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 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.
最后一句我看不懂
there’s just not as substantial amount of
it 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