因素。一些养分很稀少,环境中存在的真的是很少,但幸运的是,它们可以循环利用。当养分在环境中反复被使用时,我们称之为养分循环。由于养分的重要性和贫乏性,营养循环是我们在这门课中会讲到的最重要的生态系统过程之一。最重要的三种营养回收包括氮循环、碳循环以及今天我们要讲的磷循环。

    许多生态学家都研究了磷循环,因为正如我说的,磷循环是非常重要的养分,并不是很丰富。大量的磷存在于岩石及海底深处。

    磷是怎么到那里呢?我们来讲一下岩石中的磷。岩石在风化过程中被分解成越来越小的微粒,在很长一段时间里它们受到风雨缓慢的侵蚀。磷随着岩石分解过程中被释放出来,并扩散到土壤中。一旦磷融入泥土,就会被植物的根部所吸收。

    学生:这就是人们开采包含有大量磷的岩石来推推动农业的原因吗?

    教授:呃,人们开采岩石,将其打碎,然后将提取出来的磷放在农业化肥里。因此,人类在磷循环的前半部起到了重要的作用,岩石的分解和磷的扩散,通过加速这个自然发生的速度。明白了么?

    磷进入泥土中,会增进植物的生长。当植物死亡时,它们会分解,那么磷就再次循环到泥土中,吃那些植物的动物也一样,或者吃其它吃过这些植物的动物。我们把这称之为陆地阶段的磷循环。但是土壤中的很多磷会被雨水或者融雪冲刷到河水中。那么就开始了另一种阶段的循环。你们能猜猜叫什么吗?南希。

    学生:呃,那个既然叫做陆地阶段循环,那么这个应该叫做水阶段循环。

    教授:是的。这个问题只有这样的难度,是吧?在正常的水阶段循环中,河水会最终倾泻到海洋,一旦进入海洋,磷就会被海藻之类的植物所吸收。鱼吃了海藻,或者吃了其它吃过这些植物的鱼。

    但是水循环阶段会被过剩的肥料所影响。如果作物没有吸收所有的磷,那么大量的磷会进入河水,这就会引起水下植物的迅速生长,最终导致有机体在水中产生堵塞,这就会改变河水的流动。目前人们针对这些影响在做一些研究,我真的希望能在生态系统遭到破坏之前找到应付这个问题的方法。

    好了,当然,人们还有一种干预正常过程的方式‐‐‐‐捕鱼。捕鱼业可以帮助磷回归陆地。在正常的水阶段循环中,剩余的磷会沉淀到海洋底部,与海洋沉积物混合在一起。但是记住,这是一个循环。海洋深处的磷必须以某种方式回到表面,来完成循环,然后从头开始这个循环。

    几百万年后,强烈的地质作用,如水下火山会提升海洋沉积物并形成新的陆地。当水下火山推动海底岩石到表面时,新陆地形成。多年后,新陆地上富含磷的岩石开始侵蚀,循环再继续。

    学生:那么,您刚才说氮循环也是很重要的养分循环。而大气层中有很多的氮, 所以我在想,大气层中也有很多的磷吗?

    教授:问得好,乔治。你猜测磷会到达地球大气层的想法是对的。它可以从陆地或者海洋到达大气层中,反之亦然。然而,和氮不一样,大气层中磷的含量非常少。

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    题型分类:主旨题
    原文定位:

    The three most important nutrient cycles are the Nitrogen cycle, the Carbon cycle, and the one we are going to talk about today, the Phosphorus cycle.

    选项分析:
    在 lecture 的开头教授提到有些营养元素在自然界中的量不多,主要靠循环来得到充分利用,借此引出了 nutrient cycle 这个概念,并且全文接下来都围绕这个内容讲述,D选项正确。

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