Figure 3. Global nitrogen reservoirs, fluxes
and turnover times. Major
reservoirs are underlined, pool sizes and fluxes are given in Tg (1012 g) N and Tg N yr-1. Turnover times (reservoir divided by largest flux to or from reservoir ) are in
parentheses. To convert Tg N to moles N, multiply by 7.1 x 1010.
As with oxygen, the atmosphere, which contains 78% N2, is the
largest nitrogen reservoir. Other gaseous nitrogen species important in ozone chemistry have short lifetimes and are of local importance. Nitrous oxide, an important long-lived greenhouse
gas, is photolyzed in the stratosphere. The thermodynamically stable form of nitrogen in the presence of oxygen is not N2, but NO3-. However, conversion of relatively inert N2 to other forms is limited by the microbially mediated nitrogen fixation rate,
and fixed nitrogen is rapidly incorporated into living tissue. Note that the anthropogenic nitrogen fixation rate is about 60% that of natural fixation. Microbially mediated denitrification completes the nitrogen cycle. The nitrogen biomass reservoirs
are based on carbon reservoirs and the C:N ratio.