托福听力TPO10听力文本(Conversation+Lecture)

　　TPO10 Lecture 3

　　Ecology

　　Narrator

　　Listen to part of a lecture in an Ecology Class.

　　Professor

　　So we’ve been talking about nutrients, the elements in the environment that are essential for living organisms to develop, live a healthy life and reproduce. Some nutrients are quiet 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 eco-system processes that will cover in this course. The three most important nutrient recycles 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 cycle is a most important nutrient and it’s not so abundant. The largest quantities are found in rocks at the bottom of the ocean. How the Phosphorus get there? Well, let’s start with the Phosphorus in rocks. The rocks get broken down into smaller and smaller particles as they are weathered. They are weathered slowly by rain and wind over long periods of time. Phosphorus is slowly released as the rocks are broken down and then it gets spread around into the soil. Once it’s in the soil, plants absorb it through their roots.

　　Student

　　So that’s the reason people mine rocks that contain a lot of Phosphorus to help the agriculture?

　　Professor

　　Hum, they mined the rock, artificially break it down and put the Phosphorus into the agricultural fertilizers. So humans can play a role in a first part of the Phosphorus cycle -- the breaking down of rocks and the spreading 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 anyone guess what it is called? Nancy

　　Nancy

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

　　Professor

　　Yes, 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 sometime affected by excessive fertilizers. If not all of Phosphorus gets used by the crops and larger 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 waste 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 the 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 under water volcanoes lift up the ocean sediments to form new land. When an under volcano pushes submerge rock to the surface, a new island is created. Then over many more years the Phosphorus reach rocks of the new land begin to erode and the cycle continues.

　　Guy

　　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?

　　Professor

　　Good question, George. You’re right to guess the Phosphorus can end up in earth atmosphere. It can move from the land or from the oceans to the atmosphere, and vice versa. However, there’s just not as substantial amount of it there, like there is with nitrogen, it’s a very minimal quantity.