新托福考试必备：新托福TPO（1-24）听力原文文本TPO10

　　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.