Nitrogen is an important component of proteins and is essential for plant growth. But when acid rain carries the nitrogen oxides emitted in automobile exhaust into the ecosystem, the normally nitrogen-deficient natural world becomes saturated with it.
氮是蛋白質的重要成份之一，為植物成長必備的要素，但因汽機車排放的氮氧化物，隨著酸雨被帶到生態系，原本在自然界屬「不足」的氮，多年累積下就轉為「飽和」。