The budgets of the volatile elements hydrogen, carbon and nitrogen stem from the initial contents given by the accretion processes and the integrated fluxes out and into the mantle during the Earth's history. After the final accretion of a late veneer of either C1 chondrite or cometary material, the initial repartition of hydrogen, carbon and nitrogen, respectively expressed in units of present ocean mass, present crustal carbon mass and present atmosphere mass were the following :
Surface of the Earth : hydrogen (water), 0.5 carbon, 3 nitrogen, 3.
Upper mantle : hydrogen (water), 0.8 carbon, 2.5 nitrogen, 8.
Due to somewhat different behavior of these three elements during the outgassing at ocean ridges and recycling by the subduction the present repartition is now :
Surface of the Earth : hydrogen (water), 1 carbon, 1 nitrogen, 1.
Upper mantle : hydrogen (water), 0.2 carbon, 4.5 nitrogen, 10.
In particular , while the present C/N ratios in the surface and mantle materials are not grossly different, the carbon and nitrogen fluxes in and out of these reservoirs and the evolution of their repartition are quite different : nitrogen has low fluxes in both directions (residence time in the surface reservoir of around one billion years) but has been steadily accumulating in the upper mantle and still is. Carbon fluxes are much higher and correspond to a residence time of 400 millions years and the upper mantle-surface circulation has reached a steady state.
This is reflected by the fact that the overall isotopic composition of carbon in both reservoirs is identical : -4 /PDB, whereas the nitrogen isotopic composition is in gross disequilibrium : d15N slightly positive for the surface, -5 to 7 for the upper mantle.
This is due to the fact that the storage upper mantle form of nitrogen is much more stable than that of carbon : carbon is essentially an incompatible element while nitrogen has a rather large degree of compatibility.