It is one of nature’s great ironies that the availability of nitrogen in the soil frequently sets an upper limit on plant growth even though the plants’ leaves　are bathed in a sea of nitrogen gas.

　　=It is one of nature’s great ironies/ that the availability of nitrogen/ in the soil/ frequently sets an upper limit on plant growth/ even though the plants’ leaves　are bathed in a sea of nitrogen gas.

　　*自然界最大的矛盾之一/就是氮供给/土壤中的/通常限定了一个上限植物生长的/，尽管植物的叶子沉浸在一片氮气的海洋中。

　　*这个句子很容易理解，但是很有意思，关键词是irony，主要是搞清楚irony的内容是什么。