  Nitrogen is the most common element in Earth's atmosphere and also appears to
be present in significant amounts in the mantle. However, its long-term cycling
between these two reservoirs remains poorly understood. Here a range of biotic
and abiotic mechanisms are evaluated that could have caused nitrogen exchange
between Earth's surface and interior over time. In the Archean, biological
nitrogen fixation was likely strongly limited by nutrient and/or electron
acceptor constraints. Abiotic fixation of dinitrogen becomes efficient in
strongly reducing atmospheres, but only once temperatures exceed around 1000 K.
Hence if atmospheric N2 levels really were as low as they are today 3.0 - 3.5
Ga, the bulk of Earth's mantle nitrogen must have been emplaced in the Hadean,
most likely at a time when the surface was molten. The elevated atmospheric N
content on Venus compared to Earth can be explained abiotically by a water loss
redox pump mechanism, where oxygen liberated from H2O photolysis and subsequent
H loss to space oxidises the mantle, causing enhanced outgassing of nitrogen.
This mechanism has implications for understanding the partitioning of other
Venusian volatiles and atmospheric evolution on exoplanets.
