The global nitrogen cycle: changes and consequences

In the absence of human activities, biological N fixation is the primary source of reactive N, providing about 90-130 Tg N year(-1) (Tg = 10(12) g) on the continents. Human activities have resulted in the fixation of an additional similar to 150 Tg N year(-1) by energy production, fertilizer production, and cultivation of crops (e.g., legumes, rice). Some sinks of anthropogenic N have been estimated (e.g., N2O accumulation in the atmosphere; loss to coastal oceans), however due to the uncertainty around the magnitude of other sinks (e.g., retention in groundwater, soils, or vegetation or denitrification to N,) a possibly large portion of the N fixed by humans is missing. While we know that N is accumulating in the environment, we do not know the rate of accumulation. Due to the myriad of effects of excess N on humans, ecosystems, and the atmosphere, and their cascading nature (i.e., one atom of N can have a large number of different effects as it is transformed to different N species), this lack of knowledge is unfortunate. There are limited options available to society to reduce the amount of N mobilized by human action because there is, in effect, a N imperative - it is required for food production. As population and per capita consumption of food (especially animal products) increase, more and more N will be converted from unreactive to reactive forms in the future. This is especially true in less developed regions.