Nitrogen is not rare on Earth. In fact, the majority of our atmosphere - which you are breathing right now, is composed of nitrogen. But most of it is quite inert and not bio-available to living organisms such as plants and trees. This scarcity means that the availability of nitrogen nutrients is often the bottleneck that will control the rate of growth in many ecosystems. But what happens when humans inject more nitrogen into the nitrogen cycle (see below) via air pollution? That's what scientists from the Smithsonian Tropical Research Institute have been looking into by studying tropical forests in Panama and Thailand.
But the implications ares more complex than "more nitrogen = faster growth". According to the scientists, there is no evidence that trees are growing faster in Panama, despite the "long-term increases in nitrogen deposition and atmospheric carbon dioxide". It can also affect the ratio of different species of trees by removing the competitive advantage of species that were better at fixing nitrogen.
These are all things that climate change model should take into account to arrive at better predictions of the future, but the full effects of this nitrogen fertilization aren't yet entirely clear. This definitely requires some research, and I'll be curious to see how different the effects are at different latitudes (for example, in the boreal forest). But for know, this remains a known unknown.
Via Science Daily