Bingo.

An international group of scientists say there is an immediate need for a global assessment of the nitrogen cycle and its impact on climate.

"The nitrogen cycle is changing faster than that of any other element. In addition, the effects of reactive nitrogen are not limited to a single medium. A single molecule of reactive nitrogen may transition through many forms -- ammonia, nitrogen oxide, nitric acid, nitrate and organic nitrogen -- and may successively lead to a number of environmental, health and social impacts, including contributing to higher levels of ozone in the lower atmosphere."