Nitrogen Cycle

In order to understand how nitrogen moves throughout the earth/ocean/atmosphere environment, we need to look at the nitrogen cycle, which gives the sources and sinks of nitrogen and the fluxes between these reservoirs, as shown in Figure 6. These values may be subject to large errors and are for illustration only. The main concept to draw from this figure is that there are two components of the nitrogen cycle: the right hand side represents tropospheric interactions and interchange with the surface for $NO$_X, and the left hand side describes nitrous oxide, which is considered separately. The components on the right hand side are all part of the rapid cycle: being quite reactive, these constituents will cycle into and out of the lower atmosphere within a couple weeks. But, of course, since we continue to put in more all the time, there is this continuous supply in the atmosphere. However, if we were able to switch off all of natural and anthropogenic sources, the atmospheric concentrations of these molecules would rapidly deplete to zero within a couple of weeks.

Also shown are the different pathways and transformations throughout these various cycles. So, for instance, some nitrogen enters the atmosphere as NO, is transformed to $NO$₂, and then may go back into the soils as $NO$₂ or it may be converted to $NO$₃ in an aerosol and then be rained out or deposited out as a dry particle. The gas may form into particles directly or attach to rain droplets and eventually end up back in the soil or in the ocean. A similar description applies to the various transformations of ammonia ($NH$₃).

Nitrous oxide, on the other hand, follows a different pathway: it experiences no chemical transformation or rainout or dry deposition in the troposphere. Once emitted, the nitrous oxide molecule drifts throughout the lower atmosphere, possibly for decades, until it makes a chance visit to the lower stratosphere where it is broken down by ultraviolet light into O and N or NO. The Ns can form $N$₂, but the NO is available to participate in ozone depletion. The point of Figure 7 is that, of the nitrogen compounds emitted at the surface by natural or anthropogenic process, only nitrous oxide lives long enough to migrate to the stratosphere to create problems at that level. Oxides of nitrogen in the lower atmosphere cause acid rain problems, as we will see later, but they do not contribute significantly to global warming or ozone depletion.

There are several unanswered questions relating to nitrogen movement through the biosphere, atmosphere and soil. These questions are being addressed through intensive research on biogeochemicals in the lower troposphere.

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