托福tpo10 阅读部分 ecology

托福tpo10 阅读部分 ecology最后一段You’re right to guess the Phosphorus can end up inearth atmosphere. It can move from the land or from the oceans to theatmosphere, and vice versa. However, there’s just not as substantial amount ofit there, like there is with nitrogen, it’s a very minimal quantity.最后一句我看不懂 there’s just not as substantial amount ofit there, like there is with nitrogen, it’s a very minimal quantity.这句话怎么理解,这里的it代指什么